Prepare ConfigKit v0.2.0

CONFIGKIT_DESIGN_PROPOSAL.md

@@ -1,861 +0,0 @@
-# ConfigKit — Initial Design and Architecture
-
-## Status
-
-**Design note for initial implementation.**
-
-Audience: future maintainers of ConfigKit, including LLM coding agents assisting with implementation.
-
-This document captures the current architectural and policy decisions for ConfigKit. The goal is to establish a sufficiently opinionated v1 design that can be implemented consistently across the application suite without prematurely becoming a general-purpose configuration framework.
-
----
-
-## 1. Purpose
-
-ConfigKit is a shared Go library for configuration loading and resolution across applications such as Seriatim, Audita, Narratio, WeatherFeeder, WeatherReporter, Distributor, and related tools.
-
-Its purpose is to provide a consistent, blessed configuration lifecycle for applications that use YAML configuration files.
-
-ConfigKit should solve recurring infrastructure concerns once:
-
-- file loading;
-- YAML parsing and strictness;
-- deterministic imports / `conf.d` composition;
-- duplicate-definition detection;
-- intentional higher-precedence overlays such as profiles;
-- presence tracking;
-- source/provenance tracking;
-- typed decoding;
-- standardized diagnostics;
-- effective-configuration inspection;
-- support for application defaults, derived defaults, and validation.
-
-ConfigKit is **not** intended to become a universal configuration framework.
-
----
-
-## 2. Design Principles
-
-### 2.1 YAML is syntax; ConfigKit defines semantics
-
-The YAML parser must not implicitly define ConfigKit behavior for:
-
-- composition;
-- precedence;
-- duplicate keys;
-- sequence merging;
-- `null`;
-- imports;
-- defaults;
-- profile overlays.
-
-These behaviors must be explicit ConfigKit policy.
-
-### 2.2 Preserve information until late in the pipeline
-
-ConfigKit should retain the parsed YAML tree and source metadata through composition and overlay processing.
-
-Do **not** immediately reduce configuration to `map[string]any` or typed Go structs.
-
-The implementation should preserve enough information to answer:
-
-> Why does this effective value have this value, and where did it come from?
-
-### 2.3 Keep application-facing Go types ordinary
-
-Applications should normally use idiomatic types:
-
-```go
-type Config struct {
-    Enabled bool
-    Workers int
-    Timeout time.Duration
-}
-```
-
-ConfigKit should not require pervasive `*bool`, `*int`, or `Optional[T]` fields merely to distinguish absence from explicit zero values.
-
-Parser-level presence is metadata owned by ConfigKit.
-
-Application-level optionality should remain explicit in the application's Go type where it is genuinely part of the application's semantics.
-
----
-
-## 3. Responsibility Boundary
-
-### ConfigKit owns
-
-- configuration source loading;
-- YAML parsing;
-- YAML policy enforcement;
-- import discovery;
-- deterministic import ordering;
-- strict composition;
-- overlay application;
-- duplicate-definition detection;
-- source location tracking;
-- presence tracking;
-- definition history / provenance;
-- typed decoding;
-- standard diagnostics;
-- effective-config rendering;
-- config source reporting;
-- config fingerprinting, if implemented;
-- redaction mechanisms for secrets.
-
-### Consuming applications own
-
-- their `Config` Go types;
-- static default values;
-- derived/defaulted values;
-- semantic validation;
-- schema meaning;
-- schema migrations;
-- runtime initialization;
-- runtime resource validation;
-- hot reload and resource replacement.
-
----
-
-## 4. Configuration Resolution Model
-
-The conceptual lifecycle is:
-
-```text
-source discovery
-      ↓
-YAML parsing
-      ↓
-YAML policy validation
-      ↓
-strict base composition
-      ↓
-profile overlay
-      ↓
-static defaults / typed decode
-      ↓
-derived defaults / normalization
-      ↓
-semantic validation
-      ↓
-immutable effective configuration
-```
-
-A useful precedence model is:
-
-```text
-LOW PRECEDENCE
-
-application defaults
-        ↓
-base configuration
-  ├─ main file
-  └─ imports / conf.d     ← strict composition
-        ↓
-selected profile         ← intentional overlay
-
-HIGH PRECEDENCE
-```
-
-Future layers such as environment-variable or CLI value overrides may be added later, but are not required for v1.
-
----
-
-## 5. YAML Policy
-
-YAML remains the configuration format.
-
-ConfigKit should support a conservative YAML subset rather than arbitrary YAML semantics.
-
-Recommended v1 policy:
-
-- one YAML document per file;
-- mappings, sequences, and scalars;
-- string mapping keys;
-- comments;
-- unique mapping keys;
-- reject unknown fields when decoding into typed configuration;
-- reject custom YAML tags;
-- reject YAML merge keys (`<<`);
-- strongly consider rejecting anchors / aliases;
-- reject ambiguous or unsupported constructs rather than guessing.
-
-The intended model is approximately:
-
-> JSON's data model, with comments and more pleasant human syntax.
-
-The underlying YAML implementation must remain an internal implementation detail. Public ConfigKit APIs should not expose parser-specific node or error types.
-
----
-
-## 6. Presence Semantics
-
-ConfigKit must distinguish at least these states:
-
-| State | Meaning |
-|---|---|
-| key absent | no opinion; a lower-precedence/default value may apply |
-| `key: false`, `key: 0`, `key: ""` | explicitly configured value |
-| `key: null` | explicitly null; not equivalent to absent |
-| ordinary nonzero value | explicitly configured value |
-
-Presence information must survive configuration resolution even after the final typed Go value is produced.
-
-Applications should be able to determine, when necessary, whether an effective value was explicitly configured or defaulted.
-
-Conceptual metadata API:
-
-```go
-result.Config
-result.Metadata.IsExplicit("transcription.concurrency")
-result.Metadata.Origin("transcription.concurrency")
-result.Metadata.History("transcription.concurrency")
-```
-
-Exact API names are not yet fixed.
-
----
-
-## 7. Defaults
-
-### 7.1 Static defaults
-
-Applications define static defaults, for example:
-
-```go
-func DefaultConfig() Config {
-    return Config{
-        Enabled: true,
-        Workers: 4,
-        Timeout: 30 * time.Second,
-    }
-}
-```
-
-Explicit YAML values such as `false` and `0` must override those defaults.
-
-Absent YAML values leave defaults intact.
-
-### 7.2 Derived defaults
-
-Defaults that depend on other resolved fields should be applied by an application-owned resolver after typed decoding.
-
-Example:
-
-```go
-func ResolveConfig(cfg *Config, meta configkit.Metadata) error {
-    if !meta.IsExplicit("timeout") {
-        cfg.Timeout = cfg.PollInterval / 10
-    }
-    return nil
-}
-```
-
-ConfigKit should provide a standard lifecycle hook for this but should not own application-specific derived-default logic.
-
----
-
-## 8. `null`
-
-`null` is **not** equivalent to absence.
-
-Recommended v1 rule:
-
-> `null` is an explicit value and is valid only where the application schema permits null.
-
-For ordinary scalar fields such as:
-
-```go
-Workers int
-Enabled bool
-```
-
-this should be invalid:
-
-```yaml
-workers: null
-```
-
-Nullable application semantics may be represented using suitable Go types such as pointers.
-
-Do not use `null` in v1 to mean "reset this value to the application default."
-
-If profile-level reset-to-default semantics are eventually required, add an explicit ConfigKit mechanism rather than overloading YAML `null`.
-
----
-
-## 9. Strict Composition
-
-Base configuration may be assembled from a main file plus imported fragments, including Debian-style `conf.d` directories.
-
-Composition means:
-
-> Multiple files contribute non-conflicting portions of one logical configuration.
-
-A duplicate semantic value path across composed files is an error.
-
-Example allowed:
-
-```yaml
-# 10-server.yaml
-server:
-  host: localhost
-```
-
-```yaml
-# 20-server.yaml
-server:
-  port: 8080
-```
-
-These define different paths:
-
-```text
-server.host
-server.port
-```
-
-Example rejected:
-
-```yaml
-# A
-server:
-  port: 8080
-```
-
-```yaml
-# B
-server:
-  port: 9000
-```
-
-Both define:
-
-```text
-server.port
-```
-
-The diagnostic should identify both source locations.
-
-### 9.1 Mapping behavior
-
-Mappings compose recursively by semantic path.
-
-### 9.2 Sequence behavior
-
-Sequences do not implicitly compose or merge.
-
-If two composed files define the same sequence path, that is a collision.
-
-When configuration needs independently composable named entries, prefer mappings keyed by stable identifiers rather than lists.
-
----
-
-## 10. Intentional Overlay
-
-Overlay is a separate operation from composition.
-
-Overlay means:
-
-> A higher-precedence layer intentionally replaces values from a lower-precedence layer.
-
-For mappings, overlay recursively by key.
-
-For scalar values, the higher-precedence value wins.
-
-For sequences, the higher-precedence sequence replaces the lower-precedence sequence in full.
-
-Do not implement implicit sequence append, union, or index-wise merging in v1.
-
----
-
-## 11. Profiles
-
-Profiles are the first intended overlay mechanism.
-
-Example layout:
-
-```text
-config/
-├── narratio.yml
-├── conf.d/
-│   ├── 10-transcription.yml
-│   ├── 20-artifacts.yml
-│   └── 30-distribution.yml
-└── profiles/
-    ├── production.yml
-    └── testing.yml
-```
-
-Default invocation loads only the composed base configuration.
-
-Example:
-
-```text
-narratio
-```
-
-Explicit profile selection:
-
-```text
-narratio --config-profile=testing
-```
-
-produces:
-
-```text
-application defaults
-        ↓
-main config + imports
-        ↓
-profiles/testing.yml
-```
-
-### 11.1 `default` profile
-
-"Default" should mean **no profile overlay**.
-
-Do not require a literal `profiles/default.yml`.
-
-### 11.2 Profile names
-
-`--config-profile` should accept a profile name, not an arbitrary path.
-
-Profile names should use a constrained character set, e.g.:
-
-```text
-[a-zA-Z0-9_-]+
-```
-
-The selected profile should resolve deterministically relative to the configuration root.
-
-### 11.3 Profiles should not redefine the configuration graph
-
-Profiles should modify application configuration values, not change how ConfigKit discovers configuration.
-
-Profile overlays should therefore not introduce new imports or redefine import behavior.
-
-The base configuration graph is discovered and composed first; the selected profile is then applied to the resulting configuration tree.
-
----
-
-## 12. Import Policy
-
-Import behavior must be deterministic and explicitly specified.
-
-Recommended v1 rules:
-
-- relative imports resolve relative to the file containing the import;
-- glob results are sorted deterministically;
-- duplicate semantic values across imported fragments are errors;
-- explicit missing imports are errors;
-- an optional empty `conf.d` glob may resolve to zero files;
-- import cycles are errors;
-- cycle diagnostics should show the full cycle;
-- define file identity carefully with respect to symlinks;
-- consider constraining imports to an allowed configuration root;
-- do not support remote/network imports in v1.
-
-Remote imports create separate concerns around availability, caching, authentication, integrity, startup latency, and freshness and should remain out of scope.
-
----
-
-## 13. Provenance and Definition History
-
-Provenance is a first-class design requirement.
-
-ConfigKit should retain, for each effective semantic value path:
-
-- whether it was absent or explicitly configured;
-- its effective value source;
-- source file;
-- line and column where practical;
-- configuration layer;
-- previous definitions overridden by higher-precedence layers.
-
-Conceptually:
-
-```go
-type Definition struct {
-    Layer  Layer
-    Source Source
-    Value  NodeRef
-}
-
-type FieldHistory struct {
-    Definitions []Definition
-}
-```
-
-Possible layers:
-
-```go
-const (
-    LayerDefault Layer = iota
-    LayerBase
-    LayerProfile
-)
-```
-
-Future layers may include environment or CLI overrides.
-
-This metadata should enable diagnostics such as:
-
-```text
-transcription.concurrency = 1
-
-history:
-  application default                     4
-  conf.d/10-transcription.yml:14          8
-  profiles/testing.yml:7                  1
-
-effective source:
-  profiles/testing.yml:7
-```
-
----
-
-## 14. Typed Decoding
-
-After composition and overlays produce one effective presence-aware configuration tree, ConfigKit decodes it into the application's typed Go `Config`.
-
-Requirements:
-
-- unknown fields are errors;
-- type mismatches are errors;
-- diagnostics should preserve source location where possible;
-- parser-specific errors should be converted into ConfigKit diagnostics.
-
-Do not expose raw YAML parser types through the public API.
-
----
-
-## 15. Semantic Validation
-
-Application-specific validation remains application-owned.
-
-ConfigKit should provide a standard validation hook and diagnostic type.
-
-Validation should normally collect multiple independent errors rather than aborting after the first.
-
-Example output:
-
-```text
-configuration invalid:
-
-  weather.sources.nws.timeout
-    must be > 0
-    config.yml:19:5
-
-  outputs.public
-    duplicate definition
-    config.yml:42:3
-    conf.d/90-output.yml:8:3
-```
-
----
-
-## 16. Schema Versioning
-
-Applications should support explicit configuration schema versions.
-
-Example:
-
-```yaml
-schema_version: narratio.v1
-```
-
-The version identifies the application's configuration schema, not the ConfigKit library version.
-
-ConfigKit may later provide lifecycle support for application-provided migrations, but automatic migration is not required for the initial implementation.
-
-The design should leave a clean pipeline location for schema-version checking and migration.
-
----
-
-## 17. Secrets and Redaction
-
-ConfigKit will eventually process configuration containing secrets.
-
-Redaction must be considered before implementing:
-
-- effective-config output;
-- diagnostics;
-- config diffs;
-- logging;
-- config fingerprinting.
-
-Potential application annotation:
-
-```go
-APIKey string `yaml:"api_key" configkit:"secret"`
-```
-
-Exact mechanism is TBD.
-
-Human-facing tooling must never accidentally emit raw secrets.
-
-If fingerprints include secret values, the raw values must still never be exposed.
-
----
-
-## 18. Diagnostics and Operability
-
-ConfigKit should establish recognizable diagnostics across the application suite.
-
-Potential standardized commands enabled by ConfigKit:
-
-```text
-<app> config validate
-<app> config effective
-<app> config sources
-<app> config explain <path>
-```
-
-These commands do not all need to exist in the initial skeleton, but the metadata model should make them straightforward to implement later.
-
-`config effective` must support secret redaction.
-
----
-
-## 19. Non-Goals for v1
-
-Do not include these in the initial implementation unless a concrete application requirement appears:
-
-- hot reload;
-- runtime resource replacement;
-- remote/network imports;
-- arbitrary configuration providers;
-- TOML/JSON/HCL/CUE support;
-- Vault/S3 configuration providers;
-- sophisticated environment-variable overlays;
-- arbitrary CLI field overrides;
-- sequence append/union merge strategies;
-- arbitrary custom merge policies;
-- profile inheritance;
-- reset-to-default operators;
-- application runtime validation such as checking whether an S3 bucket exists.
-
-ConfigKit should remain opinionated.
-
----
-
-## 20. Initial Internal Architecture
-
-A reasonable internal model is:
-
-```text
-Source
-  ↓
-Document
-  ├─ parsed tree
-  └─ source locations
-  ↓
-Composer
-  strict combination
-  duplicate definitions = error
-  ↓
-Overlay
-  higher layer wins
-  mappings recurse
-  sequences replace
-  ↓
-Metadata / Provenance
-  presence
-  origins
-  definition history
-  ↓
-Decoder[T]
-  effective tree → typed Config
-  ↓
-Resolver[T]
-  application-derived defaults / normalization
-  ↓
-Validator[T]
-  application semantic validation
-  ↓
-Result[T]
-```
-
-Conceptual result:
-
-```go
-type Result[T any] struct {
-    Config      T
-    Metadata    Metadata
-    Sources     []Source
-    Diagnostics []Diagnostic
-    Fingerprint string // optional / future
-}
-```
-
----
-
-## 21. Possible Package Skeleton
-
-Exact names are not settled, but an initial repository could look roughly like:
-
-```text
-configkit/
-├── configkit.go
-├── source.go
-├── document.go
-├── compose.go
-├── overlay.go
-├── metadata.go
-├── decode.go
-├── diagnostic.go
-├── profile.go
-├── import.go
-├── schema.go
-├── secret.go
-└── internal/
-    └── yaml/
-```
-
-Public API should remain small.
-
-A future happy path might resemble:
-
-```go
-result, err := configkit.Load[Config](
-    configkit.File(path),
-    configkit.WithImports(),
-    configkit.WithProfile(profile),
-    configkit.WithDefaults(DefaultConfig()),
-    configkit.WithResolver(ResolveConfig),
-    configkit.WithValidator(ValidateConfig),
-)
-```
-
-This is illustrative only. Do not prematurely lock in option names or signatures.
-
----
-
-## 22. Suggested Initial Implementation Sequence
-
-### Stage 1 — Core document model
-
-Implement:
-
-- local file source;
-- YAML parsing;
-- conservative YAML validation;
-- source locations;
-- semantic path traversal;
-- internal presence-aware tree representation.
-
-### Stage 2 — Strict composition
-
-Implement:
-
-- recursive mapping composition;
-- duplicate semantic-path detection;
-- deterministic import ordering;
-- useful duplicate diagnostics.
-
-### Stage 3 — Profile overlay
-
-Implement:
-
-- constrained profile name resolution;
-- recursive mapping overlay;
-- scalar replacement;
-- sequence replacement;
-- provenance history across layers.
-
-### Stage 4 — Typed decoding
-
-Implement:
-
-- generic `Load[T]` or equivalent;
-- strict unknown-field rejection;
-- typed errors converted to ConfigKit diagnostics.
-
-### Stage 5 — Defaults and resolution hooks
-
-Implement:
-
-- static application defaults;
-- explicit-value preservation for `false`, `0`, and empty strings;
-- metadata query for explicitness;
-- derived-default / resolver hook.
-
-### Stage 6 — Semantic validation
-
-Implement:
-
-- application validation hook;
-- multi-error diagnostics.
-
-### Stage 7 — Introspection
-
-Implement or prototype:
-
-- source listing;
-- effective-config rendering;
-- `explain(path)`;
-- secret redaction.
-
----
-
-## 23. Tests That Define Policy
-
-ConfigKit should have policy-focused tests, not merely implementation tests.
-
-At minimum, test:
-
-- absent bool receives default;
-- explicit `false` overrides `true` default;
-- absent integer receives default;
-- explicit `0` overrides nonzero default;
-- explicit empty string remains explicit;
-- `null` differs from absent;
-- duplicate key within one YAML file fails;
-- duplicate semantic path across composed files fails;
-- sibling mapping paths compose successfully;
-- profile scalar overrides base scalar;
-- profile mapping recursively overrides only specified children;
-- profile sequence replaces base sequence;
-- base sequence collision across composed files fails;
-- profile cannot redefine import graph;
-- imports are deterministically ordered;
-- import cycles fail with useful diagnostics;
-- unknown typed-config fields fail;
-- provenance identifies effective source;
-- provenance retains overridden definitions;
-- defaults are distinguishable from explicit equal-valued configuration;
-- secrets are redacted from introspection output.
-
-These tests should become executable documentation for ConfigKit policy.
-
----
-
-## 24. Key Decisions Summary
-
-For v1:
-
-1. YAML remains the configuration language.
-2. ConfigKit defines a conservative YAML policy.
-3. Parsed configuration remains presence-aware until after composition and overlays.
-4. Application-facing Go structs remain idiomatic and strongly typed.
-5. Absent, explicit zero/false/empty, and `null` are distinct.
-6. Base imports use strict composition.
-7. Duplicate semantic values across composed files are errors.
-8. Profiles use explicit higher-precedence overlay semantics.
-9. Mapping overlays recurse.
-10. Sequence overlays replace.
-11. "Default profile" means no profile overlay.
-12. Profiles are names, not arbitrary file paths.
-13. Profiles do not redefine the import graph.
-14. Static defaults are application-owned.
-15. Derived defaults are application-owned and presence-aware.
-16. Provenance and definition history are first-class metadata.
-17. Semantic validation is application-owned but follows a common lifecycle.
-18. Parser implementation details remain private.
-19. Secrets must be redactable.
-20. Hot reload, remote providers, generalized merge strategies, and broad extensibility are non-goals until concrete requirements exist.
-
-The core architectural objective is:
-
-> **Build one deterministic, explainable configuration resolution model for the entire application suite.**

README.md

@@ -1,3 +1,39 @@
 # ConfigKit
 
 An opinionated Go library for applications that use YAML configuration files.
+
+ConfigKit's initial loader accepts one strict YAML
+source (or explicit defaults-only input), optionally traverses explicit path and glob
+imports, strictly composes their disjoint definitions, applies typed defaults, and
+can overlay one selected fixed profile and check an application schema version. It
+returns detached metadata and safe diagnostics.
+Applications may perform audited defaulting and normalization through a resolver,
+then report domain errors and warnings through read-only semantic validation. Secret
+tags and a path predicate drive deterministic redacted effective YAML. Typed secret
+references resolve environment names before validation, and explicit
+secrets-directory bootstrap is available as a separate startup operation. Explicit
+CLI override records apply above profiles. Value explanation is available through
+detached structured records and deterministic redacted output.
+
+```go
+type Config struct {
+    Enabled bool `yaml:"enabled"`
+    Workers int  `yaml:"workers"`
+}
+
+func ReadConfig(path string) (Config, error) {
+    result, err := configkit.Load[Config](configkit.File(path))
+    if err != nil {
+        return Config{}, err
+    }
+    return result.Config, nil
+}
+```
+
+The [configuration contract](docs/config.md) describes implemented behavior. The
+[consumer adoption guide](docs/consumers/README.md) provides maintained Narratio-,
+Notarius-, WeatherReporter-, and Seriatim-shaped examples and migration guidance.
+The [deferred scope](docs/roadmap/deferred.md) records deliberate exclusions. See the
+[development guide](docs/development.md) for repository orientation and the
+[release procedure](docs/release.md) for maintainer-only candidate and publication
+steps.

callback_ownership_test.go

@@ -0,0 +1,52 @@
+package configkit
+
+import "testing"
+
+func TestCallbackAssignmentsAreDetachedAcrossLoads(t *testing.T) {
+	type object struct{ Value string }
+	type cfg struct {
+		Map     map[string][]*object
+		Pointer *object
+		Secret  Secret
+	}
+	for _, validatorAssignment := range []bool{false, true} {
+		name := "resolver"
+		if validatorAssignment {
+			name = "validator"
+		}
+		t.Run(name, func(t *testing.T) {
+			external := &object{Value: "original"}
+			shared := map[string][]*object{"key": {external}}
+			definition := Definition[cfg]{Resolve: func(c *cfg, r *Resolution) error {
+				Normalize(r, Keys("map"), &c.Map, shared, "populate")
+				c.Pointer = external
+				r.Record(Keys("pointer"), ChangeNormalization, "populate")
+				c.Secret = SecretEnv("CONFIGKIT_CALLBACK_SECRET")
+				r.Record(Keys("secret"), ChangeNormalization, "reference")
+				return nil
+			}}
+			if validatorAssignment {
+				definition.Validate = func(c *cfg, v *Validation) { c.Map = shared; c.Pointer = external }
+			}
+			calls := 0
+			lookup := func(string) (string, bool) { calls++; return "credential", true }
+			first, err := loadWithSecretLookup(NoSource(), lookup, Options[cfg]{Definition: definition})
+			if err != nil {
+				t.Fatal(err)
+			}
+			second, err := loadWithSecretLookup(NoSource(), lookup, Options[cfg]{Definition: definition})
+			if err != nil {
+				t.Fatal(err)
+			}
+			first.Config.Map["key"][0].Value = "changed"
+			first.Config.Pointer.Value = "changed pointer"
+			external.Value = "external change"
+			if second.Config.Map["key"][0].Value != "original" || second.Config.Pointer.Value != "original" {
+				t.Fatal("published configuration shares callback state")
+			}
+			if second.Config.Secret.Value() != "credential" || !second.Config.Secret.IsResolved() || calls != 2 {
+				t.Fatal("publication changed secret state or repeated environment lookup")
+			}
+		})
+	}
+}

clone.go

@@ -0,0 +1,159 @@
+package configkit
+
+import (
+	"encoding"
+	"math"
+	"reflect"
+)
+
+// secretCloneMode distinguishes fresh unresolved references from publication of
+// already validated private state, which must not trigger another lookup.
+type secretCloneMode uint8
+
+const (
+	cloneUnresolvedSecrets secretCloneMode = iota
+	cloneValidatedSecrets
+)
+
+func cloneConfiguration(schema *schemaDescriptor, value reflect.Value, limits projectionLimits) (reflect.Value, *projectionIssue) {
+	return cloneConfigurationWithMode(schema, value, limits, cloneUnresolvedSecrets)
+}
+
+func cloneConfigurationWithMode(schema *schemaDescriptor, value reflect.Value, limits projectionLimits, mode secretCloneMode) (reflect.Value, *projectionIssue) {
+	if schema == nil || schema.root == nil || !value.IsValid() || value.Type() != schema.root.typeOf {
+		return reflect.Value{}, &projectionIssue{code: CodeSchemaUnsupportedType, message: "configuration value does not match its schema"}
+	}
+	operation := valueOperation{schema: schema, limits: limits, active: make(map[valueVisit]bool), cloneMode: mode}
+	return operation.clone(value, schema.root, Path{}, 0)
+}
+
+func (o *valueOperation) clone(value reflect.Value, descriptor *typeDescriptor, path Path, depth int) (reflect.Value, *projectionIssue) {
+	if issue := o.checkDepth(path, depth); issue != nil {
+		return reflect.Value{}, issue
+	}
+	if descriptor.kind == descriptorPointer {
+		if value.IsNil() {
+			return reflect.Zero(value.Type()), nil
+		}
+		leave, issue := o.enter(value, path)
+		if issue != nil {
+			return reflect.Value{}, issue
+		}
+		defer leave()
+		child, issue := o.clone(value.Elem(), descriptor.element, path, depth)
+		if issue != nil {
+			return reflect.Value{}, issue
+		}
+		result := reflect.New(value.Type().Elem())
+		result.Elem().Set(child)
+		return result, nil
+	}
+
+	switch descriptor.kind {
+	case descriptorStruct:
+		result := reflect.New(value.Type()).Elem()
+		for _, member := range descriptor.members {
+			memberPath, memberDepth := path, depth
+			if !member.inline {
+				memberPath, memberDepth = path.Key(member.name), depth+1
+			}
+			cloned, issue := o.clone(value.Field(member.index), member.descriptor, memberPath, memberDepth)
+			if issue != nil {
+				return reflect.Value{}, issue
+			}
+			result.Field(member.index).Set(cloned)
+		}
+		return result, nil
+	case descriptorMap:
+		if value.IsNil() {
+			return reflect.Zero(value.Type()), nil
+		}
+		leave, issue := o.enter(value, path)
+		if issue != nil {
+			return reflect.Value{}, issue
+		}
+		defer leave()
+		result := reflect.MakeMapWithSize(value.Type(), value.Len())
+		for _, key := range value.MapKeys() {
+			cloned, issue := o.clone(value.MapIndex(key), descriptor.element, path.Key(key.String()), depth+1)
+			if issue != nil {
+				return reflect.Value{}, issue
+			}
+			result.SetMapIndex(key, cloned)
+		}
+		return result, nil
+	case descriptorSlice:
+		if value.IsNil() {
+			return reflect.Zero(value.Type()), nil
+		}
+		leave, issue := o.enter(value, path)
+		if issue != nil {
+			return reflect.Value{}, issue
+		}
+		defer leave()
+		result := reflect.MakeSlice(value.Type(), value.Len(), value.Len())
+		for i := 0; i < value.Len(); i++ {
+			cloned, issue := o.clone(value.Index(i), descriptor.element, path.Index(i), depth+1)
+			if issue != nil {
+				return reflect.Value{}, issue
+			}
+			result.Index(i).Set(cloned)
+		}
+		return result, nil
+	case descriptorArray:
+		result := reflect.New(value.Type()).Elem()
+		for i := 0; i < value.Len(); i++ {
+			cloned, issue := o.clone(value.Index(i), descriptor.element, path.Index(i), depth+1)
+			if issue != nil {
+				return reflect.Value{}, issue
+			}
+			result.Index(i).Set(cloned)
+		}
+		return result, nil
+	case descriptorDynamic:
+		if value.IsNil() {
+			return reflect.Zero(value.Type()), nil
+		}
+		concrete := value.Elem()
+		dynamic, issue := o.dynamicDescriptor(concrete.Type(), path)
+		if issue != nil {
+			return reflect.Value{}, issue
+		}
+		cloned, issue := o.clone(concrete, dynamic, path, depth)
+		if issue != nil {
+			return reflect.Value{}, issue
+		}
+		result := reflect.New(value.Type()).Elem()
+		result.Set(cloned)
+		return result, nil
+	case descriptorSecret:
+		result := reflect.New(value.Type()).Elem()
+		reference := value.Interface().(Secret)
+		if o.cloneMode == cloneValidatedSecrets {
+			result.Set(value)
+		} else if reference.specified {
+			result.Set(reflect.ValueOf(SecretEnv(reference.EnvName())))
+		}
+		return result, nil
+	case descriptorText:
+		text, issue := marshalCanonicalText(value, path)
+		if issue != nil {
+			return reflect.Value{}, issue
+		}
+		result := reflect.New(value.Type()).Elem()
+		unmarshaler := result.Addr().Interface().(encoding.TextUnmarshaler)
+		if err := unmarshaler.UnmarshalText([]byte(text)); err != nil {
+			return reflect.Value{}, &projectionIssue{code: CodeSchemaCodec, path: path.clone(), message: "custom text scalar could not be reconstructed"}
+		}
+		return result, nil
+	case descriptorFloat:
+		if math.IsNaN(value.Float()) || math.IsInf(value.Float(), 0) {
+			return reflect.Value{}, &projectionIssue{code: CodeSchemaNonFinite, path: path.clone(), message: "configuration float must be finite"}
+		}
+		fallthrough
+	default:
+		result := reflect.New(value.Type()).Elem()
+		result.Set(value)
+		return result, nil
+	}
+}

compose.go

@@ -0,0 +1,190 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// composedDocument is the private handoff between source assembly and later
+// precedence layers. Claims remain ordered by source and declaration traversal;
+// origins retain every source that contributes to a composite mapping.
+type composedDocument struct {
+	root         *document.Node
+	claims       []compositionClaim
+	origins      map[string]Origin
+	overlays     []overlayRecord
+	applications []historyApplication
+	inputBytes   int64
+	inputNodes   int
+}
+
+// cloneComposedDocument detaches mutable composition and provenance state;
+// history application trees are immutable and retain their existing sharing.
+func cloneComposedDocument(base *composedDocument) *composedDocument {
+	return &composedDocument{
+		root: document.Clone(base.root), claims: cloneCompositionClaims(base.claims),
+		origins: cloneOriginIndex(base.origins), overlays: cloneOverlayRecords(base.overlays),
+		applications: cloneHistoryApplications(base.applications),
+		inputBytes:   base.inputBytes, inputNodes: base.inputNodes,
+	}
+}
+
+type compositionClaim struct {
+	path     Path
+	kind     document.Kind
+	location Location
+}
+
+type compositionClaims struct {
+	ordered []compositionClaim
+	byPath  map[string][]compositionClaim
+}
+
+// composeDocuments recursively joins disjoint mapping entries. Source order
+// controls only output traversal and diagnostics: every non-mapping overlap is
+// rejected before a result is assembled.
+func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic) {
+	if len(inputs) == 0 {
+		return nil, []Diagnostic{compositionInvalidDiagnostic("strict composition requires at least one document", nil)}
+	}
+
+	collected := compositionClaims{byPath: make(map[string][]compositionClaim)}
+	for _, input := range inputs {
+		if input == nil || input.Kind != document.Mapping {
+			var location *Location
+			if input != nil {
+				converted := publicLocation(input.Location)
+				location = &converted
+			}
+			return nil, []Diagnostic{compositionInvalidDiagnostic("strict composition requires mapping document roots", location)}
+		}
+		collected.append(input, Path{})
+	}
+
+	for _, claim := range collected.ordered {
+		claims := collected.byPath[claim.path.String()]
+		if len(claims) < 2 || allMappingClaims(claims) {
+			continue
+		}
+		return nil, []Diagnostic{compositionConflictDiagnostic(claim.path, claims)}
+	}
+
+	root := document.Clone(inputs[0])
+	for _, input := range inputs[1:] {
+		mergeDisjointMappings(root, input)
+	}
+	applications := make([]historyApplication, 0, len(inputs))
+	for _, input := range inputs {
+		applications = append(applications, historyApplication{layer: LayerBase, value: input})
+	}
+	return &composedDocument{
+		root:         root,
+		claims:       cloneCompositionClaims(collected.ordered),
+		origins:      compositionOrigins(collected.ordered),
+		applications: applications,
+	}, nil
+}
+
+func (c *compositionClaims) append(node *document.Node, path Path) {
+	claim := compositionClaim{
+		path:     path.clone(),
+		kind:     node.Kind,
+		location: publicLocation(node.Location),
+	}
+	c.ordered = append(c.ordered, claim)
+	pointer := path.String()
+	c.byPath[pointer] = append(c.byPath[pointer], claim)
+
+	if node.Kind != document.Mapping {
+		return
+	}
+	for _, entry := range node.Mapping {
+		c.append(entry.Value, path.Key(entry.Key))
+	}
+}
+
+func allMappingClaims(claims []compositionClaim) bool {
+	for _, claim := range claims {
+		if claim.kind != document.Mapping {
+			return false
+		}
+	}
+	return true
+}
+
+func mergeDisjointMappings(target, incoming *document.Node) {
+	for _, incomingEntry := range incoming.Mapping {
+		index := documentEntryIndex(target.Mapping, incomingEntry.Key)
+		if index < 0 {
+			cloned := incomingEntry
+			cloned.Value = document.Clone(incomingEntry.Value)
+			target.Mapping = append(target.Mapping, cloned)
+			continue
+		}
+		// Claim validation guarantees that the only remaining overlap is a
+		// mapping pair. Empty mappings naturally contribute no child entries.
+		mergeDisjointMappings(target.Mapping[index].Value, incomingEntry.Value)
+	}
+}
+
+func documentEntryIndex(entries []document.Entry, key string) int {
+	for index := range entries {
+		if entries[index].Key == key {
+			return index
+		}
+	}
+	return -1
+}
+
+func compositionOrigins(claims []compositionClaim) map[string]Origin {
+	origins := make(map[string]Origin, len(claims))
+	for _, claim := range claims {
+		pointer := claim.path.String()
+		origin := origins[pointer]
+		origin.Layer = LayerBase
+		origin.Locations = append(origin.Locations, claim.location)
+		origins[pointer] = origin
+	}
+	return origins
+}
+
+func applyCompositionOrigins(decoded *decodedConfiguration, origins map[string]Origin) {
+	for pointer, origin := range origins {
+		if _, present := decoded.presence[pointer]; present {
+			decoded.origins[pointer] = cloneOrigin(origin)
+		}
+	}
+}
+
+func cloneCompositionClaims(claims []compositionClaim) []compositionClaim {
+	cloned := make([]compositionClaim, len(claims))
+	for index, claim := range claims {
+		cloned[index] = claim
+		cloned[index].path = claim.path.clone()
+	}
+	return cloned
+}
+
+func compositionInvalidDiagnostic(message string, location *Location) Diagnostic {
+	return Diagnostic{
+		Code: CodeComposeInvalid, Severity: SeverityError, Message: message,
+		Primary: cloneLocation(location),
+	}
+}
+
+func compositionConflictDiagnostic(path Path, claims []compositionClaim) Diagnostic {
+	primary := claims[0].location
+	related := make([]Location, 0, len(claims)-1)
+	for _, claim := range claims[1:] {
+		related = append(related, claim.location)
+	}
+	return Diagnostic{
+		Code:     CodeComposeConflict,
+		Severity: SeverityError,
+		Path:     path.clone(),
+		Message:  "configuration path is defined by multiple sources",
+		Primary:  &primary,
+		Related:  related,
+	}
+}
+
+func publicLocation(location document.Location) Location {
+	return Location{SourceID: location.SourceID, Line: location.Line, Column: location.Column}
+}

compose_test.go

@@ -0,0 +1,194 @@
+package configkit
+
+import (
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestComposeDocumentsJoinsDisjointMappingsAndTracksClaims(t *testing.T) {
+	base := parseCompositionDocument(t, "source-1", `services:
+  api:
+    enabled: true
+empty: {}
+`)
+	fragment := parseCompositionDocument(t, "source-2", `services:
+  worker:
+    enabled: false
+values: [one, two]
+`)
+	baseBefore := document.Clone(base)
+	fragmentBefore := document.Clone(fragment)
+
+	composed, diagnostics := composeDocuments(base, fragment)
+	if len(diagnostics) != 0 {
+		t.Fatalf("composeDocuments() diagnostics = %#v", diagnostics)
+	}
+	for _, path := range []Path{
+		Keys("services", "api", "enabled"),
+		Keys("services", "worker", "enabled"),
+		Keys("empty"),
+		Keys("values"),
+	} {
+		if _, ok := documentAtPath(composed.root, path); !ok {
+			t.Errorf("composed document is missing %s", path.String())
+		}
+	}
+
+	serviceOrigin := composed.origins[Keys("services").String()]
+	wantLocations := []Location{
+		{SourceID: "source-1", Line: 2, Column: 3},
+		{SourceID: "source-2", Line: 2, Column: 3},
+	}
+	if !reflect.DeepEqual(serviceOrigin, Origin{Layer: LayerBase, Locations: wantLocations}) {
+		t.Fatalf("services origin = %#v, want composite origin", serviceOrigin)
+	}
+	wantClaimPaths := []string{"", "/services", "/services/api", "/services/api/enabled", "/empty", "", "/services", "/services/worker", "/services/worker/enabled", "/values"}
+	gotClaimPaths := make([]string, len(composed.claims))
+	for index, claim := range composed.claims {
+		gotClaimPaths[index] = claim.path.String()
+	}
+	if !reflect.DeepEqual(gotClaimPaths, wantClaimPaths) {
+		t.Fatalf("claim paths = %#v, want %#v", gotClaimPaths, wantClaimPaths)
+	}
+	reversed, reversedDiagnostics := composeDocuments(fragment, base)
+	if len(reversedDiagnostics) != 0 {
+		t.Fatalf("reversed composeDocuments() diagnostics = %#v", reversedDiagnostics)
+	}
+	if deltas := compareConfigurationValues(composed.root, reversed.root); len(deltas) != 0 {
+		t.Fatalf("input order changed effective values: %#v", deltas)
+	}
+
+	if !reflect.DeepEqual(base, baseBefore) || !reflect.DeepEqual(fragment, fragmentBefore) {
+		t.Fatal("composition mutated an input document")
+	}
+	composed.root.Mapping[0].Value.Mapping[0].Value.Mapping[0].Value.Text = "changed"
+	if !reflect.DeepEqual(base, baseBefore) || !reflect.DeepEqual(fragment, fragmentBefore) {
+		t.Fatal("composed document aliases an input document")
+	}
+}
+
+func TestComposeDocumentsRejectsEveryPresentNonMappingOverlap(t *testing.T) {
+	tests := []struct {
+		name  string
+		left  string
+		right string
+		path  string
+	}{
+		{name: "equal scalar", left: "value: true\n", right: "value: true\n", path: "/value"},
+		{name: "different scalar", left: "value: true\n", right: "value: false\n", path: "/value"},
+		{name: "null", left: "value: null\n", right: "value: null\n", path: "/value"},
+		{name: "sequence", left: "value: [one]\n", right: "value: [two]\n", path: "/value"},
+		{name: "mapping and scalar", left: "value:\n  nested: true\n", right: "value: text\n", path: "/value"},
+		{name: "scalar and mapping", left: "value: text\n", right: "value:\n  nested: true\n", path: "/value"},
+		{name: "nested keyed entry", left: "items:\n  shared:\n    left: 1\n", right: "items:\n  shared:\n    left: 2\n", path: "/items/shared/left"},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			left := parseCompositionDocument(t, "source-1", test.left)
+			right := parseCompositionDocument(t, "source-2", test.right)
+			_, diagnostics := composeDocuments(left, right)
+			assertCompositionConflict(t, diagnostics, test.path, []string{"source-1", "source-2"})
+		})
+	}
+}
+
+func TestComposeDocumentsReportsAllClaimantsAtExactLocations(t *testing.T) {
+	first := parseCompositionDocument(t, "source-1", "group:\n  value: 1\n")
+	second := parseCompositionDocument(t, "source-2", "group:\n  value: 2\n")
+	third := parseCompositionDocument(t, "source-3", "group:\n  value: 3\n")
+
+	_, diagnostics := composeDocuments(first, second, third)
+	assertCompositionConflict(t, diagnostics, "/group/value", []string{"source-1", "source-2", "source-3"})
+	locations := append([]Location{*diagnostics[0].Primary}, diagnostics[0].Related...)
+	for _, location := range locations {
+		if location.Line != 2 || location.Column != 10 {
+			t.Fatalf("claim location = %#v, want line 2 column 10", location)
+		}
+	}
+
+	_, diagnostics = composeDocuments(
+		parseCompositionDocument(t, "source-1", "group:\n  left: 1\n"),
+		parseCompositionDocument(t, "source-2", "group:\n  right: 2\n"),
+		parseCompositionDocument(t, "source-3", "group: blocked\n"),
+	)
+	assertCompositionConflict(t, diagnostics, "/group", []string{"source-1", "source-2", "source-3"})
+}
+
+func TestComposeDocumentsTreatsEmptyMappingsAsChildlessContributions(t *testing.T) {
+	empty, diagnostics := composeDocuments(
+		parseCompositionDocument(t, "source-1", "{}\n"),
+		parseCompositionDocument(t, "source-2", "{}\n"),
+	)
+	if len(diagnostics) != 0 || len(empty.root.Mapping) != 0 || len(empty.origins[""].Locations) != 2 {
+		t.Fatalf("empty root composition = (%#v, %#v)", empty, diagnostics)
+	}
+
+	tests := []struct {
+		name  string
+		left  string
+		right string
+	}{
+		{name: "two empty mappings", left: "value: {}\n", right: "value: {}\n"},
+		{name: "empty then populated", left: "value: {}\n", right: "value: {nested: true}\n"},
+		{name: "populated then empty", left: "value: {nested: true}\n", right: "value: {}\n"},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			composed, diagnostics := composeDocuments(
+				parseCompositionDocument(t, "source-1", test.left),
+				parseCompositionDocument(t, "source-2", test.right),
+			)
+			if len(diagnostics) != 0 {
+				t.Fatalf("composeDocuments() diagnostics = %#v", diagnostics)
+			}
+			if origin := composed.origins["/value"]; len(origin.Locations) != 2 {
+				t.Fatalf("value origin = %#v, want both mapping locations", origin)
+			}
+			if test.name != "two empty mappings" {
+				if _, ok := documentAtPath(composed.root, Keys("value", "nested")); !ok {
+					t.Fatal("populated mapping contribution was not retained")
+				}
+			}
+		})
+	}
+}
+
+func TestComposeDocumentsRejectsInvalidBoundaries(t *testing.T) {
+	if composed, diagnostics := composeDocuments(); composed != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid {
+		t.Fatalf("empty composition = (%#v, %#v)", composed, diagnostics)
+	}
+	nonMapping := &document.Node{Kind: document.String, Location: document.Location{SourceID: "source-1", Line: 4, Column: 2}}
+	if composed, diagnostics := composeDocuments(nonMapping); composed != nil || len(diagnostics) != 1 || diagnostics[0].Primary == nil || diagnostics[0].Primary.Line != 4 {
+		t.Fatalf("non-mapping composition = (%#v, %#v)", composed, diagnostics)
+	}
+}
+
+func parseCompositionDocument(t *testing.T, sourceID, input string) *document.Node {
+	t.Helper()
+	parsed, diagnostics := parseSource(Bytes(sourceID, []byte(input)), sourceID, defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("parse diagnostics = %#v", diagnostics)
+	}
+	return parsed.document
+}
+
+func assertCompositionConflict(t *testing.T, diagnostics []Diagnostic, path string, sources []string) {
+	t.Helper()
+	if len(diagnostics) != 1 {
+		t.Fatalf("diagnostics = %#v, want one conflict", diagnostics)
+	}
+	diagnostic := diagnostics[0]
+	if diagnostic.Code != CodeComposeConflict || diagnostic.Path.String() != path || diagnostic.Primary == nil {
+		t.Fatalf("diagnostic = %#v, want compose conflict at %s", diagnostic, path)
+	}
+	locations := append([]Location{*diagnostic.Primary}, diagnostic.Related...)
+	gotSources := make([]string, len(locations))
+	for index, location := range locations {
+		gotSources[index] = location.SourceID
+	}
+	if !reflect.DeepEqual(gotSources, sources) {
+		t.Fatalf("claim sources = %#v, want %#v", gotSources, sources)
+	}
+}

composition_fuzz_test.go

@@ -0,0 +1,68 @@
+package configkit
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+func FuzzCompositionAndOverlay(f *testing.F) {
+	for _, seed := range [][2]string{
+		{"{}\n", "{}\n"},
+		{"left: {value: one}\n", "right: [two]\n"},
+		{"shared: {left: one}\n", "shared: {right: two}\n"},
+		{"value: one\n", "value: two\n"},
+	} {
+		f.Add([]byte(seed[0]), []byte(seed[1]))
+	}
+
+	limits := yamladapter.Limits{Depth: 12, Nodes: 256, ParserDepth: 24}
+	f.Fuzz(func(t *testing.T, leftInput, rightInput []byte) {
+		if len(leftInput) > 4<<10 || len(rightInput) > 4<<10 {
+			return
+		}
+		left, leftIssues := yamladapter.Parse(bytes.NewReader(leftInput), "left", limits)
+		right, rightIssues := yamladapter.Parse(bytes.NewReader(rightInput), "right", limits)
+		if len(leftIssues) != 0 || len(rightIssues) != 0 {
+			return
+		}
+		leftBefore, rightBefore := document.Clone(left), document.Clone(right)
+
+		composed, diagnostics := composeDocuments(left, right)
+		if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
+			t.Fatal("composition mutated an input document")
+		}
+		if len(diagnostics) == 0 {
+			reversed, reversedDiagnostics := composeDocuments(right, left)
+			if len(reversedDiagnostics) != 0 {
+				t.Fatalf("reversed composition failed: %#v", reversedDiagnostics)
+			}
+			if differences := compareConfigurationValues(composed.root, reversed.root); len(differences) != 0 {
+				t.Fatalf("composition order changed effective values: %#v", differences)
+			}
+			composed.root.Mapping = append(composed.root.Mapping, document.Entry{Key: "fuzz-detachment"})
+			if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
+				t.Fatal("composition result aliases an input document")
+			}
+		}
+
+		base, baseDiagnostics := composeDocuments(left)
+		if len(baseDiagnostics) != 0 {
+			t.Fatalf("single accepted mapping did not compose: %#v", baseDiagnostics)
+		}
+		overlaid, overlayDiagnostics := overlayDocuments(base, right)
+		if len(overlayDiagnostics) != 0 {
+			t.Fatalf("accepted mappings did not overlay: %#v", overlayDiagnostics)
+		}
+		if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
+			t.Fatal("overlay mutated an input document")
+		}
+		overlaid.root.Mapping = append(overlaid.root.Mapping, document.Entry{Key: "fuzz-detachment"})
+		if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
+			t.Fatal("overlay result aliases an input document")
+		}
+	})
+}

decode.go

@@ -0,0 +1,470 @@
+package configkit
+
+import (
+	"encoding"
+	"math"
+	"reflect"
+	"regexp"
+	"strconv"
+	"time"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+var integerPattern = regexp.MustCompile(`^-?(0|[1-9][0-9]*)$`)
+var unsignedIntegerPattern = regexp.MustCompile(`^(0|[1-9][0-9]*)$`)
+
+type decodeIssue struct {
+	code     Code
+	path     Path
+	message  string
+	location document.Location
+}
+
+type inputPresence struct {
+	null     bool
+	location document.Location
+}
+
+type decodedConfiguration struct {
+	value    reflect.Value
+	presence map[string]inputPresence
+	origins  map[string]Origin
+	baseline *document.Node
+	final    *document.Node
+}
+
+type decodeContext struct {
+	versionPath *Path
+}
+
+func decodeConfiguration(schema *schemaDescriptor, defaults reflect.Value, input *document.Node) (decodedConfiguration, []decodeIssue) {
+	return decodeConfigurationWithContext(schema, defaults, input, decodeContext{})
+}
+
+func decodeConfigurationWithContext(schema *schemaDescriptor, defaults reflect.Value, input *document.Node, context decodeContext) (decodedConfiguration, []decodeIssue) {
+	if schema == nil || schema.root == nil || !defaults.IsValid() || defaults.Type() != schema.root.typeOf {
+		return decodedConfiguration{}, []decodeIssue{{code: CodeSchemaUnsupportedType, message: "configuration defaults do not match the schema"}}
+	}
+	baseline, projectionIssue := projectConfiguration(schema, defaults, defaultProjectionLimits())
+	if projectionIssue != nil {
+		return decodedConfiguration{}, []decodeIssue{decodeIssueFromProjection(projectionIssue)}
+	}
+	working, cloneIssue := cloneConfiguration(schema, defaults, defaultProjectionLimits())
+	if cloneIssue != nil {
+		return decodedConfiguration{}, []decodeIssue{decodeIssueFromProjection(cloneIssue)}
+	}
+
+	presence := make(map[string]inputPresence)
+	if input != nil {
+		collectPresence(input, Path{}, presence)
+		decoded, issues := decodeValue(input, working, schema.root, Path{}, schema, context)
+		if len(issues) != 0 {
+			return decodedConfiguration{}, issues
+		}
+		working = decoded
+	}
+	final, projectionIssue := projectConfiguration(schema, working, defaultProjectionLimits())
+	if projectionIssue != nil {
+		return decodedConfiguration{}, []decodeIssue{decodeIssueFromProjection(projectionIssue)}
+	}
+	origins := make(map[string]Origin)
+	collectOrigins(final, Path{}, presence, origins)
+	return decodedConfiguration{
+		value:    working,
+		presence: presence,
+		origins:  origins,
+		baseline: baseline,
+		final:    final,
+	}, nil
+}
+
+func decodeValue(node *document.Node, base reflect.Value, descriptor *typeDescriptor, path Path, schema *schemaDescriptor, context decodeContext) (reflect.Value, []decodeIssue) {
+	if node.Kind == document.Null {
+		switch descriptor.kind {
+		case descriptorPointer, descriptorMap, descriptorSlice, descriptorDynamic:
+			return reflect.Zero(descriptor.typeOf), nil
+		case descriptorSecret:
+			return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeSecretRequired, path, node, "required secret reference cannot be null")}
+		default:
+			return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeDecodeNull, path, node, "null is not allowed for this configuration type")}
+		}
+	}
+	if descriptor.kind == descriptorPointer {
+		var elementBase reflect.Value
+		if base.IsNil() {
+			elementBase = reflect.Zero(descriptor.typeOf.Elem())
+		} else {
+			elementBase = base.Elem()
+		}
+		decoded, issues := decodeValue(node, elementBase, descriptor.element, path, schema, context)
+		if len(issues) != 0 {
+			return reflect.Value{}, issues
+		}
+		result := reflect.New(descriptor.typeOf.Elem())
+		result.Elem().Set(decoded)
+		return result, nil
+	}
+
+	switch descriptor.kind {
+	case descriptorStruct:
+		return decodeStruct(node, base, descriptor, path, schema, context)
+	case descriptorMap:
+		return decodeMap(node, base, descriptor, path, schema, context)
+	case descriptorSlice:
+		return decodeSlice(node, descriptor, path, schema, context)
+	case descriptorArray:
+		return decodeArray(node, descriptor, path, schema, context)
+	case descriptorDynamic:
+		return decodeDynamic(node, base, path, schema, context)
+	case descriptorString:
+		if node.Kind != document.String && !(pathMatches(context.versionPath, path) && isIntegerVersion(node)) {
+			return typeMismatch(path, node, "string")
+		}
+		result := reflect.New(descriptor.typeOf).Elem()
+		result.SetString(node.Text)
+		return result, nil
+	case descriptorBoolean:
+		if node.Kind != document.Boolean {
+			return typeMismatch(path, node, "boolean")
+		}
+		result := reflect.New(descriptor.typeOf).Elem()
+		result.SetBool(node.Text == "true")
+		return result, nil
+	case descriptorSigned:
+		if node.Kind != document.Number || !integerPattern.MatchString(node.Text) {
+			return typeMismatch(path, node, "integer")
+		}
+		value, err := strconv.ParseInt(node.Text, 10, descriptor.typeOf.Bits())
+		if err != nil {
+			return rangeFailure(path, node, "integer")
+		}
+		result := reflect.New(descriptor.typeOf).Elem()
+		result.SetInt(value)
+		return result, nil
+	case descriptorUnsigned:
+		if node.Kind != document.Number || !unsignedIntegerPattern.MatchString(node.Text) {
+			return typeMismatch(path, node, "unsigned integer")
+		}
+		value, err := strconv.ParseUint(node.Text, 10, descriptor.typeOf.Bits())
+		if err != nil {
+			return rangeFailure(path, node, "unsigned integer")
+		}
+		result := reflect.New(descriptor.typeOf).Elem()
+		result.SetUint(value)
+		return result, nil
+	case descriptorFloat:
+		if node.Kind != document.Number {
+			return typeMismatch(path, node, "number")
+		}
+		value, err := strconv.ParseFloat(node.Text, descriptor.typeOf.Bits())
+		if err != nil || math.IsNaN(value) || math.IsInf(value, 0) {
+			return rangeFailure(path, node, "number")
+		}
+		result := reflect.New(descriptor.typeOf).Elem()
+		result.SetFloat(value)
+		return result, nil
+	case descriptorDuration:
+		if node.Kind != document.String {
+			return typeMismatch(path, node, "duration string")
+		}
+		value, err := time.ParseDuration(node.Text)
+		if err != nil {
+			return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeDecodeDuration, path, node, "duration string is invalid")}
+		}
+		result := reflect.New(descriptor.typeOf).Elem()
+		result.SetInt(int64(value))
+		return result, nil
+	case descriptorSecret:
+		if node.Kind != document.String {
+			return typeMismatch(path, node, "secret environment name")
+		}
+		return reflect.ValueOf(SecretEnv(node.Text)), nil
+	case descriptorText:
+		if node.Kind != document.String {
+			return typeMismatch(path, node, "text string")
+		}
+		result := reflect.New(descriptor.typeOf).Elem()
+		unmarshaler := result.Addr().Interface().(encoding.TextUnmarshaler)
+		if err := unmarshaler.UnmarshalText([]byte(node.Text)); err != nil {
+			return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeDecodeText, path, node, "text value is invalid for its configuration type")}
+		}
+		return result, nil
+	default:
+		return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeSchemaUnsupportedType, path, node, "configuration type is not supported")}
+	}
+}
+
+func decodeStruct(node *document.Node, base reflect.Value, descriptor *typeDescriptor, path Path, schema *schemaDescriptor, context decodeContext) (reflect.Value, []decodeIssue) {
+	if node.Kind != document.Mapping {
+		return typeMismatch(path, node, "mapping")
+	}
+	result := reflect.New(descriptor.typeOf).Elem()
+	result.Set(base)
+	var issues []decodeIssue
+	for _, entry := range node.Mapping {
+		childPath := path.Key(entry.Key)
+		if position, ok := descriptor.fieldByName[entry.Key]; ok {
+			field := descriptor.fields[position]
+			target, ok := fieldForDecode(result, field.index, true)
+			if !ok {
+				issues = append(issues, decodeIssue{code: CodeSchemaUnsupportedType, path: childPath, message: "configuration field could not be addressed", location: entry.ValueLocation})
+				continue
+			}
+			decoded, fieldIssues := decodeValue(entry.Value, target, field.descriptor, childPath, schema, context)
+			if len(fieldIssues) != 0 {
+				issues = append(issues, fieldIssues...)
+				continue
+			}
+			target.Set(decoded)
+			continue
+		}
+		if descriptor.inlineMap != nil {
+			if issue := decodeInlineMapEntry(result, *descriptor.inlineMap, entry, childPath, schema, context); issue != nil {
+				issues = append(issues, issue...)
+			}
+			continue
+		}
+		issues = append(issues, decodeIssue{code: CodeDecodeUnknownField, path: childPath, message: "configuration field is not declared", location: entry.KeyLocation})
+	}
+	return result, issues
+}
+
+func decodeInlineMapEntry(target reflect.Value, field fieldDescriptor, entry document.Entry, path Path, schema *schemaDescriptor, context decodeContext) []decodeIssue {
+	mapValue, ok := fieldForDecode(target, field.index, true)
+	if !ok || mapValue.Kind() != reflect.Map {
+		return []decodeIssue{{code: CodeSchemaInline, path: path, message: "inline map could not be addressed", location: entry.KeyLocation}}
+	}
+	if mapValue.IsNil() {
+		mapValue.Set(reflect.MakeMap(mapValue.Type()))
+	}
+	key := reflect.New(mapValue.Type().Key()).Elem()
+	key.SetString(entry.Key)
+	base := mapValue.MapIndex(key)
+	if !base.IsValid() {
+		base = reflect.Zero(mapValue.Type().Elem())
+	}
+	decoded, issues := decodeValue(entry.Value, base, field.descriptor.element, path, schema, context)
+	if len(issues) == 0 {
+		mapValue.SetMapIndex(key, decoded)
+	}
+	return issues
+}
+
+func decodeMap(node *document.Node, base reflect.Value, descriptor *typeDescriptor, path Path, schema *schemaDescriptor, context decodeContext) (reflect.Value, []decodeIssue) {
+	if node.Kind != document.Mapping {
+		return typeMismatch(path, node, "mapping")
+	}
+	result := base
+	if result.IsNil() {
+		result = reflect.MakeMap(descriptor.typeOf)
+	}
+	var issues []decodeIssue
+	for _, entry := range node.Mapping {
+		key := reflect.New(descriptor.typeOf.Key()).Elem()
+		key.SetString(entry.Key)
+		entryBase := result.MapIndex(key)
+		if !entryBase.IsValid() {
+			entryBase = reflect.Zero(descriptor.typeOf.Elem())
+		}
+		decoded, entryIssues := decodeValue(entry.Value, entryBase, descriptor.element, path.Key(entry.Key), schema, context)
+		if len(entryIssues) != 0 {
+			issues = append(issues, entryIssues...)
+			continue
+		}
+		result.SetMapIndex(key, decoded)
+	}
+	return result, issues
+}
+
+func decodeSlice(node *document.Node, descriptor *typeDescriptor, path Path, schema *schemaDescriptor, context decodeContext) (reflect.Value, []decodeIssue) {
+	if node.Kind != document.Sequence {
+		return typeMismatch(path, node, "sequence")
+	}
+	result := reflect.MakeSlice(descriptor.typeOf, len(node.Sequence), len(node.Sequence))
+	var issues []decodeIssue
+	for i, child := range node.Sequence {
+		decoded, childIssues := decodeValue(child, reflect.Zero(descriptor.typeOf.Elem()), descriptor.element, path.Index(i), schema, context)
+		if len(childIssues) != 0 {
+			issues = append(issues, childIssues...)
+			continue
+		}
+		result.Index(i).Set(decoded)
+	}
+	return result, issues
+}
+
+func decodeArray(node *document.Node, descriptor *typeDescriptor, path Path, schema *schemaDescriptor, context decodeContext) (reflect.Value, []decodeIssue) {
+	if node.Kind != document.Sequence {
+		return typeMismatch(path, node, "sequence")
+	}
+	if len(node.Sequence) != descriptor.length {
+		return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeDecodeArrayLength, path, node, "configuration array has the wrong length")}
+	}
+	result := reflect.New(descriptor.typeOf).Elem()
+	var issues []decodeIssue
+	for i, child := range node.Sequence {
+		decoded, childIssues := decodeValue(child, reflect.Zero(descriptor.typeOf.Elem()), descriptor.element, path.Index(i), schema, context)
+		if len(childIssues) != 0 {
+			issues = append(issues, childIssues...)
+			continue
+		}
+		result.Index(i).Set(decoded)
+	}
+	return result, issues
+}
+
+func decodeDynamic(node *document.Node, base reflect.Value, path Path, schema *schemaDescriptor, context decodeContext) (reflect.Value, []decodeIssue) {
+	if node.Kind == document.Mapping && base.IsValid() && !base.IsNil() {
+		concrete := base.Elem()
+		operation := valueOperation{schema: schema}
+		descriptor, issue := operation.dynamicDescriptor(concrete.Type(), path)
+		if issue == nil && acceptsMapping(descriptor) {
+			decoded, issues := decodeValue(node, concrete, descriptor, path, schema, context)
+			if len(issues) != 0 {
+				return reflect.Value{}, issues
+			}
+			result := reflect.New(base.Type()).Elem()
+			result.Set(decoded)
+			return result, nil
+		}
+	}
+	var value any
+	switch node.Kind {
+	case document.Null:
+		return reflect.Zero(base.Type()), nil
+	case document.String:
+		value = node.Text
+	case document.Boolean:
+		value = node.Text == "true"
+	case document.Number:
+		if integerPattern.MatchString(node.Text) {
+			parsed, err := strconv.ParseInt(node.Text, 10, 64)
+			if err != nil {
+				return rangeFailure(path, node, "dynamic integer")
+			}
+			value = parsed
+		} else {
+			parsed, err := strconv.ParseFloat(node.Text, 64)
+			if err != nil || math.IsNaN(parsed) || math.IsInf(parsed, 0) {
+				return rangeFailure(path, node, "dynamic number")
+			}
+			value = parsed
+		}
+	case document.Sequence:
+		sequence := make([]any, len(node.Sequence))
+		var issues []decodeIssue
+		for i, child := range node.Sequence {
+			decoded, childIssues := decodeDynamic(child, reflect.Zero(reflect.TypeFor[any]()), path.Index(i), schema, context)
+			if len(childIssues) != 0 {
+				issues = append(issues, childIssues...)
+				continue
+			}
+			sequence[i] = decoded.Interface()
+		}
+		if len(issues) != 0 {
+			return reflect.Value{}, issues
+		}
+		value = sequence
+	case document.Mapping:
+		mapping := make(map[string]any, len(node.Mapping))
+		var issues []decodeIssue
+		for _, entry := range node.Mapping {
+			decoded, childIssues := decodeDynamic(entry.Value, reflect.Zero(reflect.TypeFor[any]()), path.Key(entry.Key), schema, context)
+			if len(childIssues) != 0 {
+				issues = append(issues, childIssues...)
+				continue
+			}
+			mapping[entry.Key] = decoded.Interface()
+		}
+		if len(issues) != 0 {
+			return reflect.Value{}, issues
+		}
+		value = mapping
+	default:
+		return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeDecodeType, path, node, "dynamic configuration value has an unsupported kind")}
+	}
+	result := reflect.New(base.Type()).Elem()
+	if value != nil {
+		result.Set(reflect.ValueOf(value))
+	}
+	return result, nil
+}
+
+func acceptsMapping(descriptor *typeDescriptor) bool {
+	for descriptor != nil && descriptor.kind == descriptorPointer {
+		descriptor = descriptor.element
+	}
+	return descriptor != nil && (descriptor.kind == descriptorStruct || descriptor.kind == descriptorMap || descriptor.kind == descriptorDynamic)
+}
+
+func fieldForDecode(root reflect.Value, index []int, allocate bool) (reflect.Value, bool) {
+	current := root
+	for position, fieldIndex := range index {
+		if current.Kind() != reflect.Struct {
+			return reflect.Value{}, false
+		}
+		current = current.Field(fieldIndex)
+		if position == len(index)-1 {
+			return current, current.CanSet()
+		}
+		for current.Kind() == reflect.Pointer {
+			if current.IsNil() {
+				if !allocate || !current.CanSet() {
+					return reflect.Value{}, false
+				}
+				current.Set(reflect.New(current.Type().Elem()))
+			}
+			current = current.Elem()
+		}
+	}
+	return reflect.Value{}, false
+}
+
+func collectPresence(node *document.Node, path Path, presence map[string]inputPresence) {
+	presence[path.String()] = inputPresence{null: node.Kind == document.Null, location: node.Location}
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			collectPresence(entry.Value, path.Key(entry.Key), presence)
+		}
+	case document.Sequence:
+		for i, child := range node.Sequence {
+			collectPresence(child, path.Index(i), presence)
+		}
+	}
+}
+
+func collectOrigins(node *document.Node, path Path, presence map[string]inputPresence, origins map[string]Origin) {
+	if input, ok := presence[path.String()]; ok {
+		origins[path.String()] = Origin{Layer: LayerBase, Locations: []Location{{SourceID: input.location.SourceID, Line: input.location.Line, Column: input.location.Column}}}
+	} else {
+		origins[path.String()] = Origin{Layer: LayerDefault}
+	}
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			collectOrigins(entry.Value, path.Key(entry.Key), presence, origins)
+		}
+	case document.Sequence:
+		for i, child := range node.Sequence {
+			collectOrigins(child, path.Index(i), presence, origins)
+		}
+	}
+}
+
+func decodeIssueFromProjection(issue *projectionIssue) decodeIssue {
+	return decodeIssue{code: issue.code, path: issue.path.clone(), message: issue.message}
+}
+
+func newDecodeIssue(code Code, path Path, node *document.Node, message string) decodeIssue {
+	return decodeIssue{code: code, path: path.clone(), message: message, location: node.Location}
+}
+
+func typeMismatch(path Path, node *document.Node, expected string) (reflect.Value, []decodeIssue) {
+	return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeDecodeType, path, node, "configuration value must be a "+expected)}
+}
+
+func rangeFailure(path Path, node *document.Node, expected string) (reflect.Value, []decodeIssue) {
+	return reflect.Value{}, []decodeIssue{newDecodeIssue(CodeDecodeRange, path, node, expected+" is outside the supported range")}
+}

decode_test.go

@@ -0,0 +1,312 @@
+package configkit
+
+import (
+	"errors"
+	"math"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type namedSmall int8
+
+type validatedText string
+
+func (value validatedText) MarshalText() ([]byte, error) {
+	return []byte(value), nil
+}
+
+func (value *validatedText) UnmarshalText(text []byte) error {
+	if string(text) == "bad-secret-canary" {
+		return errors.New("CANARY_TEXT_ERROR: bad-secret-canary")
+	}
+	*value = validatedText(text)
+	return nil
+}
+
+func TestDecodeAppliesInputOverDetachedDefaults(t *testing.T) {
+	t.Parallel()
+
+	type nested struct {
+		A string `yaml:"a"`
+		B string `yaml:"b"`
+	}
+	type config struct {
+		Enabled      bool              `yaml:"enabled"`
+		Workers      int               `yaml:"workers"`
+		Label        string            `yaml:"label"`
+		Nested       nested            `yaml:"nested"`
+		EmptyMapping nested            `yaml:"empty_mapping"`
+		Objects      map[string]nested `yaml:"objects"`
+		Sequence     []string          `yaml:"sequence"`
+		Replacement  []string          `yaml:"replacement"`
+		Pointer      *nested           `yaml:"pointer"`
+		NilPointer   *nested           `yaml:"nil_pointer"`
+		Nullable     map[string]string `yaml:"nullable"`
+		Dynamic      any               `yaml:"dynamic"`
+		DynamicShape any               `yaml:"dynamic_shape"`
+		DynamicNull  any               `yaml:"dynamic_null"`
+		Fixed        [2]int            `yaml:"fixed"`
+		Small        namedSmall        `yaml:"small"`
+		Delay        time.Duration     `yaml:"delay"`
+		Custom       validatedText     `yaml:"custom"`
+	}
+	defaults := config{
+		Enabled:      true,
+		Workers:      4,
+		Label:        "default",
+		Nested:       nested{A: "keep", B: "before"},
+		EmptyMapping: nested{A: "keep-empty", B: "keep-empty"},
+		Objects: map[string]nested{
+			"existing": {A: "keep-map", B: "before"},
+		},
+		Sequence:     []string{"default"},
+		Replacement:  []string{"default"},
+		Pointer:      &nested{A: "keep-pointer", B: "before"},
+		Nullable:     map[string]string{"default": "value"},
+		Dynamic:      map[string]any{"keep": "default", "count": int64(4)},
+		DynamicShape: nested{A: "keep-dynamic", B: "before"},
+		DynamicNull:  "default",
+		Fixed:        [2]int{1, 2},
+		Small:        7,
+		Delay:        time.Second,
+		Custom:       "default",
+	}
+	input := `
+enabled: false
+workers: 0
+label: ""
+nested: {b: changed}
+empty_mapping: {}
+objects:
+  existing: {b: changed}
+  new: {b: added}
+sequence: []
+replacement: [one, two]
+pointer: {b: changed}
+nil_pointer: {b: added}
+nullable: null
+dynamic: {count: 0, fraction: 1.5}
+dynamic_shape: {b: changed}
+dynamic_null: null
+fixed: [3, 4]
+small: -8
+delay: 1.5s
+custom: accepted
+`
+	schema := mustSchemaFor[config](t)
+	documentRoot := parseDocumentForDecode(t, input)
+	decoded, issues := decodeConfiguration(schema, reflect.ValueOf(defaults), documentRoot)
+	if len(issues) != 0 {
+		t.Fatalf("decode issues = %#v", issues)
+	}
+	got := decoded.value.Interface().(config)
+	if got.Enabled || got.Workers != 0 || got.Label != "" {
+		t.Fatalf("explicit zero values lost: %#v", got)
+	}
+	if got.Nested != (nested{A: "keep", B: "changed"}) || got.EmptyMapping != defaults.EmptyMapping {
+		t.Fatalf("mapping defaults = %#v / %#v", got.Nested, got.EmptyMapping)
+	}
+	if got.Objects["existing"] != (nested{A: "keep-map", B: "changed"}) || got.Objects["new"] != (nested{B: "added"}) {
+		t.Fatalf("map defaults = %#v", got.Objects)
+	}
+	if got.Sequence == nil || len(got.Sequence) != 0 || !reflect.DeepEqual(got.Replacement, []string{"one", "two"}) {
+		t.Fatalf("sequence replacement = %#v / %#v", got.Sequence, got.Replacement)
+	}
+	if got.Pointer == defaults.Pointer || *got.Pointer != (nested{A: "keep-pointer", B: "changed"}) || got.NilPointer == nil || *got.NilPointer != (nested{B: "added"}) {
+		t.Fatalf("pointer defaults = %#v / %#v", got.Pointer, got.NilPointer)
+	}
+	if got.Nullable != nil || got.DynamicNull != nil {
+		t.Fatalf("null values = %#v / %#v", got.Nullable, got.DynamicNull)
+	}
+	dynamic := got.Dynamic.(map[string]any)
+	if dynamic["keep"] != "default" || dynamic["count"] != int64(0) || dynamic["fraction"] != float64(1.5) {
+		t.Fatalf("dynamic mapping = %#v", dynamic)
+	}
+	if got.DynamicShape != (nested{A: "keep-dynamic", B: "changed"}) {
+		t.Fatalf("dynamic concrete mapping = %#v", got.DynamicShape)
+	}
+	if got.Fixed != [2]int{3, 4} || got.Small != -8 || got.Delay != 1500*time.Millisecond || got.Custom != "accepted" {
+		t.Fatalf("scalar decoding = %#v", got)
+	}
+
+	for _, path := range []Path{Keys(), Keys("enabled"), Keys("nested"), Keys("nested", "b"), Keys("empty_mapping")} {
+		if _, ok := decoded.presence[path.String()]; !ok {
+			t.Errorf("explicit path %q missing", path.String())
+		}
+	}
+	if _, ok := decoded.presence[Keys("nested", "a").String()]; ok {
+		t.Fatal("parent mapping made omitted child explicit")
+	}
+	if decoded.origins[Keys("nested", "a").String()].Layer != LayerDefault || decoded.origins[Keys("nested", "b").String()].Layer != LayerBase {
+		t.Fatalf("origins = %#v", decoded.origins)
+	}
+	if decoded.origins[Keys("nested", "b").String()].Locations[0].Line == 0 {
+		t.Fatal("input origin lost source coordinates")
+	}
+
+	defaults.Objects["existing"] = nested{A: "mutated"}
+	defaults.Pointer.B = "mutated"
+	defaults.Sequence[0] = "mutated"
+	if got.Objects["existing"].A != "keep-map" || got.Pointer.B != "changed" || len(got.Sequence) != 0 {
+		t.Fatal("decoded value aliases defaults")
+	}
+	got.Pointer.B = "caller mutation"
+	if mappingValue(t, mappingValue(t, decoded.final, "pointer"), "b").Text != "changed" {
+		t.Fatal("final snapshot aliases returned typed value")
+	}
+	if mappingValue(t, mappingValue(t, decoded.baseline, "nested"), "b").Text != "before" {
+		t.Fatal("baseline snapshot was mutated by decode")
+	}
+}
+
+func TestDecodeDefaultsOnlyAndRepeatedResultsAreIndependent(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Values map[string][]string `yaml:"values"`
+	}
+	defaults := config{Values: map[string][]string{"one": {"a"}}}
+	schema := mustSchemaFor[config](t)
+	first, issues := decodeConfiguration(schema, reflect.ValueOf(defaults), nil)
+	if len(issues) != 0 {
+		t.Fatal(issues)
+	}
+	second, issues := decodeConfiguration(schema, reflect.ValueOf(defaults), nil)
+	if len(issues) != 0 {
+		t.Fatal(issues)
+	}
+	firstValue := first.value.Interface().(config)
+	secondValue := second.value.Interface().(config)
+	firstValue.Values["one"][0] = "changed"
+	if secondValue.Values["one"][0] != "a" || defaults.Values["one"][0] != "a" {
+		t.Fatal("repeated defaults-only results share aliases")
+	}
+	if len(first.presence) != 0 || first.origins[Keys("values", "one").String()].Layer != LayerDefault {
+		t.Fatalf("defaults-only metadata = %#v / %#v", first.presence, first.origins)
+	}
+}
+
+func TestDecodeCollectsIndependentTypeAndShapeFailures(t *testing.T) {
+	t.Parallel()
+
+	type nested struct {
+		Known int `yaml:"known"`
+	}
+	type config struct {
+		Small    int8          `yaml:"small"`
+		Unsigned uint8         `yaml:"unsigned"`
+		Number   float32       `yaml:"number"`
+		Text     string        `yaml:"text"`
+		Flag     bool          `yaml:"flag"`
+		Delay    time.Duration `yaml:"delay"`
+		Fixed    [2]int        `yaml:"fixed"`
+		Nested   nested        `yaml:"nested"`
+		Custom   validatedText `yaml:"custom"`
+		Dynamic  any           `yaml:"dynamic"`
+	}
+	input := `
+small: 128
+unsigned: -1
+number: 1e999
+text: 3
+flag: "true"
+delay: 10
+fixed: [1]
+nested: {first_unknown: 1, second_unknown: 2}
+custom: bad-secret-canary
+dynamic: 9223372036854775808
+`
+	schema := mustSchemaFor[config](t)
+	_, issues := decodeConfiguration(schema, reflect.ValueOf(config{Custom: "ok"}), parseDocumentForDecode(t, input))
+	if len(issues) != 11 {
+		t.Fatalf("issues = %d: %#v", len(issues), issues)
+	}
+	want := map[string]Code{
+		"/small":                 CodeDecodeRange,
+		"/unsigned":              CodeDecodeType,
+		"/number":                CodeDecodeRange,
+		"/text":                  CodeDecodeType,
+		"/flag":                  CodeDecodeType,
+		"/delay":                 CodeDecodeType,
+		"/fixed":                 CodeDecodeArrayLength,
+		"/nested/first_unknown":  CodeDecodeUnknownField,
+		"/nested/second_unknown": CodeDecodeUnknownField,
+		"/custom":                CodeDecodeText,
+		"/dynamic":               CodeDecodeRange,
+	}
+	for _, issue := range issues {
+		if wantCode, ok := want[issue.path.String()]; !ok || wantCode != issue.code {
+			t.Errorf("unexpected issue %#v", issue)
+		}
+		if issue.location.Line == 0 {
+			t.Errorf("issue lost input location: %#v", issue)
+		}
+		if strings.Contains(issue.message, "CANARY") || strings.Contains(issue.message, "bad-secret") {
+			t.Errorf("issue exposed value: %#v", issue)
+		}
+	}
+}
+
+func TestDecodeNullabilityAndNestedMapStrictness(t *testing.T) {
+	t.Parallel()
+
+	type nested struct {
+		Value int `yaml:"value"`
+	}
+	type config struct {
+		Scalar int               `yaml:"scalar"`
+		Object nested            `yaml:"object"`
+		Array  [1]int            `yaml:"array"`
+		Map    map[string]nested `yaml:"map"`
+	}
+	input := "scalar: null\nobject: null\narray: null\nmap:\n  item: {unknown: 1}\n"
+	_, issues := decodeConfiguration(mustSchemaFor[config](t), reflect.ValueOf(config{}), parseDocumentForDecode(t, input))
+	want := map[string]Code{
+		"/scalar":           CodeDecodeNull,
+		"/object":           CodeDecodeNull,
+		"/array":            CodeDecodeNull,
+		"/map/item/unknown": CodeDecodeUnknownField,
+	}
+	if len(issues) != len(want) {
+		t.Fatalf("issues = %#v", issues)
+	}
+	for _, issue := range issues {
+		if want[issue.path.String()] != issue.code {
+			t.Errorf("issue = %#v", issue)
+		}
+	}
+}
+
+func TestDecodeRejectsInvalidDefaultsBeforeApplyingInput(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Name   string         `yaml:"name"`
+		Extras map[string]any `yaml:",inline"`
+		Float  float64        `yaml:"float"`
+	}
+	schema := mustSchemaFor[config](t)
+	for name, defaults := range map[string]config{
+		"inline collision": {Name: "declared", Extras: map[string]any{"name": "duplicate"}},
+		"non-finite":       {Float: math.NaN()},
+	} {
+		t.Run(name, func(t *testing.T) {
+			if _, issues := decodeConfiguration(schema, reflect.ValueOf(defaults), parseDocumentForDecode(t, "{}\n")); len(issues) != 1 || !strings.HasPrefix(string(issues[0].code), "schema.") {
+				t.Fatalf("issues = %#v", issues)
+			}
+		})
+	}
+}
+
+func parseDocumentForDecode(t *testing.T, input string) *document.Node {
+	t.Helper()
+	parsed, diagnostics := parseSource(Bytes("decode.yml", []byte(input)), "source-1", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("parse diagnostics = %#v", diagnostics)
+	}
+	return parsed.document
+}

diagnostic.go

@@ -0,0 +1,248 @@
+package configkit
+
+import (
+	"cmp"
+	"fmt"
+	"io"
+	"slices"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// LoadError reports safe structured diagnostics for a failed configuration load.
+// It intentionally does not expose or unwrap parser and application errors.
+type LoadError struct {
+	diagnostics []Diagnostic
+	sources     []SourceInfo
+}
+
+// Error implements error without rendering diagnostic messages or input values.
+func (e *LoadError) Error() string {
+	if e == nil {
+		return "configkit: configuration load failed"
+	}
+	count := 0
+	for _, diagnostic := range e.diagnostics {
+		if diagnostic.Severity == SeverityError {
+			count++
+		}
+	}
+	if count == 1 {
+		return "configkit: configuration load failed with 1 error"
+	}
+	return fmt.Sprintf("configkit: configuration load failed with %d errors", count)
+}
+
+// Diagnostics returns a deep copy of the error's canonical diagnostics.
+func (e *LoadError) Diagnostics() []Diagnostic {
+	if e == nil {
+		return nil
+	}
+	return cloneDiagnostics(e.diagnostics)
+}
+
+// Sources returns a deep copy of sources discovered before loading failed.
+func (e *LoadError) Sources() []SourceInfo {
+	if e == nil {
+		return nil
+	}
+	return cloneSourceInfos(e.sources)
+}
+
+// WriteDiagnostics writes diagnostics in canonical order. It escapes terminal
+// control characters and propagates writer errors, including short writes.
+func WriteDiagnostics(output io.Writer, diagnostics []Diagnostic) error {
+	canonical := canonicalDiagnostics(diagnostics)
+	for _, diagnostic := range canonical {
+		if err := writeDiagnostic(output, diagnostic); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+type diagnosticAccumulator struct {
+	diagnostics []Diagnostic
+	sources     []SourceInfo
+}
+
+func (a *diagnosticAccumulator) add(diagnostics ...Diagnostic) {
+	for _, diagnostic := range diagnostics {
+		a.diagnostics = append(a.diagnostics, cloneDiagnostic(diagnostic))
+	}
+}
+
+func (a *diagnosticAccumulator) loadError() *LoadError {
+	return &LoadError{
+		diagnostics: canonicalDiagnostics(a.diagnostics),
+		sources:     cloneSourceInfos(a.sources),
+	}
+}
+
+func canonicalDiagnostics(diagnostics []Diagnostic) []Diagnostic {
+	canonical := cloneDiagnostics(diagnostics)
+	for i := range canonical {
+		slices.SortFunc(canonical[i].Related, compareLocation)
+	}
+	slices.SortFunc(canonical, compareDiagnostic)
+	return canonical
+}
+
+func compareDiagnostic(left, right Diagnostic) int {
+	if order := compareOptionalLocation(left.Primary, right.Primary); order != 0 {
+		return order
+	}
+	if order := cmp.Compare(left.Path.String(), right.Path.String()); order != 0 {
+		return order
+	}
+	if order := cmp.Compare(left.Code, right.Code); order != 0 {
+		return order
+	}
+	if order := cmp.Compare(left.Severity, right.Severity); order != 0 {
+		return order
+	}
+	if order := cmp.Compare(left.Message, right.Message); order != 0 {
+		return order
+	}
+	return compareLocations(left.Related, right.Related)
+}
+
+func compareOptionalLocation(left, right *Location) int {
+	if left == nil && right == nil {
+		return 0
+	}
+	if left == nil {
+		return -1
+	}
+	if right == nil {
+		return 1
+	}
+	return compareLocation(*left, *right)
+}
+
+func compareLocations(left, right []Location) int {
+	for i := 0; i < min(len(left), len(right)); i++ {
+		if order := compareLocation(left[i], right[i]); order != 0 {
+			return order
+		}
+	}
+	return cmp.Compare(len(left), len(right))
+}
+
+func compareLocation(left, right Location) int {
+	if order := compareSourceID(left.SourceID, right.SourceID); order != 0 {
+		return order
+	}
+	if order := cmp.Compare(left.Line, right.Line); order != 0 {
+		return order
+	}
+	return cmp.Compare(left.Column, right.Column)
+}
+
+func compareSourceID(left, right string) int {
+	leftOrder, leftOK := sourceSequence(left)
+	rightOrder, rightOK := sourceSequence(right)
+	if leftOK && rightOK {
+		return cmp.Compare(leftOrder, rightOrder)
+	}
+	return cmp.Compare(left, right)
+}
+
+func sourceSequence(sourceID string) (int, bool) {
+	const prefix = "source-"
+	if !strings.HasPrefix(sourceID, prefix) {
+		return 0, false
+	}
+	value, err := strconv.Atoi(strings.TrimPrefix(sourceID, prefix))
+	if err != nil || value < 1 {
+		return 0, false
+	}
+	return value, true
+}
+
+func writeDiagnostic(output io.Writer, diagnostic Diagnostic) error {
+	severity := diagnostic.Severity
+	if severity == "" {
+		severity = SeverityError
+	}
+	path := diagnostic.Path.String()
+	if path == "" {
+		path = "<root>"
+	}
+
+	var b strings.Builder
+	b.WriteString(escapeControls(string(severity)))
+	if diagnostic.Code != "" {
+		b.WriteString(" [")
+		b.WriteString(escapeControls(string(diagnostic.Code)))
+		b.WriteByte(']')
+	}
+	b.WriteByte(' ')
+	b.WriteString(escapeControls(path))
+	if diagnostic.Primary != nil {
+		b.WriteString(" (")
+		writeLocationText(&b, *diagnostic.Primary)
+		b.WriteByte(')')
+	}
+	if diagnostic.Message != "" {
+		b.WriteString(": ")
+		b.WriteString(escapeControls(diagnostic.Message))
+	}
+	b.WriteByte('\n')
+	for _, related := range diagnostic.Related {
+		b.WriteString("  related: ")
+		writeLocationText(&b, related)
+		b.WriteByte('\n')
+	}
+	return writeString(output, b.String())
+}
+
+func writeLocationText(output *strings.Builder, location Location) {
+	label := location.SourceID
+	if label == "" {
+		label = "<synthetic>"
+	}
+	output.WriteString(escapeControls(label))
+	if location.Line > 0 {
+		output.WriteByte(':')
+		output.WriteString(strconv.Itoa(location.Line))
+		if location.Column > 0 {
+			output.WriteByte(':')
+			output.WriteString(strconv.Itoa(location.Column))
+		}
+	}
+}
+
+func writeString(output io.Writer, value string) error {
+	written, err := io.WriteString(output, value)
+	if err != nil {
+		return err
+	}
+	if written != len(value) {
+		return io.ErrShortWrite
+	}
+	return nil
+}
+
+func escapeControls(value string) string {
+	var b strings.Builder
+	for _, r := range value {
+		if unicode.IsControl(r) {
+			quoted := strconv.QuoteRune(r)
+			b.WriteString(quoted[1 : len(quoted)-1])
+			continue
+		}
+		b.WriteRune(r)
+	}
+	return b.String()
+}
+
+func hasErrors(diagnostics []Diagnostic) bool {
+	for _, diagnostic := range diagnostics {
+		if diagnostic.Severity == SeverityError {
+			return true
+		}
+	}
+	return false
+}

diagnostic_test.go

@@ -0,0 +1,143 @@
+package configkit
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+	"testing"
+)
+
+func TestDiagnosticsAreCanonicalAndDetached(t *testing.T) {
+	t.Parallel()
+
+	diagnostics := []Diagnostic{
+		{
+			Code:     "decode.type",
+			Severity: SeverityError,
+			Path:     Keys("later"),
+			Message:  "later",
+			Primary:  &Location{SourceID: "source-10", Line: 1, Column: 1},
+		},
+		{
+			Code:     "schema.invalid",
+			Severity: SeverityError,
+			Path:     Keys("root"),
+			Message:  "source-less",
+		},
+		{
+			Code:     "decode.type",
+			Severity: SeverityError,
+			Path:     Keys("earlier"),
+			Message:  "earlier",
+			Primary:  &Location{SourceID: "source-2", Line: 3, Column: 4},
+			Related: []Location{
+				{SourceID: "source-10", Line: 1, Column: 1},
+				{SourceID: "source-2", Line: 9, Column: 1},
+			},
+		},
+	}
+
+	var forward bytes.Buffer
+	if err := WriteDiagnostics(&forward, diagnostics); err != nil {
+		t.Fatal(err)
+	}
+	reversed := []Diagnostic{diagnostics[2], diagnostics[1], diagnostics[0]}
+	var backward bytes.Buffer
+	if err := WriteDiagnostics(&backward, reversed); err != nil {
+		t.Fatal(err)
+	}
+	if forward.String() != backward.String() {
+		t.Fatalf("rendering depends on submission order:\n%s\n---\n%s", forward.String(), backward.String())
+	}
+	lines := strings.Split(forward.String(), "\n")
+	if !strings.Contains(lines[0], "source-less") || !strings.Contains(lines[1], "source-2:3:4") {
+		t.Fatalf("unexpected canonical order:\n%s", forward.String())
+	}
+	if !strings.Contains(lines[2], "source-2:9:1") || !strings.Contains(lines[3], "source-10:1:1") {
+		t.Fatalf("related locations not canonical:\n%s", forward.String())
+	}
+
+	accumulator := diagnosticAccumulator{
+		sources: []SourceInfo{{
+			ID:           "source-1",
+			Label:        "config.yml",
+			ImportedFrom: &Location{SourceID: "source-0", Line: 1},
+		}},
+	}
+	accumulator.add(diagnostics...)
+	loadErr := accumulator.loadError()
+	var found *LoadError
+	if !errors.As(error(loadErr), &found) || found != loadErr {
+		t.Fatal("errors.As did not find LoadError")
+	}
+
+	first := loadErr.Diagnostics()
+	first[1].Primary.SourceID = "mutated"
+	first[1].Path.segments[0].key = "mutated"
+	first[1].Related[0].SourceID = "mutated"
+	second := loadErr.Diagnostics()
+	if second[1].Primary.SourceID != "source-2" || second[1].Path.String() != "/earlier" || second[1].Related[0].SourceID != "source-2" {
+		t.Fatal("Diagnostics returned aliased records")
+	}
+	sources := loadErr.Sources()
+	sources[0].ImportedFrom.SourceID = "mutated"
+	if loadErr.Sources()[0].ImportedFrom.SourceID != "source-0" {
+		t.Fatal("Sources returned aliased records")
+	}
+	if got := loadErr.Error(); got != "configkit: configuration load failed with 3 errors" {
+		t.Fatalf("Error() = %q", got)
+	}
+}
+
+func TestWriteDiagnosticsEscapesControls(t *testing.T) {
+	t.Parallel()
+
+	diagnostic := Diagnostic{
+		Code:     "validate.error",
+		Severity: SeverityError,
+		Path:     Keys("line\nbreak"),
+		Message:  "message\r\nnext",
+		Primary:  &Location{SourceID: "source\tlabel", Line: 1},
+	}
+	var output bytes.Buffer
+	if err := WriteDiagnostics(&output, []Diagnostic{diagnostic}); err != nil {
+		t.Fatal(err)
+	}
+	got := output.String()
+	if strings.Count(got, "\n") != 1 {
+		t.Fatalf("control characters created output lines: %q", got)
+	}
+	for _, escaped := range []string{`line\nbreak`, `message\r\nnext`, `source\tlabel`} {
+		if !strings.Contains(got, escaped) {
+			t.Errorf("output %q does not contain %q", got, escaped)
+		}
+	}
+}
+
+func TestWriteDiagnosticsPropagatesWriterFailures(t *testing.T) {
+	t.Parallel()
+
+	diagnostics := []Diagnostic{{Severity: SeverityError, Message: "safe"}}
+	want := errors.New("write failed")
+	if err := WriteDiagnostics(errorWriter{err: want}, diagnostics); !errors.Is(err, want) {
+		t.Fatalf("error writer returned %v", err)
+	}
+	if err := WriteDiagnostics(shortWriter{}, diagnostics); !errors.Is(err, io.ErrShortWrite) {
+		t.Fatalf("short writer returned %v", err)
+	}
+}
+
+type errorWriter struct {
+	err error
+}
+
+func (w errorWriter) Write(p []byte) (int, error) {
+	return min(3, len(p)), w.err
+}
+
+type shortWriter struct{}
+
+func (shortWriter) Write(p []byte) (int, error) {
+	return max(0, len(p)-1), nil
+}

directives.go

@@ -0,0 +1,229 @@
+package configkit
+
+import (
+	"path"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+const directiveKey = "configkit"
+
+type importDeclaration struct {
+	path     string
+	glob     string
+	optional bool
+	location Location
+}
+
+func splitImportDirectives(root *document.Node) (*document.Node, []importDeclaration, []Diagnostic) {
+	body := document.Clone(root)
+	var declarations []importDeclaration
+	directiveIndex := -1
+	for index, entry := range body.Mapping {
+		if entry.Key != directiveKey {
+			continue
+		}
+		directiveIndex = index
+		parsed, diagnostics := parseImportDirective(entry.Value, Keys(directiveKey))
+		if len(diagnostics) != 0 {
+			return nil, nil, diagnostics
+		}
+		declarations = parsed
+		break
+	}
+	if directiveIndex >= 0 {
+		body.Mapping = append(body.Mapping[:directiveIndex], body.Mapping[directiveIndex+1:]...)
+	}
+	if diagnostics := nestedDirectiveDiagnostics(body, Path{}); len(diagnostics) != 0 {
+		return nil, nil, diagnostics
+	}
+	return body, declarations, nil
+}
+
+func parseImportDirective(node *document.Node, directivePath Path) ([]importDeclaration, []Diagnostic) {
+	if node.Kind != document.Mapping {
+		return nil, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, directivePath, "configuration directive must be a mapping", node)}
+	}
+	var declarations []importDeclaration
+	for _, entry := range node.Mapping {
+		fieldPath := directivePath.Key(entry.Key)
+		if entry.Key != "imports" {
+			return nil, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "configuration directive field is not recognized", entry.Value)}
+		}
+		if entry.Value.Kind != document.Sequence {
+			return nil, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "imports must be a sequence", entry.Value)}
+		}
+		for index, item := range entry.Value.Sequence {
+			declaration, diagnostics := parseImportDeclaration(item, fieldPath.Index(index))
+			if len(diagnostics) != 0 {
+				return nil, diagnostics
+			}
+			declarations = append(declarations, declaration)
+		}
+	}
+	return declarations, nil
+}
+
+func parseImportDeclaration(node *document.Node, declarationPath Path) (importDeclaration, []Diagnostic) {
+	if node.Kind != document.Mapping {
+		return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, declarationPath, "import declaration must be a mapping", node)}
+	}
+	declaration := importDeclaration{location: publicLocation(node.Location)}
+	seenTarget := false
+	for _, entry := range node.Mapping {
+		fieldPath := declarationPath.Key(entry.Key)
+		switch entry.Key {
+		case "path", "glob":
+			if seenTarget || entry.Value.Kind != document.String || entry.Value.Text == "" || strings.TrimSpace(entry.Value.Text) != entry.Value.Text {
+				return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "import declaration requires exactly one nonempty path or glob string", entry.Value)}
+			}
+			seenTarget = true
+			declaration.location = publicLocation(entry.Value.Location)
+			if entry.Key == "path" {
+				declaration.path = entry.Value.Text
+			} else {
+				declaration.glob = entry.Value.Text
+			}
+		case "optional":
+			if entry.Value.Kind != document.Boolean {
+				return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "optional must be a boolean", entry.Value)}
+			}
+			declaration.optional = entry.Value.Text == "true"
+		default:
+			return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, fieldPath, "import declaration field is not recognized", entry.Value)}
+		}
+	}
+	if !seenTarget {
+		return importDeclaration{}, []Diagnostic{nodeImportDiagnostic(CodeImportInvalid, declarationPath, "import declaration requires exactly one path or glob", node)}
+	}
+	return declaration, nil
+}
+
+func nestedDirectiveDiagnostics(node *document.Node, current Path) []Diagnostic {
+	if node.Kind == document.Mapping {
+		for _, entry := range node.Mapping {
+			childPath := current.Key(entry.Key)
+			if entry.Key == directiveKey {
+				location := publicLocation(entry.KeyLocation)
+				return []Diagnostic{{Code: CodeImportInvalid, Severity: SeverityError, Path: childPath, Message: "configuration directives are permitted only at the document root", Primary: &location}}
+			}
+			if diagnostics := nestedDirectiveDiagnostics(entry.Value, childPath); len(diagnostics) != 0 {
+				return diagnostics
+			}
+		}
+	} else if node.Kind == document.Sequence {
+		for index, child := range node.Sequence {
+			if diagnostics := nestedDirectiveDiagnostics(child, current.Index(index)); len(diagnostics) != 0 {
+				return diagnostics
+			}
+		}
+	}
+	return nil
+}
+
+func fragmentVersionClaim(root *document.Node, rule *VersionRule) *document.Node {
+	if rule == nil {
+		return nil
+	}
+	current := root
+	if len(rule.Path.segments) == 0 {
+		return current
+	}
+	for _, segment := range rule.Path.segments {
+		if current.Kind != document.Mapping {
+			return current
+		}
+		if segment.kind == pathSegmentIndex {
+			return nil
+		}
+		key := segment.key
+		index := documentEntryIndex(current.Mapping, key)
+		if index < 0 {
+			return nil
+		}
+		current = current.Mapping[index].Value
+	}
+	return current
+}
+
+func validImportPath(value string) bool {
+	if value == "" || strings.TrimSpace(value) != value || path.IsAbs(value) || strings.HasPrefix(value, "//") {
+		return false
+	}
+	return !hasImportScheme(value)
+}
+
+func hasImportScheme(value string) bool {
+	colon := strings.IndexByte(value, ':')
+	slash := strings.IndexByte(value, '/')
+	if colon <= 0 || (slash >= 0 && slash < colon) {
+		return false
+	}
+	for index := 0; index < colon; index++ {
+		character := value[index]
+		if index == 0 {
+			if (character < 'a' || character > 'z') && (character < 'A' || character > 'Z') {
+				return false
+			}
+			continue
+		}
+		if (character < 'a' || character > 'z') && (character < 'A' || character > 'Z') &&
+			(character < '0' || character > '9') && character != '+' && character != '-' && character != '.' {
+			return false
+		}
+	}
+	return true
+}
+
+func nodeImportDiagnostic(code Code, path Path, message string, node *document.Node) Diagnostic {
+	location := publicLocation(node.Location)
+	return Diagnostic{Code: code, Severity: SeverityError, Path: path, Message: message, Primary: &location}
+}
+
+func profileDirectiveDiagnostics(node *document.Node, current Path) []Diagnostic {
+	if node.Kind == document.Mapping {
+		for _, entry := range node.Mapping {
+			childPath := current.Key(entry.Key)
+			if entry.Key == directiveKey {
+				location := publicLocation(entry.KeyLocation)
+				return []Diagnostic{{
+					Code: CodeProfileDirective, Severity: SeverityError, Path: childPath,
+					Message: "profiles cannot contain configuration directives", Primary: &location,
+				}}
+			}
+			if diagnostics := profileDirectiveDiagnostics(entry.Value, childPath); len(diagnostics) != 0 {
+				return diagnostics
+			}
+		}
+	} else if node.Kind == document.Sequence {
+		for index, child := range node.Sequence {
+			if diagnostics := profileDirectiveDiagnostics(child, current.Index(index)); len(diagnostics) != 0 {
+				return diagnostics
+			}
+		}
+	}
+	return nil
+}
+
+func disabledDirectiveDiagnostics(node *document.Node, path Path) []Diagnostic {
+	var diagnostics []Diagnostic
+	if node.Kind == document.Mapping {
+		for _, entry := range node.Mapping {
+			childPath := path.Key(entry.Key)
+			if entry.Key == directiveKey {
+				location := Location{SourceID: entry.KeyLocation.SourceID, Line: entry.KeyLocation.Line, Column: entry.KeyLocation.Column}
+				diagnostics = append(diagnostics, Diagnostic{
+					Code: CodeImportDisabled, Severity: SeverityError, Path: childPath,
+					Message: "configuration directives require imports to be enabled", Primary: &location,
+				})
+			}
+			diagnostics = append(diagnostics, disabledDirectiveDiagnostics(entry.Value, childPath)...)
+		}
+	} else if node.Kind == document.Sequence {
+		for index, child := range node.Sequence {
+			diagnostics = append(diagnostics, disabledDirectiveDiagnostics(child, path.Index(index))...)
+		}
+	}
+	return diagnostics
+}

doc.go

@@ -0,0 +1,10 @@
+// Package configkit provides deterministic, application-owned configuration
+// loading for Go programs.
+//
+// Load currently supports one file, byte, reader, or explicit no-input source with
+// strict YAML, typed defaults, schema-version checks, detached metadata, safe
+// diagnostics, recorded application resolution, read-only semantic validation, and
+// structured load errors. Composition, profiles, overrides, secrets, and effective
+// rendering are not available yet. The configuration contract is documented in
+// docs/config.md.
+package configkit

docs/adr/0001-separate-composition-and-overlay.md

@@ -0,0 +1,37 @@
+# ADR 0001: Separate Composition and Overlay
+
+## Status
+
+Accepted. This records a design decision, not implementation completion.
+
+## Date
+
+2026-09-06
+
+## Context
+
+Configuration fragments allow independent parts of an application to contribute
+settings. Treating every file as a precedence layer makes accidental duplicates
+silently depend on file order. Profiles, however, need intentional replacement.
+
+## Decision
+
+Use distinct strict-composition and intentional-overlay operations. Composition
+rejects conflicting definitions; overlay provides explicit precedence. Keep the
+operations opinionated rather than adding arbitrary per-field merge strategies.
+The [architecture policy](../policy/architecture.md) owns the governing boundary;
+the [current configuration contract](../config.md#strict-document-composition) owns
+the operation table and selected-profile overlay behavior.
+
+## Alternatives considered
+
+- Last-wins loading for all files: convenient but conceals accidental conflicts.
+- Strict loading for every layer: catches conflicts but cannot express profiles.
+- User-defined merge policies: flexible but weakens consistency and expands the API.
+
+## Consequences
+
+Consumers must distinguish modular contributions from intentional overrides.
+Diagnostics need both conflicting sources. File order remains useful for stable
+traversal and reporting but cannot silently authorize replacement. Named mappings
+are the natural representation for independently contributed entries.

docs/adr/0002-own-yaml-semantics.md

@@ -0,0 +1,39 @@
+# ADR 0002: Own YAML Semantics Behind a Private Parser Boundary
+
+## Status
+
+Accepted. This records a design decision, not implementation completion.
+
+## Date
+
+2026-09-06
+
+## Context
+
+A shared library cannot provide consistent configuration semantics if each
+consumer inherits parser conversions, node types, and extension behavior.
+Exposing parser types also makes dependency changes part of the public API.
+
+## Decision
+
+ConfigKit defines its supported YAML policy and keeps parser integration private.
+Application types and diagnostics must not require parser-specific public types.
+Use a small internal boundary rather than a general parser/provider framework.
+The [architecture policy](../policy/architecture.md) owns this constraint; the
+[current YAML document](../config.md#yaml-documents) and
+[scalar](../config.md#scalars) contracts own the implemented syntax.
+
+## Alternatives considered
+
+- Expose parser nodes and hooks: easy integration but couples consumers to parser
+  behavior and permits bypassing library policy.
+- Adopt the parser's entire YAML behavior: reduces initial code but leaves core
+  semantics outside ConfigKit's control.
+- Abstract many configuration languages immediately: adds unsupported scope.
+
+## Consequences
+
+Parser changes require behavior-level compatibility verification. ConfigKit must
+translate errors safely and enforce its own supported subset. Some existing
+consumer custom decoders will need replacement with library-supported mechanisms
+or application resolution. No particular parser dependency is selected here.

docs/adr/0003-preserve-presence-and-resolution-history.md

@@ -0,0 +1,44 @@
+# ADR 0003: Preserve Presence and Record Application Resolution
+
+## Status
+
+Accepted. This records a design decision, not implementation completion.
+
+## Date
+
+2026-09-06
+
+## Context
+
+Go zero values cannot distinguish omitted input from explicitly configured false,
+zero, or empty text. Decoding immediately also loses information needed to explain
+imports and overlays. Application normalization can subsequently make retained
+input metadata disagree with runtime values.
+
+## Decision
+
+Preserve input presence and provenance separately from ordinary application Go
+values. Keep application defaults and normalization application-owned, but record
+resolution changes so final inspection and metadata describe the actual resolved
+snapshot. Do not promise language-enforced immutability of exported Go values.
+The [architecture policy](../policy/architecture.md) owns these invariants; the
+[current resolution contract](../config.md#recorded-application-resolution) and
+[ownership contract](../config.md#typed-projection-and-ownership) own the implemented
+recording mechanism and ownership semantics.
+
+## Alternatives considered
+
+- Infer presence from zero values: cannot preserve explicit zero-valued settings.
+- Require pointers or optional wrappers everywhere: burdens application types and
+  confuses input presence with domain nullability.
+- Retain only the composed input tree: cannot explain post-decode changes.
+- Automatically diff mutations without reasons: detects changes but cannot explain
+  application intent or derivation dependencies.
+
+## Consequences
+
+Metadata and snapshot ownership are core concerns rather than optional debugging
+features. Resolution hooks must participate in recording changes. Verification
+must compare final values, origins, and rendering, including secret treatment.
+This costs more than plain unmarshalling but removes repeated presence machinery
+from consumers and supports trustworthy inspection.

docs/adr/0004-secret-references-and-explicit-bootstrap.md

@@ -0,0 +1,44 @@
+# ADR 0004: Use secret references and explicit environment bootstrap
+
+Status: accepted
+
+Date: 2026-09-06
+
+## Context
+
+Consumers need credentials without storing them in YAML and need a consistent
+way to load locally provisioned secret files. WeatherReporter already implements
+useful directory-loading code. Ordinary configuration loading must remain free
+of implicit process mutation, and operators must understand blocked secret files.
+
+## Decision
+
+Represent secrets as ConfigKit-owned typed references to environment-variable
+names. Resolve only surviving references; retain names, never credentials, in
+configuration trees and inspection. Offer directory bootstrap as a separate,
+explicit startup operation, reusing WeatherReporter code where applicable.
+Existing environment variables win even when empty. Report skipped files and
+partial failures without values. Keep bootstrap provenance separate from later
+lookup provenance rather than imply an unchanged environment between calls.
+General environment-variable configuration overlays are not on the roadmap.
+The [current reference contract](../config.md#typed-secret-references) and
+[bootstrap contract](../config.md#explicit-secrets-directory-bootstrap) own the
+implemented behavior.
+
+## Alternatives considered
+
+- Literal secrets or interpolated YAML: familiar but allows credentials into
+  configuration artifacts and expands conventions and redaction obligations.
+- Application-only directory loaders: keeps process mutation outside the library
+  but preserves duplicated infrastructure and inconsistent skip/error semantics.
+- Implicit bootstrap during loading: convenient but hides process mutation and
+  makes repeated or concurrent loads difficult to reason about.
+- Directory values overriding existing environment: simple, but defeats explicit
+  deployment environment choices and obscures why a credential was selected.
+
+## Consequences
+
+Applications migrate secret fields, choose startup ordering, and retain bootstrap
+reports. ConfigKit owns bounded loading, reference validation, safe reporting and
+inspection; applications own credential use. No multi-variable transaction or
+protection against concurrent process-environment mutation is promised.

docs/adr/0005-explicit-overrides-and-safe-explanation.md

@@ -0,0 +1,42 @@
+# ADR 0005: Record explicit CLI overrides and safe value explanation
+
+Status: accepted
+
+Date: 2026-09-06
+
+## Context
+
+Consumers need deliberate per-invocation configuration changes and an explanation
+of effective values. Presence and origin records alone cannot show which values
+were replaced. Adding CLI parsing or arbitrary environment overlays would broaden
+the library beyond its consistent configuration model.
+
+## Decision
+
+Use parser-neutral explicit path/value/flag records above selected profiles.
+Defaults remain below strictly composed base input. Apply overrides to the owned
+node tree before typed decoding, retaining the existing parser adapter boundary.
+Recorded domain resolution follows input selection. Provide safe structured
+historical values and an explanation renderer in the initial release, building
+on ADR 0003's presence and audited resolution decision. Applications own commands.
+The [current override contract](../config.md#explicit-cli-overrides) and
+[explanation contract](../config.md#historical-value-explanation) own the implemented
+behavior.
+
+## Alternatives considered
+
+- Retire configuration flags in favor of profiles: smaller API, but removes useful
+  explicit invocation choices that fit a single fixed precedence model.
+- Mutate structs after loading: easy locally, but bypasses validation and leaves
+  metadata and effective inspection inconsistent.
+- Defer historical values: reduces initial implementation scope but cannot answer
+  why an effective value differs from defaults or an earlier file.
+- Publish raw history trees: flexible but couples consumers to internals and risks
+  leaking sensitive values. Detached, redacted snapshots preserve the boundary.
+
+## Consequences
+
+The library owns override validation and source attribution without choosing a
+CLI framework. History requires bounded retained state and tests for replacement,
+deletion, composite origins and sensitivity. Credentials never enter history;
+secret reference names remain explainable unless explicitly classified sensitive.

docs/config.md

@@ -0,0 +1,746 @@
+# Configuration Input Contract
+
+This document owns ConfigKit's currently implemented source-input, YAML syntax,
+schema, typed-decoding, loading, metadata, redaction, rendering, and ownership
+contracts. The parser, owned document representation, and decoder remain private
+behind the public loading lifecycle.
+
+## Sources
+
+ConfigKit exposes opaque sources through these constructors:
+
+| Constructor | Current contract |
+| --- | --- |
+| `File(path)` | Requires a nonempty path, opens a regular file for each consumption, and closes only the library-owned file handle. Missing, inaccessible, unreadable, directory, and other nonregular inputs fail. |
+| `Bytes(label, data)` | Requires a nonempty, non-secret diagnostic label and snapshots bytes during construction. Later caller mutation cannot affect the source. |
+| `Reader(label, input)` | Requires a nonempty, non-secret label and non-nil reader. The reader is claimed once across all copies of the source, consumed but never closed, and cannot be reused. |
+| `NoSource()` | Selects defaults-only loading, creates no source record, and marks no paths explicit. |
+
+The zero `Source` is invalid. Constructor argument failures are retained inside the
+opaque source and become safe structured diagnostics when the source is consumed.
+File sources can observe file changes between consumptions; byte sources cannot.
+
+Each source is limited to 8 MiB. A load using imports or an active profile is limited
+to 64 MiB across all accepted sources, 1,024 accepted sources, 64 active import
+edges, and 1,000,000 input nodes in aggregate. Glob expansion separately permits
+1,000,000 visited directory entries, including nonmatches. Reads request at most the
+remaining allowance plus one byte so excess data is rejected rather than silently
+truncated. Data returned together with a read error is not accepted as partial input.
+
+## Single-source loading
+
+`Load[T](source, options...)` accepts zero or one `Options[T]`. The configuration
+root `T` must satisfy the application schema contract below. With no options it
+starts from the Go zero value. A `Definition[T].Defaults` factory, when supplied,
+runs exactly once after successful source parsing, schema construction, optional
+schema-version checking, and selected-profile loading. Its result is detached before
+input is applied. Resolver-provided mutable values are detached before secret lookup
+and validation, and a final detached copy prevents equal-valued aliases introduced
+by validators from escaping into the returned configuration. The publication copy
+preserves already validated secret state without rereading the environment.
+
+The current lifecycle is:
+
+```text
+validate options → parse the selected source → traverse/compose enabled imports
+  → build schema → check the optional application version → apply a selected profile
+  → apply explicit CLI overrides → create and clone defaults → strictly decode
+  → recorded application resolution
+  → resolve typed secret references → semantic validation → classify secrets
+  → snapshot configuration and metadata
+```
+
+`File`, `Bytes`, and `Reader` each contribute the first base source with deterministic
+ID `source-1`; enabled imports continue in depth-first order as `source-2` onward.
+`NoSource` skips parsing, supplies an absent version to a configured version checker,
+contributes no source, and continues through defaults, decode, and projection. A
+missing file remains an error; it is never interpreted as no input.
+
+The current `Options` boundary accepts a defaults factory, `VersionRule`, resolver,
+semantic validator, explicit path and glob imports, one selected profile, and
+filesystem context. It also accepts explicit CLI override records, static secret
+tags, and a dynamic path predicate. `NoSource` rejects imports, active profiles,
+and filesystem context as invalid combinations but accepts overrides. Input
+containing a reserved `configkit` mapping key is rejected while imports are
+disabled.
+
+Every source, options, schema, version, defaults, decode, resolution, or validation
+failure returns a nil result and a `*LoadError`. Successful results contain caller-owned `Config`, detached
+`Metadata`, a defensive source list, and defensive nonfatal diagnostics. Mutating
+the returned configuration or inspection records cannot alter ConfigKit's private
+snapshot. ConfigKit performs no logging. `Result.WriteEffective` exposes only the
+frozen redacted projection.
+
+### Application schema versions
+
+`VersionRule` declares a semantic path and non-nil checker. Before defaults or typed
+decoding, the checker receives a `Version` preserving absent, explicit null, string,
+or strict decimal-integer input. Strings and integers supply their exact spelling in
+`Version.Text`; ConfigKit does not impose a machine-integer range. Fractional or
+exponent numbers, booleans, mappings, and sequences produce `version.type`, and the
+checker does not run. ConfigKit does not interpret the text or impose a universal
+version path.
+
+Integer versions use the strict grammar `^-?(0|[1-9][0-9]*)$`. Their spelling is
+preserved exactly, including `-0`, and application policy decides which identifiers
+are supported. A numeric version has no built-in `int64` range limit. Existing parser
+policy still rejects leading zeroes, leading plus signs, hexadecimal notation, and
+other non-JSON scalar forms before version checking.
+
+At exactly the configured semantic path, accepted integer input can decode into a
+plain Go string or named string without a text codec, including pointer wrappers.
+This is a load-local exception: siblings and unrelated fields, dynamic values,
+integer and float destinations, durations, `Secret`, and text codecs retain ordinary
+strict decoding. The retained source and history stay numeric while typed projection
+and effective YAML contain the resulting string; explanation records this through
+the normal decoding reconciliation event. Fragment, profile, and CLI ownership
+protections apply before this conversion.
+
+The checker reports safe, value-free errors and warnings through `Validation.Error`
+and `Validation.Warning`; ConfigKit resolves primary and related paths to exact or
+nearest input locations. Reported errors fail loading. If the checker returns an
+error without reporting one, ConfigKit emits a generic `version.hook_failed`
+diagnostic and suppresses the returned error text. Warnings alone are retained on a
+successful result. Callback panics remain programming failures.
+
+### Layered metadata
+
+`Result.Metadata` describes every final canonical path. Queries retain explicit
+key/index distinctions using actual effective and historical container shapes;
+only parsed numeric tokens are resolved contextually. A descendant that never
+existed is not historical merely because an atomic ancestor was replaced. Exact surviving input paths
+are explicit and have a base or profile origin; omitted paths have a default origin.
+An explicit parent mapping does not make omitted children explicit. Null state and
+available source coordinates are retained. `History` records the Go-zero or application
+default baseline followed by exact base or profile definitions, including
+equal-valued input. Profile replacement removes displaced descendants from effective
+metadata while retaining their base definition and the parent replacement operation
+in history. All metadata queries and returned nested records are detached.
+
+`Metadata.HistoricalPaths` enumerates observed paths, including removed paths,
+without reloading input or resolving unselected credentials. Consumers can use
+it with `History` to validate ownership of declarations hidden by an overlay;
+domain ownership rules remain application responsibilities. The method's Go
+documentation in [metadata.go](../metadata.go) defines enumeration and path-ordering
+details.
+
+History is chronological: a path can have a synthetic default record, its exact base
+definition, a profile replacement, and a recorded resolution operation. A descendant
+query uses the ancestor's path for a whole-container or whole-sequence replacement;
+the descendant can therefore retain history after `Lookup` stops reporting it as
+effective. Equal-valued input is still an explicit base definition, while a derived
+value for absent input remains non-explicit.
+
+A mapping assembled from the selected file and imports has one composite base origin
+whose locations retain source traversal order. A mapping also visited by a profile
+has a profile-layer composite origin with the lower contributing locations followed
+by the profile location; an empty mapping is still an explicit container even though
+it contributes no children. Resolution origins are synthetic: they have layer
+`resolution`, no fabricated file location, and a `derived_default` or `normalization`
+operation. Reasons and dependency paths retain first occurrence order with duplicates
+removed. Source records remain ordered as selected source, depth-first imports, the
+selected profile, then synthetic CLI records in supplied order.
+
+### Secret classification and effective rendering
+
+Secret classification combines `configkit:"secret"` field tags with the optional
+pure `Definition.Secret(Path) bool` predicate. Tags on concrete structs inside
+supported dynamic values are retained in typed projections, including superseded
+defaults and pre-resolution values. A tagged or predicate-classified
+container makes all descendants sensitive. ConfigKit evaluates final and historical
+paths, so replacing or deleting a definition does not erase its classification. The
+predicate can add sensitivity but cannot declassify a tagged path.
+
+Recorded resolution dependencies propagate sensitivity to their derived target.
+Propagation reaches a fixed point, so record/map iteration order does not affect the
+result and dependency cycles terminate. A dependency on a classified descendant or
+on a container that contains one is treated conservatively as sensitive. Applications
+must classify secret-producing transformations whose dependencies are not recorded.
+
+`Result.WriteEffective` writes the frozen resolved snapshot as deterministic redacted
+YAML. Mappings sort by decoded key string, sequences retain order, indentation is two
+spaces with noncompact sequence indentation, line endings are LF, the document has no
+start or end marker, and output ends with a newline. Nil pointers/maps/slices remain
+`null`; non-nil empty maps and sequences remain `{}` and `[]`. A secret root is
+replaced by the quoted string `'[REDACTED]'`. Redacted output is intended for
+inspection and is not promised to be reloadable.
+
+Rendering uses only a safe adapter tree built from the already completed canonical
+typed projection. It does not invoke application YAML marshal hooks or text codecs;
+all text-codec projection has succeeded before `Load` returns. Caller mutation of
+`Result.Config` cannot change later output. Writer failures, including short writes,
+are returned and may leave partial redacted output; internal dumper failures are
+translated to a fixed safe error. There is no unredacted rendering option and
+ConfigKit performs no implicit logging.
+
+`Result.Config` is the explicit trusted boundary and contains ordinary typed values,
+including secrets. Metadata records, diagnostics, and load errors contain no
+configuration values. Explanation snapshots deliberately expose non-secret
+configuration history after applying the redaction contract below. Applications
+must still keep source labels, resolution reasons, and diagnostic messages value-free
+because ConfigKit cannot infer and sanitize secrets embedded in arbitrary application
+prose.
+
+### Historical-value explanation
+
+`Result.Explain(path)` returns a detached `Explanation` for any known effective or
+historical path. Its effective value and each chronological before/after value are
+`ValueSnapshot` records containing deterministic YAML text rather than mutable Go or
+parser values. `Present=false` with empty YAML means absence or deletion; explicit
+null is present and renders as `null`. The history starts with the canonical typed
+application-default or Go-zero baseline, then includes only relevant base sources in
+traversal order, the selected profile, supplied CLI records, and audited net
+resolution operations. Overlapping parent/child resolution records are rendered
+as one net subtree event with combined reasons and dependencies, rather than
+pretending that the recorded assignments ran in lexical order. Explicit equal
+writes remain visible. When input replay
+differs from the final typed input, a `ChangeDecoding` event records the transition
+before domain resolution. Its origin retains the decoded path's source/default
+attribution; its reason identifies static-default retention and canonical decoding.
+This built-in inspection operation is not accepted by `Resolution.Record`.
+
+Base, profile, and CLI history preserves the policy-validated input representation,
+including a superseded value that could not have decoded into the final Go field.
+Baseline, effective, and resolution values use canonical typed projection. A
+decoding event bridges replayed input and canonical typed values, so intermediate
+null/scalar replacements cannot incorrectly imply permanent default removal. Mapping
+queries reconstruct each source or override contribution, while sequences remain
+atomic: a descendant history identifies its presence before and after the containing
+sequence replacement without implying element-wise merging. Removed paths remain
+explainable; unknown and malformed paths return false.
+
+Every snapshot applies the same historical tag, predicate, and recorded-dependency
+sensitivity closure as effective rendering before YAML is exposed. A redacted scalar
+is the quoted marker `'[REDACTED]'`, and a container snapshot has `Redacted=true` if
+it contains a masked descendant. Typed `Secret` snapshots contain only the configured
+environment name. `SecretReference` reports that name only for a successfully
+resolved effective reference, and is omitted when its path is classified. Credential
+values are never retained in explanation history.
+
+`Result.WriteExplanation(output, path)` renders the safe structured result with the
+canonical JSON-pointer path, canonical-versus-input representation labels, effective
+source, composite locations, chronological values, resolution reasons and
+dependencies, and successful secret-reference lookup attribution. Control characters
+in paths, source identifiers and labels, reasons, dependencies, and reference names
+are escaped. An unknown path returns a fixed safe error before writing anything;
+writer errors and short writes are returned and may leave partial output. There is no
+unredacted mode.
+
+Retained history consists of immutable source applications and audited subtree
+deltas, rather than a full tree copy per field. Shared nodes count once toward a
+4,000,000-node retained-history limit; exceeding it fails loading with
+`explain.limit_exceeded`. Replaying one requested path is performed on demand, and
+the returned YAML strings and nested records cannot mutate later explanations.
+
+Application commands should render the separate `SecretsReport` immediately after
+an explicit `LoadSecretsDir` bootstrap, then use `WriteDiagnostics`,
+`WriteEffective`, or `WriteExplanation` on the later load result as appropriate.
+ConfigKit does not infer bootstrap outcomes from a secret reference and does not
+bundle these independent reports into a command framework.
+
+## Typed secret references
+
+Credential-bearing fields use the opaque `Secret` type and store an environment
+variable name in YAML, such as `api_key: OPENROUTER_API_KEY`. `SecretEnv(name)`
+constructs the same unresolved reference for application defaults or recorded
+resolution. Names must match `[A-Za-z_][A-Za-z0-9_]*`; they are public
+configuration and remain visible through `EnvName`, canonical projection, history,
+and effective output unless the path is separately classified for redaction.
+
+A non-pointer `Secret` is required. An absent, null, or invalid reference, or an
+unset or empty environment value, fails with a safe `secret.*` diagnostic at the
+configuration path. `*Secret` is optional only when nil; every non-nil reference
+must resolve to a nonempty value. Secret references work in typed structs, maps,
+pointers, slices, and arrays. A dynamic `any` value does not establish secret
+semantics and cannot contain `Secret`; applications must use a typed slot.
+
+ConfigKit resolves only the effective names after recorded application resolution
+and before semantic validation. Each unique name is read with `os.LookupEnv` once
+per load. Validation can inspect the credential through the deliberately named
+`Value()` accessor and distinguish success with `IsResolved()`. Later loads read
+the environment again, and cloned defaults are rebuilt from names so a previously
+resolved result cannot retain a stale credential. `Load` never writes to the
+environment.
+
+Credential bytes are held only in the per-load lookup state and the trusted returned
+configuration, and are returned only by `Value`. Secret formatting uses a safe
+public-name representation, JSON marshals only the name, and projection, metadata,
+histories, diagnostics, effective output,
+and resolution snapshots never retain the credential. Validation remains read-only:
+in addition to ordinary projected-value comparison, ConfigKit checks the private
+resolved state against this load's lookup result. Validator mutation checks also
+reject changes to projected field sensitivity, even when a replacement dynamic
+concrete type preserves the same textual values. Applications must keep their
+validation messages value-free and must not include `Value()` in diagnostics.
+
+For example, a typed pipeline field is declared as:
+
+```go
+type PipelineConfig struct {
+    APIKey configkit.Secret `yaml:"api_key"`
+}
+```
+
+The executable `ExampleSecretEnv` demonstrates loading and inspecting the safe
+reference state without printing its value.
+
+## Explicit secrets-directory bootstrap
+
+`LoadSecretsDir(directory)` is a separate, opt-in startup operation. It is never
+called by `Load`, is not a YAML option, and does not participate in configuration
+validation. Applications choose the native directory path and should invoke the
+operation once before starting concurrent work. The selected path may itself be a
+symlink; ConfigKit resolves it to an existing canonical directory before examining
+its immediate entries.
+
+The bootstrap enumerates at most 1,024 entries and sorts them by filename. Every
+entry must have an ASCII environment-variable name and be a regular, nonsymlink
+file. Metadata for all entries is validated before file content is read or process
+state is changed. Existing environment variables win, including variables whose
+value is empty; their files are not opened. Each eligible file is limited to 1 MiB,
+and eligible content is limited to 8 MiB in total. ConfigKit removes exactly one
+terminal LF or CRLF, preserves all other whitespace, accepts an empty value, and
+rejects NUL bytes.
+
+All eligible files are read successfully before application begins. Presence is
+checked again immediately before each `os.Setenv`: a variable populated since
+preflight is skipped. A write failure stops application without rollback, so prior
+writes remain and later eligible entries are `not_applied`. This operation is not
+atomic and does not provide compare-and-set semantics for concurrent environment
+mutation.
+
+`SecretsReport.Entries` returns detached, filename-ordered `SecretFileResult`
+records with `loaded`, `skipped_existing`, `not_applied`, or `failed` status.
+`ExistingEmpty` distinguishes an empty existing winner. `SecretsReport.WriteTo`
+renders only quoted names, canonical paths, statuses, and that empty-state flag;
+it never retains or renders file contents. Failures return the non-nil partial
+report with a `*SecretsError` containing detached `secrets.*` diagnostics and no
+raw filesystem or environment error chain. Applications should retain the report
+alongside the later configuration result and explicitly present blocked and failed
+files when appropriate; ConfigKit stores no implicit global report.
+
+### Recorded application resolution
+
+An optional `Definition.Resolve` callback runs after successful typed decoding. It
+receives the working configuration and a per-load `Resolution` context. The context's
+`IsExplicit` query uses surviving input presence, so explicit false, zero, empty, and
+null are never confused with absence or static defaults.
+
+`Default` assigns and records a derived value only when its path is not explicit.
+`Normalize` always assigns and records. For transformations performed directly by
+application code, `Resolution.Record` declares a derived-default or normalization
+operation; a recorded container covers changes to its descendants. Records require
+a schema-valid path, valid kind, safe nonempty reason, and schema-valid dependency
+paths. Repeated records at one path are coalesced: reasons and dependencies retain
+their first occurrence, the last helper value is authoritative, and any normalization
+makes the combined operation a normalization.
+
+ConfigKit projects the configuration before and after the callback. Every net change
+must have an exact or ancestor record. Helper-assigned values must agree with the
+final value at their declared path, which supports ordinary map-entry copy/writeback
+while detecting forgotten writeback or unrelated targets. Invalid paths, kinds,
+values, records, helper targets, and unrecorded mutations fail the load. Equal-valued
+derived assignments may still record application intent even when there is no
+structural change.
+
+`Resolution.Error` and `Resolution.Warning` attach exact or nearest available input
+locations. Reported errors fail the load; warnings survive on a successful result. A
+returned resolver error adds one generic `resolve.hook_failed` error only when the
+callback did not report an error, and its text is never exposed. Callback panics are
+programming failures. Resolution and validation contexts are callback-scoped; using
+a nil, zero, or retained context outside its active callback panics with fixed,
+value-free text.
+
+Successful records update effective origins and append detached metadata history
+without changing input explicitness. Removed paths leave effective metadata but
+retain history. ConfigKit privately snapshots each recorded before/after subtree for
+later safe explanation; caller mutations of `Result.Config` cannot alter those
+snapshots.
+
+### Semantic validation
+
+An optional `Definition.Validate` callback runs after successful resolution and
+receives the resolved configuration plus a per-load `Validation` context. It can
+collect multiple independent `validate.error` and `validate.warning` reports.
+Warnings continue the lifecycle and are returned on success; any error returns a nil
+result with the full safe diagnostic and source report.
+
+Primary and related paths must be valid for the application schema. Omitted declared
+fields and missing string-map or dynamic keys are valid report targets; unknown
+struct fields and malformed paths produce `validate.invalid_path`. ConfigKit attaches
+the exact surviving input location when one exists, otherwise the nearest input
+container location, or no primary location for defaults-only synthetic state.
+
+Validation is read-only. ConfigKit projects the configuration immediately before and
+after the callback and returns `validate.mutation` at every detected net-change path,
+including nested mutable state. A value that can no longer be projected is also a
+mutation failure. No mutated or otherwise invalid configuration is published.
+Callback panics remain programming failures, and retained contexts use the same
+fixed closed-context panic as version and resolution callbacks.
+
+Only schema-bound exported state participates in loading, resolution audit,
+validation audit, metadata, and snapshots. Ignored or unexported ordinary fields are
+zero in returned detached values. Applications retain responsibility for domain
+rules and runtime initialization; validators should not start clients or depend on
+live services as a substitute for configuration validation.
+
+## Strict document composition
+
+ConfigKit strictly composes parsed base documents when `Options.Imports` is true.
+Application defaults do not participate in this operation.
+
+| Existing node | Incoming node | Result |
+| --- | --- | --- |
+| Absent | Absent | Absent |
+| Absent | Present | Retain the incoming node |
+| Present | Absent | Retain the existing node |
+| Mapping | Mapping | Recurse by mapping key |
+| Any other pair of present nodes | Any other pair of present nodes | `compose.conflict` |
+
+Every non-mapping overlap is a conflict, even when values are equal. Nulls and
+sequences are present atomic values, so they also conflict when repeated; sequences
+are never joined by index. Disjoint entries under shared mappings compose, making
+stable keyed mappings the natural representation for independently contributed
+entries.
+
+An empty mapping contributes no child definitions. Two empty mappings compose, and
+an empty mapping neither clears nor conflicts with another mapping's children.
+Shared mapping containers retain all contributing value locations as their composite
+base origin. A conflict reports the first claim as its primary location and every
+remaining claim as related locations, in deterministic source and declaration
+traversal order. Traversal order never authorizes replacement. Composition clones
+the resulting tree and its ordered claim records rather than mutating or aliasing an
+input document.
+
+## Selected profiles
+
+`Options.Profile` selects at most one overlay by name. Names must match
+`[a-zA-Z0-9_-]+`; they are not paths. The empty name and exact lowercase name
+`default` select no profile, and `profiles/default.yml` is never loaded implicitly.
+Every other name resolves only to `<configuration-root>/profiles/<name>.yml`, with
+no extension fallback, case normalization, inheritance, or profile stack. A selected
+missing profile is an error.
+
+The profile is a separately policy-validated YAML mapping. It is loaded after the
+base import graph is composed and its schema version is checked, but before defaults
+and typed decoding. Profile bytes, nodes, and source records count toward the same
+aggregate limits as base inputs. Its canonical path must remain within the configured
+root and must not duplicate a file already present in the base graph.
+
+Overlay is intentionally distinct from strict composition:
+
+| Existing lower node | Incoming profile node | Result |
+| --- | --- | --- |
+| Absent | Value | Retain the profile value |
+| Value | Absent | Retain the lower value |
+| Mapping | Mapping | Recurse by mapping key |
+| Any other pair of present nodes | Any other pair of present nodes | Replace the entire lower node |
+
+Scalars, sequences, and null are atomic. Thus a profile sequence replaces rather
+than extends a base sequence, `[]` clears it, null is an explicit replacement, and
+cross-kind replacement is allowed when the final application schema accepts it.
+Replacing a subtree removes all displaced descendants from effective presence and
+origin data; history retains the displaced definitions and identifies the parent
+replacement. Surviving profile paths have profile origins, while recursively shared
+mapping containers retain their contributing locations.
+
+A profile may not contain `configkit` anywhere, contribute the configured schema
+version, or replace an ancestor of that version with a scalar or null. These rules
+apply even if the supplied version text would be unchanged. Profiles cannot mask a
+base parse, composition, or version failure because those checks run first.
+
+## Explicit CLI overrides
+
+`Options.Overrides` accepts only records the application knows were explicitly set
+by its CLI parser. Each `Override` supplies a non-root mapping `Path`, a detached
+`Value`, and a required safe display `Flag` such as `--workers`; ConfigKit has no CLI
+parser dependency. Parser defaults must not be submitted as overrides, and `Flag`
+must never contain the raw argument or configured value. General environment
+variable overlays are an explicit non-goal: environment access is reserved for
+typed secret references.
+
+Overrides apply after the selected profile and before typed defaults/decoding, so
+the input precedence is application defaults < composed base < selected profile <
+CLI. Recorded resolution, typed secret lookup, and semantic validation follow the
+effective override tree. Overrides also work with `NoSource`.
+
+Values are converted directly into ConfigKit's owned tree without YAML parsing or
+string coercion. Supported values are `nil`, built-in strings and booleans, all
+built-in signed and unsigned integer widths except `uintptr`, finite `float32` and
+`float64`, and recursive `[]any`/`map[string]any`. Strings stay strings; duration and
+secret fields therefore receive string values. Nil is explicit null; empty strings,
+false, and zero remain intentional; empty sequences replace whole lists; and mapping
+values recursively overlay existing mappings. Map keys are sorted canonically and
+sequence order is preserved.
+
+All records are detached and validated before any is applied. Root and explicit
+sequence-index paths are invalid; parsed numeric tokens remain mapping keys. A path
+may create missing mapping ancestors but cannot traverse an existing scalar, null,
+or sequence. Duplicate and ancestor/descendant-overlapping records are rejected.
+Neither paths nor values may create `configkit`, and no record may overlap the
+configured schema-version path. Unknown fields and effective type mismatches remain
+ordinary strict-decoder errors.
+
+At most 1,024 records are accepted, and each record creates one synthetic CLI
+source after file/profile sources in supplied order. Override sources share the
+1,024-source ceiling. A record has an 8 MiB string-byte limit; override strings and
+source bytes share the 64 MiB aggregate limit. Synthetic mapping keys and values
+count toward the existing 1,000,000-node and depth-256 ceilings. Cycles, named or
+unsupported types, nonfinite floats, and limit excesses fail with safe `override.*`
+diagnostics. Diagnostics and provenance identify the safe `Flag` source, never raw
+arguments or values.
+
+Override origins are explicit CLI origins. History retains replaced base/profile
+definitions and records equal-value replacements as well as structural changes.
+Caller-owned maps and slices are snapshotted during loading and must not be mutated
+concurrently with the call. The executable `ExampleOverride` shows translating a
+parser's explicit-set state into records while preserving an explicitly supplied
+zero.
+
+## Explicit imports
+
+Imports are opt-in. With `Options.Imports` enabled, a document may reserve its root
+`configkit` mapping for an `imports` sequence. ConfigKit removes that mapping before
+application decoding. Each sequence item is a mapping with exactly one nonempty
+string `path` or `glob` plus an optional boolean `optional`; all other shapes and
+fields are `import.invalid`. A `configkit` key below the root is invalid. With
+imports disabled, any occurrence remains `import.disabled`.
+
+The directive key is `configkit`, not the `_configkit` spelling used in v0.1.0.
+There is no legacy alias: `_configkit` is ordinary application data, subject to
+normal schema validation, and does not trigger imports. When upgrading, rename
+directive keys in root files and imported fragments, and rename any application
+fields previously named `configkit` to avoid the newly reserved key.
+
+Path declarations use slash-separated relative paths. Absolute paths and network
+URLs are invalid. Each path resolves from its declaring file's canonical directory,
+and files are traversed depth-first in declaration order: a file contributes its own
+body before its imports. Order stabilizes source records and diagnostics but never
+permits replacement. Required missing paths fail; `optional: true` suppresses only
+target nonexistence, not invalid paths, containment failures, directories,
+permissions, parse errors, or composition errors.
+
+Glob declarations use the slash-based Go `path.Match` grammar: `*`, `?`, character
+classes, and backslash escapes are supported, while recursive `**` syntax is invalid.
+Backslashes escape pattern characters rather than acting as directory separators.
+ConfigKit validates the complete pattern before filesystem traversal, walks only
+directories selected by preceding segments, and reads wildcard directories in
+bounded batches. Every examined entry consumes the separate visit budget, including
+nonmatches. Matches are sorted lexicographically by slash-relative spelling within
+each declaration before depth-first source traversal. A required zero-match glob is
+`import.missing`; `optional: true` suppresses only zero matches. Invalid patterns,
+walk failures, matched directories, invalid documents, escapes, and repeated targets
+remain errors. Final candidates are bounded by the remaining source limit before
+their contents are accumulated.
+
+The configuration root defaults to a selected file's canonical directory. A file
+source may supply `FilesystemContext.Root` to widen that root but may not supply
+`BaseDir`. Whenever byte and reader sources supply filesystem context, they must
+provide both `BaseDir` and `Root` as absolute existing directories; their diagnostic
+labels, including URI-looking labels, never imply filesystem paths. The base and
+selected file must be inside the canonical root. Nonregular targets are rejected before opening to avoid blocking on FIFOs;
+post-open regular-file checks remain in place under the stable-input assumption.
+ConfigKit canonicalizes symlinks,
+checks containment with relative paths, and opens the canonical root-relative target
+through a library-owned root handle. Contained relative or absolute symlink targets
+are accepted; escapes are rejected. Canonical paths define identity, so hard links at
+distinct paths remain distinct sources.
+
+An active-stack repeat is `import.cycle` and reports the complete cycle's import
+sites. A previously visited canonical path is `import.repeated` and reports both
+inclusion routes, including diamonds and aliases. Each accepted file is read once.
+Imported fragments may contain their own root directives, but may not claim the
+application's schema-version path or replace one of its ancestors; the version must
+come from the selected source. No directory is scanned implicitly, including one
+named `conf.d`, and ConfigKit performs no remote fetching.
+
+## YAML documents
+
+The private YAML boundary currently accepts exactly one nonempty document whose root
+is a mapping. Comments and block or flow collections are supported. It rejects:
+
+- empty input or an empty document;
+- every additional document, including an empty one;
+- duplicate decoded mapping keys;
+- non-string mapping keys;
+- explicit tags, anchors, aliases, and merge keys;
+- a quoted or plain mapping key equal to `<<`; and
+- input exceeding the fixed depth or node limits.
+
+Mapping key comparison is exact: it performs no case folding or Unicode
+normalization. The private document tree retains declaration order, exact validated
+number spelling, source identifiers, and separate key/value line and column
+locations. It contains no upstream parser nodes or types.
+
+Root depth is zero. Input depth is limited to 256 and each parsed input tree to
+1,000,000 nodes, counting mapping keys as input nodes. The upstream parser also has
+a private calibrated depth guard; passing that guard never bypasses ConfigKit's tree
+limits.
+
+## Scalars
+
+Scalar meaning is determined by ConfigKit rather than upstream coercion:
+
+| Input form | Meaning |
+| --- | --- |
+| Quoted, literal, or folded scalar | String |
+| Plain `true` or `false` | Boolean |
+| Plain `null`, `~`, or an empty mapping value | Null |
+| JSON decimal number | Number, retaining its exact spelling |
+| Date/time-looking or other ordinary plain text | String |
+
+JSON numbers permit an optional minus, a zero or nonzero decimal integer without a
+leading zero, an optional fraction with at least one digit, and an optional decimal
+exponent. Hexadecimal, octal, digit separators, leading plus, leading/trailing-dot
+forms, incomplete exponents, and non-finite values are rejected. Legacy boolean
+words (`yes`, `no`, `on`, and `off`) and non-lowercase variants of the reserved
+boolean/null words are rejected unless quoted.
+
+Mapping keys use the same scalar classification and must classify as strings. Quote
+keys that otherwise classify as a boolean, number, or null.
+
+## Application schema binding
+
+ConfigKit builds a private reflection descriptor graph for each schema operation;
+it keeps no global mutable descriptor cache. A configuration root must be a struct
+or string-keyed map. Nested supported shapes are:
+
+- booleans, strings, signed and unsigned integers, and finite floating-point types;
+- `time.Duration`, typed `Secret` references, and paired
+  `encoding.TextMarshaler`/`TextUnmarshaler` scalars;
+- pointers, structs, string-keyed maps, slices, fixed arrays, and `any`; and
+- recursive type definitions whose eventual application values are acyclic.
+
+Named scalar types retain their Go identity. `uintptr`, complex numbers, functions,
+channels, unsafe pointers, non-string map keys, and nonempty interfaces are not
+configuration types. A custom text scalar must provide both directions on its value
+or pointer method set. Building a descriptor only inspects method sets; it never
+executes application methods or parser-specific hooks.
+
+Exported struct fields use an explicit `yaml` name or the lowercase Go field name.
+Unexported fields and `yaml:"-"` are ignored. `omitempty` is accepted but does not
+change binding or projection. An explicit `yaml:",inline"` flattens a struct
+or pointer-to-struct, or designates at most one string-keyed inline map for otherwise
+unmatched names. Ambiguous names, named inline fields, multiple inline maps, and
+unsupported or malformed tag options are schema errors. Inline pointers must
+terminate in a struct; pointer-to-map and recursive pointer-only inline declarations
+are rejected before the defaults factory runs. Embedded fields are ordinary
+named fields unless explicitly inlined. `configkit:"secret"` is the only current
+ConfigKit field tag and marks its bound field or container for secret classification
+and redacted inspection.
+
+Schema path traversal uses these same bindings. Explicit key segments address
+mappings, explicit index segments address sequences, and parsed numeric pointer
+tokens become indices only in a sequence. Sequence indices must use canonical
+nonnegative decimal spelling: no signs or leading zeroes except `0` itself.
+Tokens such as `+0` and `-0` remain literal keys in mappings but do not identify
+sequence elements. Fixed-array indices are bounds checked. Dynamic `any` and inline-map entries
+remain schema-valid for application-defined descendants, but dynamic values cannot
+introduce typed secret references.
+
+Descriptors are reused by typed decoding, projection, and cloning so those operations
+cannot acquire different field bindings.
+
+### Canonical module options
+
+Applications with a known module catalog represent each configured item as an object
+with an explicit module identifier and an options mapping. Ordinary typed structs,
+including a small union of optional option fields, use the same strict binding above;
+application validation decides which fields and combinations apply to each module.
+String shorthand, a library module registry, and custom structural decoder hooks are
+not supported alternatives.
+
+When a runtime-selected catalog genuinely requires dynamic options,
+`map[string]any` retains ConfigKit's canonical dynamic scalars. In particular,
+integers are `int64`; application validation owns allowed keys, value constraints,
+range checks, and conversion to runtime `int` or catalog objects. ConfigKit does not
+provide generic subtree conversion, and consumers must not add a YAML marshal/decode
+round trip to exercise supported behavior. The
+[consumer adoption guide](consumers/README.md) contains compiled typed and dynamic
+examples.
+
+## Defaults and typed decoding
+
+The private decoder starts from an independently owned clone of the application's
+typed defaults and applies only explicitly present input nodes. Defaults therefore
+never become explicit, while equal-valued input remains explicit. Successful decode
+produces detached typed state, baseline and final canonical trees, an input-presence
+index, and default/input origins for the later public loading lifecycle.
+
+| Configured input | Effect on the typed baseline |
+| --- | --- |
+| Absent field | Retain the default |
+| Explicit false, zero, or empty string | Replace the default |
+| Partial mapping or string-keyed map | Recurse and retain omitted default children or entries |
+| Empty mapping | Retain existing mapping children |
+| Sequence, including an empty sequence | Replace the entire sequence |
+| Pointer object | Recurse into a non-nil default; otherwise allocate an object whose omitted children have ordinary Go zero values |
+| Allowed null | Replace the value with nil and suppress its default |
+
+Null is allowed only for pointers, maps, slices, and dynamic `any` values. It is an
+error for scalars, structs, and arrays. New map entries and sequence elements begin
+from their Go zero values; ConfigKit does not invent entry-level defaults.
+
+Decoding is strict at every typed depth. Unknown struct fields, type mismatches,
+invalid nulls, numeric overflow, and arrays with the wrong length are errors at the
+original semantic path and source coordinates. String-keyed maps accept arbitrary
+keys but preserve the strict schema of their values; inline maps accept unmatched
+keys according to their element descriptor. Multiple independent field failures are
+collected without publishing the partially decoded object.
+
+Booleans and strings are not generally coerced. At exactly a configured
+`VersionRule.Path`, strict decimal-integer input may decode into a plain `string`, a
+named string without a text codec, or pointer wrappers around those types. The
+exception is load-local and does not apply to unrelated fields or loads without a
+version rule. Integer destinations require integer syntax
+and a value in the destination type's range. Float destinations accept finite JSON
+numbers representable by their destination precision. Dynamic numbers become
+`int64` for integer syntax and `float64` for fractional or exponent syntax, with
+range and finiteness checks. Named scalar types retain their identity. Durations
+decode only from strings accepted by `time.ParseDuration`; integers are rejected.
+Paired text scalars decode only from strings through `encoding.TextUnmarshaler`, and
+codec failures are classified without exposing application error text or input
+values.
+
+This decoder is available only through `Load`; ConfigKit exposes no generic document
+conversion API.
+
+## Typed projection and ownership
+
+ConfigKit can internally project a schema-valid application value directly into its
+owned canonical tree without YAML serialization. Projection includes every bound
+field, including ordinary zero values; `omitempty` has no effect. Struct fields keep
+binding order, dynamic map keys sort by decoded string, and sequences keep their
+order. Integers retain exact base-10 values, finite floats use their destination
+precision, durations use `time.Duration.String`, and paired text scalars use their
+canonical marshaled text.
+
+Nil pointers, maps, slices, and dynamic values project as null. Non-nil empty maps
+and slices remain empty containers, preserving the distinction. Recursive acyclic
+objects are supported. Cyclic values, non-finite floats, unsupported dynamic concrete
+types, invalid codec output, and depth/node-limit violations fail with safe schema
+classifications that omit codec error text.
+
+The same descriptor graph creates detached typed copies. Pointers, maps, slices,
+arrays, nested dynamic values, and shared references are recursively copied; shared
+pointer identity is not preserved as configuration meaning. Paired text scalars are
+rebuilt by unmarshaling their canonical text into fresh values, detaching private
+backing storage. Ignored and unexported struct fields are zeroed rather than copied.
+
+Private structural comparison operates on canonical trees. Mapping additions,
+removals, and descendant replacements retain semantic paths. A sequence is atomic:
+any sequence change reports the sequence path and never invents correspondence
+between old and new indices. Cross-kind and null/container changes replace the
+affected path.
+
+These mechanisms support decoding, resolution auditing, metadata,
+and rendering. They are not a public generic conversion or cloning API.
+
+## Safe failures
+
+Source failures use stable `source.*` codes and YAML failures use stable `yaml.*`
+codes declared by the root package. Diagnostics contain safe classifications,
+semantic paths when applicable, and available source coordinates. Raw parser,
+conversion, reader-error, and input text is not forwarded. Source labels and
+diagnostic messages are application-visible and therefore must never contain
+secrets; terminal control characters are escaped by `WriteDiagnostics`.

docs/consumers/README.md

@@ -0,0 +1,104 @@
+# Consumer Adoption Guide
+
+ConfigKit removes parser setup, strict decoding, presence bookkeeping, import and
+profile traversal, precedence, provenance, secret lookup, and safe inspection from
+applications. Consumers still own their configuration structs, defaults, semantic
+validation, schema-version policy, CLI parsing, runtime matching and conversion,
+resource checks, remote acquisition, and service initialization.
+
+The maintained [`examples/consumers`](../../examples/consumers) package demonstrates
+four sanitized shapes using only ConfigKit and the Go standard library:
+
+| Shape | Demonstrated fit | Application code deliberately retained |
+| --- | --- | --- |
+| Narratio-like | Fresh defaults, explicit partial imports, artifact maps, recorded inherited defaults, normalization, and source ownership | Artifact meaning and the separate `model_profile` domain field |
+| Notarius-like | Canonical object bindings, a string-backed schema version accepting string or integer YAML, strict dynamic options, and `int64` dynamic integers | Catalog selection, allowed option keys, range-checked conversion to runtime `int` |
+| WeatherReporter-like | Defaults-only loading, report maps, canonical module objects, typed option unions, explicit CLI records, and typed secret startup | Report/module compatibility, rejection of explicitly empty domain lists, CLI parsing, and startup ordering |
+| Seriatim-like | Independent strict ordered speaker and autocorrect rule files | First-match and ordered replacement algorithms; no central application configuration file |
+
+The YAML under [`testdata`](../../examples/consumers/testdata) is loaded by the
+executable examples and ordinary tests. It contains no private application data,
+credentials, network calls, or dependency on a sibling checkout.
+
+The Notarius-shaped fixtures deliberately load both `version: 4` and
+`version: "4"` into the same string-backed application field. Its version checker
+still owns the supported identifier; fractional numeric input and unsupported
+integer identifiers remain negative examples.
+
+## Happy path
+
+An application defines ordinary types and passes one explicit source. Optional
+features appear only when the application needs them:
+
+```go
+result, err := configkit.Load[Config](configkit.File(path), configkit.Options[Config]{
+    Definition: rules,
+    Imports:    useDeclaredImports,
+    Profile:    selectedConfigProfile,
+    Overrides:  explicitlySetFlags,
+})
+if err != nil {
+    return err
+}
+return start(result.Config)
+```
+
+Simple consumers use neither imports nor profiles. A complex consumer can opt into
+both without redefining their syntax. ConfigKit profiles are configuration overlays;
+they do not select or replace an application's LLM, model, pipeline, report, or
+execution profile concepts.
+
+Known module catalogs should use canonical object entries with a module identifier
+and typed options. A small union struct is sufficient when the available option
+fields are known; validation selects which fields apply to each module. Runtime-
+selected catalogs may retain `map[string]any`, whose integers are `int64`; applications
+validate keys and ranges and convert only at their runtime boundary. Neither shape
+needs YAML reserialization, a parser hook, a ConfigKit registry, or a generic subtree
+conversion API.
+
+## Intentional migration changes
+
+The examples preserve domain meaning while adopting one configuration convention:
+
+- Explicit empty strings, lists, zeroes, false values, and nulls remain intentional.
+  ConfigKit accepts an empty list structurally; WeatherReporter-shaped validation
+  rejects an explicitly or programmatically supplied empty module/template list as a
+  domain rule. Loaded and constructed values use the same validator.
+- Canonical module and object bindings replace string shorthand. Unknown fields and
+  incompatible option shapes fail at their semantic paths.
+- Schema versions remain string-backed in application types. A configured
+  `VersionRule` accepts string or strict decimal-integer YAML at its exact path and
+  supplies the spelling to both the checker and a plain string destination.
+- Integers inside dynamic options are portable `int64` values. Runtime `int`
+  conversion is explicit and range checked.
+- Strict YAML intentionally rejects duplicate keys, aliases, merge keys, unsupported
+  tags/scalars, trailing documents, and unknown typed fields. Diagnostics expose
+  stable classifications and safe paths, not copied parser text.
+- General environment-variable configuration overlays retire. Application CLI
+  parsers translate only explicitly set configuration flags into `Override` records.
+  Separate invocation state remains outside configuration.
+- Credential fields migrate to environment-name `Secret` references. Applications
+  explicitly call `LoadSecretsDir` before `Load` when they choose file bootstrap;
+  blocked empty variables remain blocked, and ordinary `Load` never changes the
+  process environment.
+
+These examples are migration evidence, not completed changes to Narratio, Notarius,
+WeatherReporter, or Seriatim. Updating those repositories, their deployed YAML, CLI
+wiring, runtime catalogs, and operational documentation remains separate downstream
+work.
+
+## Maintained API examples
+
+Every nontrivial initial API sketch has a compiled example or a
+focused behavioral owner:
+
+| Roadmap surface | Maintained owner |
+| --- | --- |
+| `Load`, `NoSource`, validation, versions, resolution, `SecretEnv`, overrides, and explanation | Executable examples in [`example_test.go`](../../example_test.go) |
+| Source/import/profile pipeline rules and artifact defaults | Narratio-shaped executable example and fixture |
+| Canonical module objects and dynamic option handling | WeatherReporter- and Notarius-shaped examples and migration tests |
+| `LoadSecretsDir`, bootstrap reporting, typed lookup, CLI explanation | `Example_weatherStartup` |
+| Results, metadata, diagnostics, effective output, and writer failures | Focused root-package load, provenance, rendering, and explanation tests |
+
+The package-level import-boundary test covers the consumer examples too: supported
+usage names no YAML dependency, parser type, parser option, or parser error chain.

docs/development.md

@@ -1,54 +1,114 @@
 # Development
 
-This is the contributor entry point for thsi application. Use the task-specific
-reading guide below before making changes. Canonical architecture, contracts,
-component behavior, and policies remain in their owning documents.
+Start here when contributing to ConfigKit. Read the task-specific owners below
+before making changes. The [component overview](internal/overview.md) maps
+implementation responsibilities; the [configuration contract](config.md) owns
+public behavior.
 
-## Initial Orientation
+## Initial orientation
 
-Before starting work:
+1. Inspect the working tree and preserve unrelated changes.
+2. Read the architecture policy for code or design work.
+3. Read the policy, contract, and internal documents applicable to the task.
+4. Inspect the relevant implementation and behavioral tests before changing them.
 
-1. inspect the working tree and preserve unrelated changes;
-2. read the architecture policy for code or design work;
-3. read the policy, contract, and internal documents listed for the task;
-4. inspect the relevant implementation and tests before deciding how to change
-   them.
+## Task-specific reading guide
 
-Start with:
-
-- [Architecture policy](policy/architecture.md) for system boundaries,
-  invariants, and non-goals;
-- [Internal component overview](internal/overview.md) for the current package
-  and component map;
-- [Documentation policy](policy/documentation.md) before changing
-  documentation;
-- [Testing policy](policy/testing.md) before adding, rewriting, or deleting
-  tests.
-
-## Task-Specific Reading Guide
-
-| Task | Read before changing |
+| Task | Read |
 | --- | --- |
-| Repository orientation or component responsibility | [Internal component overview](internal/overview.md) and [architecture policy](policy/architecture.md) |
-| Examples or copyable assets | The owning contract for the demonstrated behavior and the related files under `examples/` |
-| Architecture decisions or future work | The [documentation policy](policy/documentation.md) and relevant accepted ADRs |
+| Repository orientation or component responsibility | [Architecture policy](policy/architecture.md) and [component overview](internal/overview.md) |
+| Documentation changes | [Documentation policy](policy/documentation.md) and the owning document |
+| Architecture decisions or future work | [Documentation policy](policy/documentation.md), [deferred scope](roadmap/deferred.md), and relevant ADRs listed below |
+| Code and test changes | [Architecture policy](policy/architecture.md), [testing policy](policy/testing.md), and the owning contract and tests from the map below |
+| Examples or copyable assets | The owning implemented contract and [consumer adoption guide](consumers/README.md) |
+| Release preparation | [Release policy](policy/release.md) and [release procedure](release.md) |
 
-For cross-cutting changes, follow every applicable row. Internal component
-documents own detailed subsystem change recipes.
+Accepted decisions:
 
-## Baseline Validation
+- [Separate composition and overlay](adr/0001-separate-composition-and-overlay.md)
+- [Own YAML semantics behind a private parser boundary](adr/0002-own-yaml-semantics.md)
+- [Preserve presence and record application resolution](adr/0003-preserve-presence-and-resolution-history.md)
+- [Secret references and explicit bootstrap](adr/0004-secret-references-and-explicit-bootstrap.md)
+- [Explicit overrides and safe explanation](adr/0005-explicit-overrides-and-safe-explanation.md)
 
-Use focused checks while iterating, then run validation proportionate to the
-change and the risks described by the testing policy.
+For cross-cutting changes, follow every applicable row. Completed implementation
+and remediation plans are preserved in Git history at `cb5b435`; use current
+contracts and the component overview for ongoing work. For example:
 
-The repository-level baseline for code changes is:
-
-```bash
-go test ./...
-go vet ./...
-go build ./cmd/scriptorium
+```sh
+git show cb5b435:docs/roadmap/remediation.md
 ```
 
-Documentation-only work does not require the full Go suite unless it changes
-commands, examples, generated output, or another behavior that the suite
-validates. Always check changed links, paths, examples, and canonical ownership.
+Release-candidate, publication, and verification work follows the operational
+[release procedure](release.md).
+
+The completed string-and-integer schema-version implementation and remediation
+records are preserved in Git history at `244104f`. The
+[configuration contract](config.md#application-schema-versions) owns current behavior.
+
+## Baseline validation
+
+Install Go 1.25.0 or a supported patched/newer Go toolchain. Acquire declared
+dependencies explicitly before entering an offline development or validation
+environment:
+
+```sh
+go mod download
+```
+
+Run the everyday check with Bash 3.2 or newer and Git:
+
+```sh
+./scripts/check.sh
+```
+
+[The script](../scripts/check.sh) runs from the repository root regardless of the
+invocation directory. It checks Go formatting without rewriting files, checks
+staged/unstaged and untracked whitespace, then runs `go test ./...`,
+`go vet ./...`, and `go build ./...`, stopping on the first failure. It uses the
+installed Go toolchain by default, honors an explicit `GOTOOLCHAIN`, and disables
+dependency downloads. Acquire dependencies and any selected toolchain beforehand.
+Format reported Go files with `gofmt -w <files>`.
+
+The default suite includes executable examples, maintained consumer fixtures,
+local documentation-link checks, and the dependency import-boundary check.
+It must stay deterministic and runnable without network access or credentials.
+Still inspect new/untracked content and review paths, examples, ownership, and
+current-versus-planned claims; automated checks do not replace content review.
+
+Use focused checks while iterating, with validation proportionate to the change
+under the testing policy. Documentation-only work normally needs the local-link
+check below and whitespace review; behavior-bearing examples and commands also
+need execution. Use `go test -race ./...` for ownership or concurrency changes.
+The separate [release checker](release.md#4-validate-the-source-candidate) adds
+two-toolchain, module-hygiene, race, and cross-build gates.
+
+## Task-to-test map
+
+Commands run from the repository root after dependency acquisition. These are
+starting points for focused iteration; run the everyday check before handing off
+code changes. Use the component overview to find the corresponding implementation.
+
+| Task | Behavioral test owners | Focused command |
+| --- | --- | --- |
+| YAML syntax, diagnostics, parser limits | [Adapter tests](../internal/yaml/parse_test.go), [source translation](../parse_test.go) | `go test ./internal/yaml` and `go test . -run '^TestParseSource'` |
+| File/reader access, imports, globs, containment | [Sources](../source_test.go), [FIFO](../source_fifo_test.go), [imports](../imports_test.go), [globs](../glob_test.go) | `go test . -run 'Source\|Import\|Glob\|Filesystem\|Nonregular\|RegularFile'` |
+| Composition, profiles, CLI precedence | [Composition](../compose_test.go), [overlay](../overlay_test.go), [profiles](../profile_test.go), [overrides](../override_test.go) | `go test . -run 'Compose\|Overlay\|Profile\|Override'` |
+| Schema, defaults, projection, cloning | [Schema](../schema_test.go), [decoding](../decode_test.go), [projection](../projection_test.go) | `go test . -run 'Schema\|Decode\|Projection\|Clone\|Inline\|CompareDocument'` |
+| Lifecycle, resolution, validation, callback ownership | [Loading](../load_test.go), [resolution](../resolution_test.go), [validation](../validation_test.go), [ownership](../callback_ownership_test.go), [independence](../independence_test.go) | `go test . -run 'Load\|Resolution\|Validation\|Validator\|Hook\|Callback\|Parallel'` |
+| Paths, metadata, provenance | [Paths](../path_test.go), [metadata](../metadata_test.go), [provenance](../provenance_test.go) | `go test . -run 'Path\|Metadata\|Provenance\|Queries'` |
+| Secrets, bootstrap, redaction, explanation | [References](../secret_reference_test.go), [bootstrap](../secrets_test.go), [rendering](../render_test.go), [explanation](../explain_test.go) | `go test . -run 'Secret\|Bootstrap\|Effective\|Explain\|Explanation\|History\|Sensitivity'` |
+| Consumer examples and migration fixtures | [Consumer tests](../examples/consumers/consumers_test.go), [API examples](../example_test.go) | `go test ./examples/consumers` and `go test . -run '^Example'` |
+| Documentation links and dependency isolation | [Link checks](../documentation_test.go), [import boundary](../import_boundary_test.go) | `go test . -run '^(TestLocalDocumentationLinks\|TestDependencyImportBoundary)$'` |
+| Check-command behavior | [Shell regression](../release_check_test.go) | `go test . -run '^TestCheckScripts'` |
+
+## Fuzz validation
+
+Run the bounded fuzz targets independently from the repository root. These exact
+commands keep each campaign short and constrain concurrent workers:
+
+```sh
+go test . -run='^$' -fuzz='^FuzzParsePath$' -fuzztime=30s -parallel=2
+go test ./internal/yaml -run='^$' -fuzz='^FuzzParseDocument$' -fuzztime=30s -parallel=2
+go test . -run='^$' -fuzz='^FuzzCompositionAndOverlay$' -fuzztime=30s -parallel=2
+```

docs/internal/extraction.md

@@ -0,0 +1,414 @@
+# Source Extraction Inventory
+
+This inventory records reusable implementation evidence from sibling applications.
+Those checkouts are review inputs only: ConfigKit does not import them, and its
+build and tests must not require them. ConfigKit's current contracts and roadmap
+govern every adaptation.
+
+## Inspected revisions
+
+| Application | Revision | Worktree | Role |
+| --- | --- | --- | --- |
+| Narratio | `b68435128be1e24b94fc7ec259d6e9093669fd99` | Clean `main` tracking `origin/main` | Primary parsing, composition, import, profile, presence, defaults, resolution, and inspection source |
+| WeatherReporter | `5d306a202471e10fe98923a1df27d0b0fce5c666` | Clean `main` tracking `origin/main` | Secrets bootstrap and simple-consumer/defaults/options evidence |
+| Notarius | `77ae132e5609916da40dcd4fcc749c000a63cbac` | Clean `main` tracking `origin/main` | Canonical binding, string-version, and dynamic-option migration evidence |
+| Seriatim | `0fc92f364316c874d366b00cdc884a33e1900b25` | Clean `main` tracking `origin/main` | Independent ordered speaker/autocorrect rule evidence |
+
+These revisions were inspected on 2026-09-06. Paths below are relative to each
+application repository. Record later source revisions and adaptations here when
+the owning implementation work extracts a symbol or case.
+
+## Adaptation rules
+
+- Copy and generalize only the smallest applicable mechanism or behavioral case.
+- Replace upstream YAML nodes, application types, commands, logging, digests, and
+  provider registries with ConfigKit-owned boundaries.
+- Preserve strictness, deterministic behavior, and useful failure coverage while
+  changing expectations that conflict with ConfigKit's accepted contracts.
+- Use sanitized local fixtures only. Audio, transcripts, credentials, private
+  configuration, and whole application package trees are application-only.
+- Record the exact copied/generalized symbols and tests in the owning section when
+  extraction occurs. Assignment prose records the original work boundary; dated
+  implementation records state what was actually generalized.
+
+## Narratio evidence
+
+### Source parsing and strict input
+
+Assigned to the YAML/source implementation:
+
+- Production: `internal/config/load.go` functions `decodeStrictYAML`,
+  `decodeStrictYAMLFromReader`, and `LoadSessionBytesWithOptions`; and
+  `internal/config/composition.go` functions `parseCompositionDocument`,
+  `parseCompositionBytes`, and `buildCompositionNode`.
+- Cases: `TestLoadersRejectTrailingYAMLDocuments`,
+  `TestStrictYAMLRejectsParserExposedEmptyTrailingDocument`, and
+  `TestStrictYAMLAcceptsSingleDocumentAndRejectsUnknownFields` in
+  `internal/config/loader_strict_test.go`; plus duplicate-key, alias, non-mapping,
+  malformed-input, scalar spelling, presence, declaration-order, and source-label
+  coverage in `TestParseCompositionDocumentRetainsPresenceOwnershipAndDeclarationOrder`
+  and `TestParseCompositionDocumentRejectsAmbiguousOrMalformedYAML`.
+- Preserve: labeled reader/byte input, strict unknown-field intent, rejection of
+  trailing documents (including an empty second document), duplicate detection,
+  and source-aware failures.
+- Adapt: translate v4 parser nodes into ConfigKit's private document tree, apply
+  ConfigKit's scalar grammar and limits, and replace raw parser errors with safe
+  diagnostics. Narratio's application decoders and YAML node types are not copied.
+
+Implementation record (2026-09-06): the labeled byte/reader ownership and strict
+single-document cases above were generalized into `source.go`, `parse.go`, and
+`internal/yaml`. No production line was copied verbatim. ConfigKit adds byte and
+tree ceilings, shared atomic reader claims, regular-file checks, exact numeric
+lexemes, parser-depth calibration, ConfigKit-owned locations, safe diagnostic
+codes, and an AST import-boundary regression. Application typed decoding, parser
+errors, YAML node types, and session-specific loaders were not retained.
+
+### Strict composition
+
+Assigned to document composition:
+
+- Production: `mergeAdditiveComposition`, `mergeAdditiveCompositions`,
+  `validateAdditiveClaims`, `appendCompositionClaims`, `mergeAdditiveNodes`,
+  `newCompositionConflict`, and cloning/path/source helpers in
+  `internal/config/composition.go`.
+- Cases: `TestMergeAdditiveCompositionJoinsOnlyDisjointMappings`,
+  `TestMergeAdditiveCompositionRejectsEveryDuplicateClass`, and
+  `TestMergeAdditiveCompositionReportsAllClaimingSources`.
+- Preserve: recursive joining of disjoint mappings, collisions for every overlapping
+  non-mapping value (including equal values and sequences), all claimant sources,
+  deterministic traversal, and nonmutation of inputs.
+- Adapt: empty mappings follow ConfigKit's no-child operation rule; paths use the
+  ConfigKit semantic path model; both precise locations and composite origins are
+  retained. Default values never participate in strict composition.
+
+Implementation record (2026-09-06): `composeDocuments`, its pre-order claim
+collection, conflict construction, and recursive mapping join in `compose.go` adapt
+the assigned production symbols. `TestComposeDocumentsJoinsDisjointMappingsAndTracksClaims`,
+`TestComposeDocumentsRejectsEveryPresentNonMappingOverlap`, and
+`TestComposeDocumentsReportsAllClaimantsAtExactLocations` adapt the assigned cases.
+The intentional empty-mapping difference is covered separately: empty mappings retain
+container provenance but contribute no child claims. No other semantic deviation
+from the extracted additive composer was required.
+
+### Imports
+
+Assigned to explicit import traversal and glob handling:
+
+- Production: `loadPipelineCompositionSources`,
+  `splitPipelineCompositionEnvelope`, `loadPipelineImports`, and related ownership
+  structures in `internal/config/pipeline_composition.go`.
+- Cases: `TestSplitPipelineCompositionEnvelopePreservesInputAndDeclarationOrder`,
+  `TestSplitPipelineCompositionEnvelopeRejectsMalformedShapesWithoutMutation`,
+  `TestLoadPipelineCompositionImportsDisjointFieldsAndTracksOwnership`,
+  `TestLoadPipelineCompositionMergesDisjointKeyedEntries`,
+  `TestLoadPipelineCompositionRejectsBaseConflictsWithAllSources`,
+  `TestLoadPipelineCompositionRejectsUnsafeOrInvalidImports`,
+  `TestLoadPipelineCompositionRejectsSameFileAliasesAndImportedComposition`,
+  `TestLoadPipelineCompositionReportsImportedParseAndSchemaSources`, and
+  `TestLoadPipelineCompositionKeepsRelativePathsRootBased`.
+- Preserve: explicit declarations, declaration-order traversal, disjoint
+  composition, conflict attribution, malformed/unreadable target failures, directive
+  removal, and imported parse/schema source labels.
+- Adapt: replace the root-only `composition` envelope with recursive
+  `_configkit.imports`; add path and glob declarations, bounded depth-first graph
+  traversal, canonical containment, cycle/diamond routes, and fixed source IDs.
+  Canonical paths—not `os.SameFile`—define identity, so hard links remain distinct.
+
+Path-import implementation record (2026-09-06): `splitImportDirectives`,
+`parseImportDeclaration` (now in `directives.go`), `importFilesystem` (now in
+`filesystem.go`), and `importTraversal.visitPath`/`loadImportedDocuments` in
+`imports.go` adapt the assigned envelope, loading, source,
+and failure mechanics. The corresponding ConfigKit tests adapt the assigned
+declaration-order, disjoint composition, keyed-entry, all-source conflict, unsafe
+target, alias, imported parse/schema, and relative-resolution cases. ConfigKit adds
+recursive bounded DFS, `os.OpenRoot` confinement, canonical symlink identity,
+cycle/diamond route locations, optional-path semantics, aggregate budgets, and
+fragment version-placement enforcement. That checkpoint parsed glob syntax behind an
+explicit unsupported gate that the subsequent glob implementation removed; hard
+links intentionally replace Narratio's same-file rejection with path identity.
+
+Glob-import implementation record (2026-09-06): ConfigKit retains the extracted
+declaration order, import failures, composition, and identity cases while adding the
+new `expandImportGlob`, `readGlobMatches`, and pattern-validation mechanisms in
+`glob.go`. Focused cases cover lexical ordering, multiple declarations, overlap,
+directories, hard links, contained and escaping symlinks, unmatched declarations,
+malformed and recursive patterns, escaped metacharacters, character classes,
+permission failures, explicit `conf.d` use, and entry/candidate budgets. This work
+has no Narratio production equivalent; it uses the selected standard-library
+`path.Match` and bounded `File.ReadDir` adaptation rather than its flat loader.
+
+### Profiles and overlay
+
+Assigned to selected-profile overlay:
+
+- Production: `mergeOverlayComposition`, `mergeOverlayNodes`,
+  `selectPipelineProfile`, `loadPipelineProfileOverlays`, and
+  `loadedProfileOverlay`.
+- Cases: `TestLoadPipelineProfilesSelectDefaultAndExplicitOverlay`,
+  `TestLoadPipelineProfileSelectionErrors`,
+  `TestPipelineProfileNamesRejectControlCharacters`,
+  `TestLoadPipelineProfilesValidateEveryDeclaredOverlay`,
+  `TestLoadPipelineProfileOverlaySemantics`,
+  `TestLoadPipelineProfileRejectsNullAndKindChanges`, and
+  `TestLoadPipelineProfilePairResolvesIndependentEffectivePipelines`.
+- Preserve: recursive mapping overlay, atomic scalar/sequence replacement,
+  deterministic selection failures, source ownership, and independent results.
+- Adapt: load only the selected fixed profile path; lowercase `default` means no
+  overlay. ConfigKit permits null and cross-kind replacement and does not support
+  declared defaults, inheritance, arbitrary profile paths, or validating every
+  unselected overlay.
+
+Implementation record (2026-09-06): `overlayDocuments`, `mergeOverlayMappings`,
+`loadProfileDocument`, and `profileDirectiveDiagnostics` adapt the assigned merge,
+selection, loading, and ownership mechanics. `TestOverlayDocumentsRecursesMappingsAndReplacesAtomicSubtrees`,
+`TestLoadAppliesSelectedProfileAfterBaseAndBeforeDefaults`, profile selection/error,
+directive/version, containment, and budget cases adapt the assigned tests. ConfigKit
+uses exactly one fixed selected `.yml` source, permits null and cross-kind replacement,
+retains displaced descendant history, rejects any profile directive or version-path
+contribution, and counts the profile in aggregate budgets. Narratio's declared
+default, multi-overlay validation, inheritance-like application conventions, and
+null/kind restrictions were not retained.
+
+### Effective inspection and provenance
+
+Assigned to layered metadata, redaction, rendering, and explanation:
+
+- Production: `EffectivePipelineValues`, `EffectivePipelineSources`,
+  `MarshalEffectivePipeline`, `effectivePipelineFieldPaths`,
+  `effectivePipelineOwners`, and composition `semanticRecords`/
+  `compactSemanticRecords`.
+- Cases: `TestCompositionCanonicalOutputsAreDeterministic`,
+  `TestEffectivePipelineValuesIgnoreEquivalentSourceLayout`,
+  `TestConfigValidateAndShowPipelineOnlyAreSideEffectFree`,
+  `TestConfigSourcesReportsStableOwnership`,
+  `TestConfigSourcesReportsCanonicalFamilyAndPublishOrigins`,
+  `TestConfigSourcesPreservesProfileOwnershipThroughFamilyExpansion`, and
+  `TestConfigSourcesReportsRootImportProfileAndDefaultOwnership`.
+- Preserve: deterministic effective projection, stable logical paths, source
+  ownership, side-effect-free inspection, and domain-attribution evidence.
+- Adapt: project directly from ConfigKit-owned typed values without YAML round trips;
+  add line/column locations, composite origins, complete operation history, detached
+  records, and redaction. Narratio commands, diff/digest APIs, generated artifact
+  attribution, campaign logic, and party expansion remain application-owned.
+
+Layered-provenance implementation record (2026-09-06): ConfigKit integrated its
+previously extracted source claims, overlay displacement records, typed baseline,
+and audited resolution snapshots in `load_metadata.go`. The representative
+`TestLayeredProvenanceAgreesWithResolvedSnapshot` adapts Narratio's stable ownership,
+profile ownership, deterministic effective projection, and side-effect-free
+inspection cases across one default/import/profile/resolution workflow. ConfigKit
+adds chronological descendant history for parent and whole-sequence operations,
+composite empty-mapping origins, canonical resolution reasons/dependencies, private
+snapshot agreement, defensive nested copies, and concurrent load isolation. Value
+That checkpoint deliberately left rendering, redaction, explanation, commands,
+generated attribution, diffs, and digests outside its scope.
+
+Effective-rendering implementation record (2026-09-06): ConfigKit adapts Narratio's
+deterministic effective-output and side-effect-free inspection cases in
+`classifySecretPaths`, `redactedDocument`, `Result.WriteEffective`, and the private
+`internal/yaml.Write` adapter. Tests cover stable sorted mappings, preserved sequence
+order, explicit indentation/line/document formatting, tagged containers, dynamic
+current and historical paths, fixed-point dependency cycles, nil/empty distinctions,
+resolved values, text-scalar snapshot reuse, application YAML-hook isolation,
+non-disclosure through failures and record formatting, caller mutation, and writer
+failures. Unlike Narratio's marshal/reparse path, ConfigKit renders only its owned
+frozen tree and substitutes redaction before the upstream dumper boundary. Historical
+value explanation and secret references were implemented at their later owning
+boundaries and are exercised by the consumer startup example below.
+
+### Defaults, presence, and domain resolution
+
+Assigned across schema, decoding, resolution, validation, and consumer examples:
+
+- Production: `NormalizeConfig.UnmarshalYAML`, `outputPathWasSet`,
+  `applyPipelineDefaults` and its component default helpers, `Resolve`,
+  `ResolveLoadedPipelineCampaign`, `mergeCampaignSession`, `selectStableInput`, and
+  application-supplied byte loading in `internal/app/config_loader.go`.
+- Cases: `TestNormalizeLoadAndValidate`,
+  `TestRepresentativeLoadDefaultsAndValidation`,
+  `TestMarshalEffectivePipelineRendersStablePublicConfiguration`,
+  `TestPipelineCampaignRegistryStrictDecode`,
+  `TestCampaignSessionMergeFillsStableInputs`,
+  `TestCampaignSessionMergeSessionOverridesStableInputs`,
+  `TestLoadWithSessionOptionsCarriesExplicitProfilePresence`, and the maintained
+  extraction-example coverage in `TestMaintainedExtractionExamplesPreservePublishedContracts`.
+- Preserve: fresh defaults, explicit-presence-sensitive domain decisions, strict
+  nested types, stable input selection, application-supplied labeled bytes, and
+  validation after resolution.
+- Adapt: presence comes from metadata rather than custom YAML hooks or pointer/zero
+  guesses; explicit empty and null values are retained; defaults are cloned and
+  application-owned; every domain mutation is recorded. Campaign/session fallback,
+  search paths, fetching, storage, and concrete default values remain in Narratio.
+
+Implementation record (2026-09-06): ConfigKit generalized Narratio's fresh-default,
+strict nested-type, and explicit-presence cases into `decode.go` and
+`decode_test.go`. The decoder applies the owned document tree over a detached typed
+baseline, treats mappings recursively and sequences atomically, preserves explicit
+false/zero/empty/null, rejects unknown nested fields, and returns private presence
+and default/input origin artifacts. Unlike Narratio's custom YAML hooks and
+pointer/zero inference, presence comes directly from the parsed tree and equal-valued
+input remains explicit. ConfigKit also adds destination-width numeric checks,
+dynamic `int64`/`float64`, exact arrays, duration strings, paired text codecs, safe
+multi-error collection, and repeated-result isolation. Resolution, validation,
+consumer defaults, and all campaign/session algorithms remain assigned to later
+work.
+
+Lifecycle integration record (2026-09-06): the public single-source `Load` boundary
+combines the already extracted labeled-source and strict-decoding mechanisms with
+Narratio's fresh-default and presence expectations. ConfigKit adds an application-
+defined pre-decode string-version check, defaults-only `NoSource` operation,
+detached base/default metadata and source reports, and explicit gates for every
+later lifecycle option. No application loader or production line was copied.
+Narratio's resolution, validation, import/profile, campaign/session, and command
+behavior remains assigned to its later owning work.
+
+Resolution implementation record (2026-09-06): ConfigKit adapted the intent of
+Narratio's `Resolve`, `applyPipelineDefaults`, component default helpers,
+`mergeCampaignSession`, and `selectStableInput` into a generic recorded resolver in
+`resolution.go`. The synthetic timeout/artifact cases preserve explicit-input
+decisions, derived dependencies, normalization, and map copy/writeback without
+copying campaign/session algorithms or concrete defaults. ConfigKit adds structural
+mutation auditing, container coverage, helper/path consistency checks, equal-value
+intent, coalesced repeated writes, private before/after snapshots, deterministic
+record ordering, and safe hook diagnostics. Semantic validation remains assigned to
+the next lifecycle work.
+
+Validation implementation record (2026-09-06): ConfigKit generalized Narratio's
+`TestNormalizeLoadAndValidate`, `TestRepresentativeLoadDefaultsAndValidation`,
+strict registry decoding, and maintained extraction-example expectations into the
+post-resolution `Validation` lifecycle and synthetic consumer examples. ConfigKit
+keeps domain rules application-owned, resolves schema-valid missing/dynamic report
+paths to nearest input locations, collects independent warnings/errors, and audits
+the full typed projection to reject validator mutation. Live resources, registries,
+campaign/session logic, commands, and application runtime initialization were not
+extracted.
+
+### Secrets bootstrap
+
+Assigned to the explicit bootstrap operation:
+
+- Production: `loadSecretsFromConfigWithDependencies`, `validateSecretDirectory`,
+  `validateSecretFile`, `readAPIKeyFile`, and
+  `internal/fileops/read_bounded.go` function `ReadFileBounded`.
+- Cases: `TestLoadSecretsFromConfigLoadsValidFiles`,
+  `TestLoadSecretsFromConfigPreservesExistingEnv`,
+  `TestLoadSecretsFromConfigRelativeDirUsesCWD`,
+  `TestLoadSecretsFromConfigMissingDirFails`,
+  `TestLoadSecretsFromConfigUnreadableValidEntryFails`,
+  `TestLoadSecretsFromConfigRejectsUnsafeModes`,
+  `TestLoadSecretsFromConfigRejectsNonRegularAndOversizedEntries`, and
+  `TestLoadSecretsFromConfigRejectsSymlinkAndAncestorReplacement`.
+- Preserve: bounded reads, valid environment names, existing variables (including
+  empty values) winning, regular-file checks, symlink rejection, and failure cases.
+- Adapt: use ConfigKit's deterministic report/preflight/apply state machine and
+  fixed budgets. Logging, configuration-driven implicit invocation, private-mode
+  conventions, and application API-key plumbing remain in Narratio.
+
+Implementation record (2026-09-06): `LoadSecretsDir`, `readSecretsEntries`,
+`readSecretValue`, and the private environment/file seams in `secrets.go` adapt
+Narratio's bounded-read and preserve-existing mechanisms at revision
+`b68435128be1e24b94fc7ec259d6e9093669fd99`. The corresponding tests retain its
+valid-file, existing-variable (including empty), missing/permission/read,
+nonregular, symlink, and byte-limit cases. ConfigKit validates invalid entries
+rather than skipping them, permits ordinary deployment permission modes, resolves
+the selected directory symlink, performs complete preflight before mutation, and
+emits per-file safe records without application configuration or logging.
+
+## WeatherReporter evidence
+
+### Secrets bootstrap
+
+Assigned to the explicit bootstrap operation:
+
+- Production: `loadSecrets`, `applySecrets`, `restoreEnvironment`, `secretValue`,
+  and `environmentValue` in `internal/config/secrets.go`.
+- Cases: `TestLoadSecretsDisabledLeavesEnvironmentUnchanged`,
+  `TestLoadSecretsOverwritesExistingEnvironment`,
+  `TestLoadSecretsTrimsOneTrailingLineEnding`,
+  `TestLoadSecretsRejectsInvalidDirectoryEntries`,
+  `TestLoadSecretsRejectsMissingDirectory`,
+  `TestLoadFileSecretDirectoryFailureLeavesEnvironmentUnchanged`,
+  `TestLoadFileValidationFailureLeavesSecretEnvironmentUnset`, and
+  `TestApplySecretsRollsBackOnEnvironmentFailure`.
+- Preserve: separate read/preflight and apply mechanisms, one-newline handling,
+  invalid-name/type/read coverage, no writes after preflight failure, and controlled
+  environment-write failure coverage.
+- Adapt: existing variables—including empty ones—are skipped, never overwritten;
+  application failures produce an accurate partial report and no rollback. Skip
+  entries are not read. Bootstrap is explicitly invoked and remains independent of
+  YAML loading and semantic validation.
+
+Implementation record (2026-09-06): the same `secrets.go` state machine adapts
+WeatherReporter's `loadSecrets`/`applySecrets` separation, environment-name check,
+regular-file rejection, and exact one-LF/CRLF removal at revision
+`5d306a202471e10fe98923a1df27d0b0fce5c666`. Tests adapt its newline matrix,
+invalid-entry and preflight-no-write cases, and controlled application failure.
+The overwrite/rollback expectation is intentionally replaced by existing-winner,
+presence-recheck, and partial-progress statuses; no configuration loader invokes
+the bootstrap.
+
+### Defaults, explicit presence, and module options
+
+Assigned to consumer-shaped examples and migration checks:
+
+- Production: `Load`, `mergeFile`, `normalizeReportModules`,
+  `traverseReportModules`, `normalizeModuleOptions`, and `decodeKnownOptions`.
+- Cases: `TestDefaults`, `TestOutputDirectoryLoading`,
+  `TestOutputDirectoryValidationIsConsistentForLoadedAndConstructedConfigs`,
+  `TestLoadMinimalExampleConfig`,
+  `TestReportDistributorPathTemplatesSetTracksExplicitEmptyList`,
+  `TestValidateReportModuleKeysWithoutMutatingOptions`,
+  `TestReportModuleOverridesNormalizesConstructedOptionsWithoutMutatingConfig`,
+  `TestReportModuleOverridesCanonicalizesTypedPointerOptions`,
+  `TestReportDistributorPathOverridesConsistentForLoadedAndConstructedConfig`,
+  `TestReportModuleValidationConsistentForLoadedAndConstructedConfig`,
+  `TestConstructedReportOverridesUseExportedFields`, and
+  `TestConstructedExplicitEmptyReportOverridesAreRejected`.
+- Preserve: defaults-only use, consistent loaded/constructed outcomes, explicit
+  empty-list domain rejection, typed module-specific options, normalization without
+  mutating caller input, and unknown-field rejection.
+- Adapt: ConfigKit permits empty lists structurally and exposes their presence to
+  application validation. Object bindings replace shorthand, explicitly set flags
+  become CLI override records, and general environment overlays retire. Report
+  semantics, registries, rendering, and module option validation remain application
+  responsibilities.
+
+Consumer-example implementation record (2026-09-06): at Narratio revision
+`b68435128be1e24b94fc7ec259d6e9093669fd99`, the maintained consumer cases adapt
+`TestMaintainedExtractionExamplesPreservePublishedContracts`,
+`TestConfigValidateAndShowPipelineOnlyAreSideEffectFree`, and the stable source-
+ownership tests into a sanitized partial-import/artifact fixture. ConfigKit replaces
+application composition envelopes and command output with `_configkit.imports`,
+typed defaults, recorded resolution, metadata, and explanation. Artifact meaning,
+model/pipeline profiles, commands, and generated-domain attribution remain in the
+example application layer; no production function was copied.
+
+At WeatherReporter revision `5d306a202471e10fe98923a1df27d0b0fce5c666`,
+`TestDefaults`, explicit-empty-list cases, loaded/constructed consistency cases, and
+nonmutating module-option normalization were adapted into the WeatherReporter-shaped
+fixtures and tests in [`examples/consumers`](../../examples/consumers). ConfigKit's
+detached defaults replace source mutation, canonical module objects replace string
+shorthand, ordinary validation retains empty-list and option compatibility rules,
+and explicit `Override` records replace external configuration-field overlays. The
+startup example composes the separately implemented bootstrap report, typed secret
+lookup, and explanation APIs without copying WeatherReporter's registries or
+rendering/runtime modules.
+
+Notarius revision `77ae132e5609916da40dcd4fcc749c000a63cbac` supplied
+binding/version/dynamic-option migration evidence only. The local example uses an
+ordinary canonical object, a string `VersionRule`, ConfigKit's `int64` dynamic
+integer convention, and application range checking; it does not copy a catalog or
+environment overlay. Seriatim revision
+`0fc92f364316c874d366b00cdc884a33e1900b25` supplied ordered rule-file evidence.
+The local examples load speaker and autocorrect files independently and keep their
+matching/replacement algorithms outside ConfigKit. Neither source is a build or test
+dependency.
+
+## Application-only material
+
+Do not extract application commands, filesystem discovery, logging, runtime
+initialization, remote clients, LLM/provider setup, audio/transcript fixtures,
+configuration digests, report rendering, campaign/session algorithms, or consumer
+resource validation into ConfigKit. Maintained consumer examples use only small
+synthetic types and fixtures to prove that these responsibilities remain
+straightforward.

docs/internal/overview.md

@@ -1,13 +1,89 @@
 # Internal Component Overview
 
-## Purpose
+ConfigKit exposes the root `configkit` package and keeps its document and YAML
+representations private. The implemented components and their owners are:
 
-This is the inventory of this application's implemented components for contributors.
-The [architecture policy](../policy/architecture.md) owns normative boundaries
-and invariants; public behavior belongs in the linked contracts.
+- immutable semantic `Path` construction and strict JSON Pointer parsing;
+- detached effective presence/origin and chronological history queries in
+  `metadata.go`, with provenance and diagnostic record types;
+- canonical, control-safe diagnostic rendering and structured `LoadError` access;
+- opaque file, byte, reader, and no-input source construction in `source.go`,
+  with shared regular-file checks and bounded read mechanics in `source_file.go`;
+- a parser-independent ordered tree in `internal/document`, with root-private
+  semantic lookup, indexing, and value traversal in `document_walk.go`;
+- the sole upstream YAML integration in `internal/yaml`, which translates validated
+  input into that owned tree and exposes no parser types outside its boundary;
+  `parse.go` owns shared translation into detached, ordered public diagnostics;
+- a private strict composer in `compose.go` that joins only disjoint document-tree
+  definitions, retains ordered claims and composite mapping origins, reports every
+  conflicting location, and never aliases an input tree;
+- fixed selected-profile loading in `profile.go` and recursive, non-aliasing overlay
+  with displacement records in `overlay.go`; overlay records retain origins,
+  while value explanation uses separate history applications;
+- CLI override validation/preparation in `override.go`, parser-neutral values in
+  `override_value.go`, and synthetic-source application in `override_apply.go`,
+  sharing overlay, source, byte, node, and depth policies;
+- a per-operation private reflection descriptor graph in `schema.go` for supported
+  types, YAML field binding, inlining, secret tags, recursion, and semantic paths;
+- descriptor-driven canonical projection in `projection.go`, detached typed
+  cloning with explicit secret-state modes in `clone.go`, shared reflection and
+  cycle/limit state in `value_operation.go`, and configuration-value comparison
+  in `document_compare.go`; validation separately audits sensitivity and secrets;
+- strict schema-driven decoding over detached defaults in `decode.go`, including
+  mapping/sequence semantics, null and numeric policy, safe codec errors, presence,
+  baseline/final trees, and initial default/input origins;
+- public lifecycle coordination in `load.go`, option selection in `options.go`,
+  input assembly in `load_input.go`, typed phases/publication in `load_result.go`,
+  bounded canonical import DFS in `imports.go`, filesystem context/containment in
+  `filesystem.go`, directive parsing/restrictions in `directives.go`,
+  pattern-directed glob expansion in `glob.go`, schema-version checking and report
+  contexts in `validation.go`, and layered metadata/history assembly in
+  `load_metadata.go`;
+- recorded application defaulting and normalization in `resolution.go`, including
+  explicitness queries, helper verification, structural mutation audit, coalesced
+  intent, private before/after snapshots, and resolution metadata integration;
+- post-resolution schema-aware report collection and validator mutation detection in
+  `validation.go`, completing the public callback lifecycle after composed input;
+- opaque typed environment references and per-load name resolution in
+  `secret_reference.go`, including credential-free projection/cloning and private
+  validator-state auditing;
+- explicit bounded startup environment bootstrap and safe partial reporting in
+  `secrets.go`;
+- fixed-point tag/predicate/dependency sensitivity classification in `secret.go`;
+- canonical effective output in `render.go`, using frozen-snapshot redaction
+  shared with explanation in `snapshot.go`;
+- detached public explanation records and coordination in `explain.go`, immutable
+  input applications/replay and retained-node budgeting in `history.go`, shared
+  safe snapshot projection in `snapshot.go`, and deterministic human output in
+  `explain_render.go`; the only upstream dumper integration remains isolated in
+  `internal/yaml/write.go`;
+- a separate maintained `examples/consumers` package with four sanitized consumer
+  shapes, fixtures, domain validators, and runtime-boundary functions; and
+- an everyday single-toolchain check in `scripts/check.sh` and a stricter
+  source-candidate checker in `scripts/release-check.sh`, with local
+  documentation-link and public import-boundary gates owned by root-package tests.
 
-TODO: Add tables below, using the following format:
+Dependency direction remains inward: the public lifecycle coordinates private
+source, document, schema, and inspection helpers. Semantic paths stay in the root
+package; `internal/document` owns only representation and cloning and never imports
+the root package. Only `internal/yaml` imports the upstream YAML package. Maintained
+consumer examples use the public package and standard library.
 
-| Component | Implemented responsibility | References |
-| --- | --- | --- |
-| | | |
+The [configuration contract](../config.md) owns implemented input, schema, decoding,
+loading lifecycle, path imports, composition, profile/CLI overlay, version checking,
+recorded resolution, typed secret references/bootstrap, semantic validation,
+metadata, secret classification, redacted effective rendering, historical
+explanation, and ownership behavior.
+The [consumer adoption guide](../consumers/README.md) owns current application-fit
+and migration guidance.
+
+The source-extraction inventory records the application foundations used by the
+implementation:
+
+- [Extraction inventory](extraction.md)
+
+The [architecture policy](../policy/architecture.md) owns binding boundaries and
+invariants. The [deferred scope](../roadmap/deferred.md) records deliberate
+exclusions. See the
+[development guide](../development.md) for contributor orientation and validation
+and the [release procedure](../release.md) for source-candidate gates.

docs/policy/architecture.md

@@ -2,29 +2,141 @@
 
 **Base Policy Version:** 1.0
 
-**Last Updated**: 2026-09-06
+**Last Updated:** 2026-09-06
 
-## Base Policy Template
+## Purpose and applicability
 
-This document is the base template for the canonical architecture policy,. Repositories that implement this policy should:
+ConfigKit is an opinionated Go library for consistent YAML configuration loading
+and resolution across consuming applications. This policy governs contributions.
+The [component overview](../internal/overview.md) owns implementation inventory, the
+[configuration contract](../config.md) owns implemented behavior, and the
+[deferred scope](../roadmap/deferred.md) preserves deferred scope.
 
-1. Revise this document to include an architectural policy tailored to the implementing repository, including a description of the repository's purpose, overall intended system shape, and any architectural and/or engineering invariants that must be maintained during future development;
-2. Ensure that any supporting documents referenced in the architecture policy are present on the filesystem, even if the initial content is simply `TODO`;
-3. Update the `Last Updated` field to the current date; and
-4. Remove this heading (`Base Policy Template`) and the associated text. 
+## Success criteria
 
-As compared to other base policy documents, the Architecture Policy is the document that should be most heavily customized and tailored to the intended architecture of a particular application.
+1. **Reduce application infrastructure:** Can applications delete substantial
+   configuration infrastructure while retaining clear ownership of their domain
+   rules?
+2. **Unify configuration conventions:** Do consumers adopt one consistent
+   configuration model, with only the flexibility needed to express genuine
+   application requirements?
 
-## Purpose
+Both criteria must hold. ConfigKit should not preserve divergent consumer
+conventions merely for compatibility, nor impose restrictions that prevent useful
+domain information from being expressed. Prefer a canonical representation using
+existing primitives before adding another mechanism. Extend the model when a
+concrete requirement cannot be accommodated cleanly, not simply because a
+consumer already implements a different convention.
 
-This policy defines the purpose of this repository and the architectural principles that contributors
-must follow during development.
+Use both criteria to evaluate public API changes and consumer adoption. Shared
+loading, presence tracking, source attribution, and diagnostic plumbing should
+move out of applications; application types, defaults, normalization, and semantic
+validation should remain recognizable application code. Replacing that plumbing
+with equally repetitive callback bookkeeping does not meet the first criterion.
+Offering alternate parsing, precedence, or composition conventions through escape
+hatches does not meet the second.
 
-TODO: Complete this document.
+Consumer migrations are expected and may intentionally change configuration
+syntax. Existing consumer code supplies implementation foundations and evidence
+of needs; it does not define ConfigKit's compatibility obligations. Once ConfigKit
+publishes its own contracts, changes to those contracts remain governed by the
+[release policy](release.md).
 
-Non-limiting examples of topics that may be addressed by an architectural policy include:
-- Adherence to a hexagonal architecture, including whether external integrations/APIs must be encapsulated within internal adapters (strongly recommended)
-- Whether a library is intended to provide a highly flexible API that supports a variety of downstream consumers, or a strongly opinionated API that is intended to enforce consistency across downstream consumers
-- Whether an application should support a full DAG, or alternatively a strongly ordered sequence of discrete stages
-- Whether the application should use specific external libraries to provide key functionality (e.g., FeedKit, ConfigKit, PromptKit)
-- Any explicit architectural non-goals
+Require representative simple and complex consumer examples to substantiate API
+designs. Proposed extensions must identify the domain requirement, explain why
+existing primitives cannot accommodate it cleanly, and show how the extension
+preserves a coherent model. Application-specific interpretation and validation
+remain appropriate. Reuse Narratio's applicable infrastructure under these
+boundaries, and keep simple consumers independent of optional composition
+features. The [consumer adoption guide](../consumers/README.md) demonstrates the
+implemented surface against representative simple and complex shapes.
+
+## Responsibility boundaries
+
+ConfigKit owns configuration input processing and the coordination of decoding,
+resolution, validation, and inspection. Applications own their configuration
+types, field meanings, defaults, normalization, semantic validation, and schema
+version policy. Applications also own main-file search conventions, CLI wiring,
+remote acquisition, runtime initialization, and resource checks.
+
+Library code must not depend on consuming applications or initialize their
+runtime resources. It must not perform implicit network access, discover
+application-specific paths, or change process-global state during ordinary `Load`.
+The explicit secrets-directory bootstrap is a narrow, opt-in exception: applications
+invoke it during startup, and it reports writes, skips and partial failures. It
+must never run implicitly as part of configuration loading.
+Remote data can enter through application-supplied input without introducing a
+network provider into the library.
+
+## Public API and dependency direction
+
+Keep one small, coherent public surface for the supported lifecycle. Simple
+single-document consumers must not need import, profile, or provider machinery.
+Do not expose parser-specific nodes, errors, or extension hooks through the
+public API. Parser integration belongs behind a private boundary, as recorded
+in [ADR 0002](../adr/0002-own-yaml-semantics.md).
+
+Separate input access, document processing, composition, decoding, metadata, and
+rendering responsibilities without prescribing an interface or package for each.
+Dependencies must preserve those boundaries; package splits and abstractions
+must earn their complexity. Do not impose a full application or hexagonal
+framework on this library.
+
+## Configuration integrity
+
+Composition and intentional overlay are distinct operations. Composition must
+not silently become last-wins override through file ordering. The decision and
+alternatives are recorded in [ADR 0001](../adr/0001-separate-composition-and-overlay.md).
+
+ConfigKit must define supported YAML semantics explicitly rather than inherit
+parser defaults. Processing order and diagnostics must be deterministic for the
+same inputs and options. Invalid input must not produce a successful usable
+configuration. Do not run application hooks on invalid typed configuration.
+
+User-controlled precedence is application defaults, composed base, selected
+profile, then explicitly supplied CLI overrides. Applications own flag parsing;
+ConfigKit owns overlay semantics and provenance. Domain resolution follows these
+inputs and must remain recorded, not become a hidden override channel.
+
+Preserve node-based composition until the effective input is ready for typed
+decoding. Preserve presence and source information through processing. Ordinary Go zero
+values must not be used to infer whether input was absent. Final configuration,
+metadata, and effective inspection must agree, including application resolution
+changes; see [ADR 0003](../adr/0003-preserve-presence-and-resolution-history.md).
+Safe historical values and an explanation renderer are part of the implemented contract,
+including superseded/deleted paths and effective source attribution. Inspection
+must agree with the final typed snapshot and apply historical sensitivity rules.
+See [ADR 0005](../adr/0005-explicit-overrides-and-safe-explanation.md).
+Detailed syntax and operation tables belong to the
+[current configuration contract](../config.md).
+
+## Ownership and confidentiality
+
+Returned configuration and metadata must not share mutable state across loads.
+ConfigKit must not mutate a result after returning it. Ordinary exported Go
+values are not language-enforced immutable objects; document caller ownership
+and the lifetime of captured metadata without promising otherwise.
+
+Secret values enter configuration through typed environment-name references;
+literal credentials are not a supported configuration convention. Only an explicit
+accessor exposes a resolved credential to application code. Credential bytes must
+never enter composition trees, history, or inspection. General environment-variable
+configuration overlays are an explicit non-goal. Bootstrap must preserve already
+set variables, including empty ones, and report blocked files without contents.
+See [ADR 0004](../adr/0004-secret-references-and-explicit-bootstrap.md).
+
+Secrets must be considered throughout error handling and inspection. Library
+renderers must apply secret classification, and parser errors must not be
+forwarded blindly when they can include raw input. Do not log configuration
+implicitly. Applications remain responsible for classifying their secrets and
+for safe messages supplied by their hooks.
+
+## Scope boundaries
+
+Do not introduce automatic directory discovery, network providers, arbitrary
+merge strategies, additional configuration languages, or application runtime
+management without concrete consumer evidence and an explicit design decision.
+The [deferred scope](../roadmap/deferred.md#explicit-non-goals-and-deferred-scope)
+owns the deferred-feature inventory. Significant boundary
+changes require an ADR and corresponding policy/contract updates; implementation
+convenience alone is not a reason to broaden the public API.

docs/policy/documentation.md

@@ -2,170 +2,124 @@
 
 **Base Policy Version:** 1.0
 
-**Last Updated**: 2026-09-06
-
-## Base Policy Template
-
-This document is the base template for the canonical documentation policy. Repositories that implement this policy should:
-
-1. Review the list of documents under the `Canonical Ownership` heading, remove any entries that do not currently apply and are not expected to apply to the current project, and add any additional entries that may be required by the current project;
-2. Ensure that each referenced document is present on the filesystem, even if the initial content is simply `TODO`;
-3. Update the `Last Updated` field to the current date; and
-4. Remove this heading (`Base Policy Template`) and the associated text.
+**Last Updated:** 2026-09-06
 
 ## Purpose
 
-This policy assigns each documentation topic to one canonical owner. Its goal is
-to keep this application's documentation accurate, concise, discoverable, and resistant
-to drift for users, operators, developers, integrators, and LLM coding agents.
+Assign each documentation topic one canonical owner so ConfigKit's documentation
+remains accurate and useful to consumers, maintainers, and implementation agents.
 
-## Core Rules
+## Core rules
 
-### One Canonical Owner
+### One canonical owner
 
-Each authoritative fact belongs in one document. A non-owning document may give
-a short, stable summary for orientation, but it must link to the canonical owner
-instead of repeating volatile details.
+Each authoritative fact belongs in one owning document or Go declaration's
+reference documentation. Other documents may summarize and link to that owner;
+they must not maintain competing definitions of commands, API contracts, YAML
+semantics, defaults, paths, diagnostics, or compatibility guarantees.
 
-Volatile details include commands, flags, configuration fields and defaults,
-module keys, schemas, file names, paths, status codes, retry behavior, and
-runtime guarantees. If readers could reasonably treat a statement as a
-contract, maintain it only in the owning document.
+### Policies, current behavior, and future work
 
-### Current And Future Behavior
+- Policies state currently binding contributor rules, including constraints on
+  future implementation. They do not imply that features already exist.
+- Current API/configuration contracts, consumer guidance, and component
+  documentation describe implemented behavior only, including partial boundaries.
+- ADRs record accepted decisions and rationale. Acceptance is not evidence of
+  implementation; execution status belongs in roadmaps.
+- Roadmaps own unimplemented targets, deferred work, and execution status.
 
-Outside `docs/roadmap/`, documentation describes implemented behavior only.
-Partial features may be described only to their implemented boundary.
+Future implementation plans own their task order, acceptance gates, and progress;
+`docs/roadmap/deferred.md` owns deliberate exclusions, not execution instructions.
+As behavior lands, move its normative definition into the current contract and
+replace the roadmap definition with a link. Retire completed roadmap material
+as part of release preparation; ADRs preserve significant decision history.
 
-ADRs are the narrow exception: an ADR may record an accepted architectural
-decision before implementation, but acceptance must not be presented as proof
-that the behavior exists. The roadmap owns implementation status and sequencing
-until the decision is implemented. Current architecture, user, operator,
-integration, and internal documentation are updated when the behavior lands.
+### Audience and examples
 
-### Audience And Detail
+Consumer documentation explains how to use the library and what consumers own.
+Internal documents explain mechanisms and dependencies. Keep implementation
+mechanics out of consumer guidance unless they affect a meaningful choice.
 
-Write for the document's stated audience and include only the detail needed for
-its owned topic. User and operator docs should not expose implementation detail.
-Developer docs should link to user-facing and external contracts rather than
-restate them.
+Complete copyable artifacts belong in `examples/`; Go API examples may live in
+executable Go example tests beside the relevant package. Prose should use the
+smallest useful excerpt and link to the maintained artifact. Examples must be
+secret-free and checked against implemented behavior. Clearly label illustrative
+future syntax in roadmaps; do not present it as a runnable current example.
 
-### Examples
+### Security and privacy
 
-Complete copyable files belong in `examples/`. Documentation may use the
-smallest illustrative snippet needed to explain its owned topic, but should link
-to maintained examples instead of embedding a second complete copy.
+Do not include real credentials, private keys, environment dumps, sensitive input,
+or private infrastructure details unless intentionally public. Document secret
+handling, not secret values.
 
-Examples must be valid, secret-free, and tested where practical. Commands and
-configuration used in documentation should match the application.
+## Canonical ownership
 
-### Security And Privacy
+| Topic | Owner | Boundary |
+| --- | --- | --- |
+| Product orientation | `README.md` | Current status, purpose, navigation, and a minimal working example |
+| Contributor entry point | `docs/development.md` | Reading guide and actual baseline validation commands |
+| Architecture rules | `docs/policy/architecture.md` | Responsibilities, dependency constraints, invariants, and scope boundaries |
+| Documentation rules | `docs/policy/documentation.md` | Ownership and document lifecycle |
+| Testing rules | `docs/policy/testing.md` | Risk-based sufficiency, test boundaries, and maintenance principles |
+| Release rules | `docs/policy/release.md` | Compatibility, integrity, gates, immutability, and exceptions |
+| Release procedure | `docs/release.md` | Exact maintainer commands, prerequisites, supported targets, and verification |
+| Exported Go API | Go documentation beside declarations | Symbol contracts, errors, ownership, and lifecycle API behavior |
+| YAML and resolution semantics | `docs/config.md` | Syntax, composition, precedence, conversion, secret references/bootstrap, and explanation semantics; Go reference docs link here for shared rules |
+| Consumer walkthroughs | `docs/consumers/`, when useful | Task-oriented usage and consumer responsibilities, not duplicate API reference |
+| Component inventory | `docs/internal/overview.md` | Implemented components and links to their owners |
+| Component mechanics | Other `docs/internal/` files, when needed | Detailed implementation behavior and change recipes |
+| Architectural decisions | `docs/adr/` | Context, decisions, alternatives, consequences, and supersession history |
+| Deferred scope | `docs/roadmap/deferred.md` | Deliberate exclusions and the conditions for reconsidering them |
+| Future implementation plans | Task-specific files under `docs/roadmap/` | Unfinished work, task order, acceptance gates, and progress; completed plans retire to Git history |
+| Historical release notes | `docs/releases/` | Version-specific historical summaries required by release policy |
+| Complete examples | `examples/` and executable Go examples | Maintained artifacts; prose links instead of duplicating full examples |
 
-Documentation and examples must not contain real credentials, private keys,
-private environment dumps, sensitive source material, or private infrastructure
-details unless intentionally public. Document secret-handling mechanisms, not
-secret values.
+A listed owner is not a requirement to create an empty file before it has useful
+content. Create API, consumer, example, and component documents when the corresponding behavior
+exists and there is useful content to own. Link only to existing files; use plain
+code paths for planned files. Unvalidated candidate and publication work has the
+explicit temporary exception recorded in the release policy.
 
-## Canonical Ownership
+ConfigKit has no owned HTTP service, application CLI, or runtime operations
+interface. Do not create those reference documents for consuming applications.
 
-| Topic | Canonical owner | Owned content | Content owned elsewhere |
-| --- | --- | --- | --- |
-| Product orientation and minimal end-to-end quickstart | `README.md` | What this application is, why it is useful, one shortest successful invocation, and links onward. | Complete command reference, configuration reference, operational procedures, implementation detail. |
-| Contributor entry point | `docs/development.md` | Task-oriented reading guide, minimal contributor orientation, baseline validation commands, and links to canonical docs. | Package inventory, architecture rules, subsystem behavior, detailed change recipes. |
-| Application architecture policy | `docs/policy/architecture.md` | System shape, normative ownership, dependency direction, architectural boundaries, invariants, safety properties, and non-goals. | Concrete package inventory, implementation mechanics, contributor procedures, decision history, future work. |
-| Documentation policy | `docs/policy/documentation.md` | Documentation ownership, audience boundaries, maintenance rules, and ADR/document lifecycle. | Application architecture or product behavior. |
-| Testing policy | `docs/policy/testing.md` | Test philosophy, risk-based sufficiency, test boundaries, doubles, coverage guidance, regression-test policy, and criteria for adding, rewriting, or deleting tests. | Subsystem behavior, application contracts, subsystem-specific test inventories, and implementation plans. |
-| Release policy | `docs/policy/release.md` | Versioning and compatibility rules, release integrity and immutability requirements, release-documentation policy, required gates, publication controls, verification requirements, and exception policy. | Repository-specific commands, product installation guidance, CLI version semantics, historical release summaries, and CI implementation detail. |
-| Repository-specific release procedure | `docs/release.md` | Maintainer prerequisites, candidate preparation, concrete validation and publication commands, repository and registry guards, deliverable publication, consumer-facing verification, and failure recovery. | General release requirements, product installation guidance, historical release summaries, and current compatibility contracts. |
-| CLI contract (if applicable) | `docs/cli.md` | Commands, arguments, flags, invocation semantics, and exit codes. | End-to-end operating procedures, configuration field definitions, runtime filesystem layout, module implementation details. |
-| Configuration contract (if applicable) | `docs/config.md` | Discovery and precedence, file schema, fields, defaults, environment overrides, validation rules, and user-selectable module or validator keys. | Complete example files, CLI syntax, runtime state lifecycle, module implementation details. |
-| Operations (if applicable) | `docs/operations.md` | Runtime workflows, physical filesystem and state layout, output, cache, and debug handling, resume, cleanup, permissions, recovery, and operational limits. | CLI flag syntax, configuration field definitions, logical output schemas, implementation mechanics. |
-| Public HTTP contract (if applicable) | `docs/api.md` | Routes, authentication, media types, request and response schemas, status codes, pagination, caching, idempotency, rate limits, and HTTP retry semantics. | Client walkthroughs, upstream or downstream integration internals, implementation detail. |
-| Consumer guidance for the public Go API (if applicable) | `docs/consumers/` | Task-oriented use of the public interface, minimal client examples, and consumer responsibilities. | HTTP wire semantics, external protocol contracts, internal implementation detail. |
-| Integration contracts for external components (if applicable) | `docs/integrations/` | External file formats and protocols, upstream and downstream contracts, logical output bundle paths and schemas, media types, and compatibility behavior. | Physical runtime placement and lifecycle, internal transformations, CLI syntax, configuration defaults. |
-| Inventory of implemented internal packages | `docs/internal/overview.md` | Current packages and components, their implemented responsibilities, and links to focused internal docs. | Normative architecture, contributor reading policy, external contracts. |
-| Behavior and architecture of internal packages | Other files under `docs/internal/` | Implementation flow, internal collaborators and state transitions, package-local guarantees and failures, and relevant tests. | Global architecture invariants, configuration definitions and defaults, external schemas, operator procedures. |
-| Architectural decision records | `docs/adr/` | Significant decisions, context, alternatives, rationale, consequences, and supersession history. | Current behavior reference, implementation status, task sequencing. |
-| Future work and implementation status | `docs/roadmap/` | Proposed, accepted, deferred, or rejected work; implementation status; sequencing; and task breakdowns. | Implemented behavior reference and architectural decision rationale. |
-| Release-note history | `docs/releases/` | One checked-in historical summary for each source release made under the procedure. The note at the immutable tag is that release's record. | Current commands, behavior, contracts, and compatibility definitions. |
-| Complete copyable artifacts | `examples/` | Maintained configuration, inputs, and other files intended to be copied or run. | Field-by-field reference, command reference, prose explanation. |
+## Boundary rules
 
-Documents that do not exist are required only when the corresponding interface
-or responsibility exists. Do not create placeholder API, consumer, integration,
-or operations documents for behavior the application does not have.
+Architecture owns governing constraints; ADRs explain why; contracts describe
+implemented semantics; internal documentation explains how. A roadmap may link
+to all of them but must not become a second current reference.
 
-## Boundary Rules
+Release policy owns guarantees; the release procedure owns commands. A release
+note cannot redefine a compatibility contract or replace a current-documentation
+update. The note at an immutable tag is that release's historical record.
 
-### Orientation
+This policy owns what documentation may disclose. Architecture owns security
+invariants. Current configuration and Go API contracts own secret-classification
+and rendering behavior when implemented; internal docs own their mechanisms.
 
-The README owns product orientation. The developer guide routes contributors.
-Architecture owns normative structure. Internal overview owns the current
-concrete component map. These documents may link to one another but should not
-maintain parallel package or behavior descriptions.
+## Architectural decision records
 
-### Commands, Configuration, And Operations
+Use sequential filenames, such as `0001-separate-composition-and-overlay.md`,
+and the lightweight Nygard structure: title, status, date, context, decision,
+alternatives considered, and consequences.
 
-CLI documentation answers how to invoke the application. Configuration
-documentation answers what settings mean. Operations answers what happens to
-runtime state and how to operate or recover the application. When a workflow
-crosses these topics, choose the document that owns the task and link to the
-other contracts.
+Accepted decision content is immutable. A changed decision requires a new ADR
+and a superseded status/link in the old one. Navigation links may be repaired
+when specification ownership moves without rewriting historical rationale.
+Rejected architectural alternatives belong in ADRs; deferred or rejected product
+features belong in roadmaps. Do not create an ADR for every routine choice.
 
-### Releases
+## Maintenance and verification
 
-The release policy defines requirements and invariants that every release must
-satisfy. The repository-specific release procedure defines the commands and
-workflow maintainers use to satisfy that policy. Release notes are immutable
-historical summaries, not current-state contract owners: the checked-in note at
-an immutable tag records that release, while current canonical documentation
-must change with the behavior it describes. Do not use a release procedure or
-release note to weaken policy, redefine current behavior, or defer required
-documentation updates.
-
-### Contracts And Implementation
-
-Integration and API documents define externally observable shapes and
-semantics. Internal documents explain how this application implements or consumes those
-contracts. Internal docs may name a field, file, or protocol to identify a
-dependency, but must link to its canonical contract for the definition.
-
-### Security Topics
-
-This policy owns what documentation and examples may contain. Architecture owns
-application security invariants. Configuration owns credential-supply
-mechanisms. Operations owns permissions and handling of sensitive runtime
-artifacts. Internal docs own implementation mechanisms only.
-
-## Architecture Decision Records
-
-Use sequentially numbered ADR filenames such as
-`0001-record-architecture-decisions.md`. Follow the lightweight Nygard format:
-
-1. title;
-2. status;
-3. date;
-4. context;
-5. decision;
-6. alternatives considered;
-7. consequences.
-
-Treat the decision content of an accepted ADR as immutable. When a decision
-changes, create a new ADR and update the earlier ADR's status to superseded.
-Rejected architectural alternatives belong in the ADR; rejected product ideas
-belong in the roadmap.
-
-## Maintenance
-
-When behavior changes, update its canonical owner in the same change. If
-ownership moves, remove the old definition and replace it with a link where
-navigation remains useful.
+Update the owning current documentation in the same change that implements or
+changes behavior. When ownership moves, remove the old definition and preserve
+useful navigation links.
 
 Before completing documentation work:
 
-- verify affected behavior and examples;
-- check commands, flags, fields, defaults, schemas, and paths against their
-  implementation;
-- keep unimplemented behavior in the roadmap, subject to the ADR exception;
-- remove stale references and validate links;
-- confirm that non-owning documents summarize and link rather than redefine;
-- confirm that no secrets or sensitive private data were added.
+- verify claims against the implementation or clearly identify future design;
+- check changed local links, paths, fragments, examples, and commands;
+- preserve the distinction between policy, decisions, target behavior, and status;
+- check canonical ownership and remove stale or duplicate definitions;
+- inspect formatting and whitespace;
+- ensure no sensitive data was introduced.

docs/policy/release.md

@@ -2,30 +2,7 @@
 
 **Base Policy Version:** 0.1.0
 
-**Last Updated**: 2026-09-06
-
-## Base Policy Template
-
-This document is the base template for the canonical release policy.
-Repositories that implement this policy should:
-
-1. Retain the substantive requirements of this policy unless a documented
-   repository constraint requires an exception;
-2. Define the repository-specific release procedure, validation commands,
-   supported deliverables, and verification commands in `docs/release.md`;
-3. Ensure that every document and script required by the adopted policy and
-   procedure is present on the filesystem;
-4. Add stricter repository-specific requirements where the product, delivery
-   channels, or threat model require them;
-5. Record every exception under `Policy Exceptions`, including its rationale
-   and compensating controls;
-6. Update the `Last Updated` field to the current date; and
-7. Remove this heading (`Base Policy Template`) and the associated text.
-
-The base policy is a standard, not a collection of optional recommendations.
-An implementing repository should customize its release mechanics without
-silently weakening the common integrity, validation, documentation,
-verification, or immutability guarantees below.
+**Last Updated:** 2026-09-07
 
 ## Purpose
 
@@ -38,6 +15,26 @@ The repository-specific commands that implement these requirements belong in
 `docs/release.md`. Historical release summaries belong in `docs/releases/`.
 Neither document may weaken this policy.
 
+## ConfigKit applicability
+
+ConfigKit adopts a source-only Go module release model. The source tag and
+version-matched release note are the declared deliverables. Standalone binaries,
+container images, and separate binary/package-registry publication are not
+required. This does not waive source compilation, module hygiene, testing, or
+consumer-facing module retrieval and build verification.
+
+The release procedure must define the module path, authoritative remote,
+supported Go versions and operating-system/architecture targets, prerequisite
+dependency acquisition, and the exact verification commands once established.
+The default tests themselves remain deterministic and offline. Additional
+publication channels require documented deliverables and verification before use.
+
+The Go API, documented YAML/resolution semantics, and structured diagnostic
+contracts all participate in compatibility review. A signature-preserving change
+can still break consumers. Human diagnostic wording is stable only where a
+contract expressly promises it. Minimum Go version and platform support changes
+also require an explicit compatibility assessment.
+
 ## Release Model
 
 A release consists of:
@@ -138,7 +135,7 @@ The required gates are:
 - validation of maintained examples, configuration, schemas, generated
   sources, and documentation commands where applicable;
 - review of changed local documentation links;
-- removal of obsolute roadmap documents under `docs/roadmap/` that describe
+- removal of obsolete roadmap documents under `docs/roadmap/` that describe
   completed work included in the release;
 - review for unintended files, generated output, credentials, private data,
   and unrelated changes; and
@@ -220,3 +217,21 @@ name the primary branch, authoritative remote, tag form, published
 deliverables, supported build targets, release automation responsibilities,
 and consumer verification path. It must link to this policy instead of
 restating or redefining its requirements.
+
+## Policy Exceptions
+
+For v0.1.1 only, the maintainer approved renaming the reserved `_configkit` key to
+`configkit` as an error-correcting point release because there are no current
+consumers. This intentionally changes the v0.1.0 configuration contract without a
+minor-version increment. The release note must explain the renamed directive,
+the lack of a legacy alias, and the newly reserved application key. This exception
+does not apply to subsequent releases or waive any validation or publication gate.
+
+For v0.2.0 only, the maintainer accepted the successful native macOS check at
+`244104f` for the final candidate, provided subsequent changes are documentation
+only. The final candidate still requires the complete Linux release checker.
+This exception waives only repeating the macOS check after those documentation
+changes; it does not waive other publication or verification gates.
+
+Candidate selection, publication, and post-publication verification remain
+mandatory steps in the [release procedure](../release.md).

docs/policy/testing.md

@@ -2,17 +2,7 @@
 
 **Base Policy Version:** 1.0
 
-**Last Updated**: 2026-09-06
-
-## Base Policy Template
-
-This document is the base template for the canonical testing policy,. Repositories that implement this policy should:
-
-1. Review the policy, remove any rules that must not apply to the current project, and add any additional rules that are required by the current project;
-2. Update the `Last Updated` field to the current date; and
-3. Remove this heading (`Base Policy Template`) and the associated text. 
-
-Most projects will not require any revisions to the substantive testing policy rules as written below.
+**Last Updated:** 2026-09-06
 
 ## Purpose
 
@@ -20,6 +10,38 @@ Our tests exist to make **incorrect changes expensive and correct changes cheap*
 
 We do not optimize for test count, line coverage, exhaustive isolation, or the fewest possible tests. We optimize for sufficient confidence in important behavior while imposing as little unnecessary friction as possible on future development.
 
+## ConfigKit-specific priorities
+
+Configuration semantics are compatibility contracts, even when Go signatures do
+not change. Apply the risk rules below especially to presence/default behavior,
+composition, provenance, and error classification. Keep detailed behavior cases
+with their owning contracts and tests rather than duplicating them in this policy.
+
+Use real temporary files for import traversal, symlinks, and source identity.
+Document platform prerequisites for filesystem cases and make any unsupported
+case explicit rather than silently dropping its coverage. Use labeled in-memory
+input where filesystem behavior is irrelevant.
+
+Verify agreement among final resolved values, presence, provenance, and effective
+rendering. Protect against secret disclosure on both successful inspection and
+failure paths, and against mutable defaults or results leaking across loads.
+Use focused fuzzing for parser boundaries and semantic/filesystem path handling;
+keep deterministic seed regressions in the ordinary suite. Extended fuzz runs
+need a documented bounded invocation when introduced.
+
+Secret bootstrap tests must cover existing variables including empty values,
+preflight without writes, partial apply failures, and safe blocked-file reporting.
+Use controlled environment names and restore state; process-mutating tests must
+not run in parallel. Keep real filesystem coverage and use narrow private seams
+for deterministic environment/write failures. Verify full input precedence and
+safe historical values, including removed and replaced secrets, at their owning
+boundaries. No credential bytes may appear in inspection or failure fixtures'
+expected output; use disposable canaries to prove non-disclosure.
+
+Baseline commands belong in the [development guide](../development.md). Release
+gates belong in the [release policy](release.md) and [maintainer runbook](../release.md). This
+policy does not imply that those tools or tests already exist.
+
 ## Every test has a cost
 
 Testing is not an unqualified good. Every test imposes both an immediate cost and a continuing lifetime cost.
@@ -59,7 +81,8 @@ Use a **classical/Detroit-style** approach:
 - Prefer package-level behavioral tests over tests coupled to private helpers or internal call sequences.
 - Treat exact collaborator interactions as testable behavior only when the interaction itself is a requirement.
 
-Examples of appropriate seams include clocks, randomness, subprocesses, remote APIs, object storage, email, and paid LLM calls.
+For ConfigKit, controlled input readers and filesystem failures are useful seams.
+Prefer actual parsing, composition, and decoding over mocks of their call order.
 
 ## Test execution requirements
 
@@ -176,13 +199,15 @@ Use a test-controlled limit and measure the behavior relative to that limit. Do
 
 Each behavior should have a clear test owner.
 
-- Parser tests own parsing cases.
-- Validator tests own validation rules.
-- Domain tests own transformations and invariants.
-- Adapter tests own external integration behavior.
-- Orchestrator tests own coordination and failure propagation.
-- CLI tests own argument and configuration mapping.
-- End-to-end tests prove that representative assembled workflows work.
+- YAML boundary tests own syntax and policy enforcement.
+- Composition and overlay tests own their operation semantics.
+- Decoding tests own type binding, defaults, and strictness.
+- Metadata and rendering tests own provenance queries and safe inspection.
+- Filesystem tests own source access and import identity behavior.
+- Lifecycle tests own hook ordering, failure propagation, and representative
+  agreement across the assembled configuration pipeline.
+
+These are responsibility boundaries, not a required package layout.
 
 Higher-level tests should not repeat every lower-level case. A single intentional policy change should not require unrelated edits across many test files.
 
@@ -215,7 +240,6 @@ Use:
 
 - table-driven tests for meaningful behavioral categories and boundaries;
 - `t.TempDir()` for real filesystem behavior;
-- `httptest.Server` for realistic HTTP interactions;
 - fuzz tests for parsers, normalization, path handling, and broad input spaces;
 - golden files only when the complete output is intentionally stable;
 - integration tests where correctness depends on component interaction; and
@@ -223,7 +247,8 @@ Use:
 
 Avoid exact error-string assertions unless the wording is itself contractual. Prefer `errors.Is`, `errors.As`, typed errors, or structured error fields.
 
-At CLI boundaries, prefer exit classifications, structured output, and the smallest stable semantic fragment needed to identify the error. Do not snapshot complete diagnostic wording unless it is contractual.
+At the public library boundary, prefer diagnostic codes, paths, and source
+locations over complete message snapshots. Assert wording only when contractual.
 
 Golden-file updates must require an explicit local flag. CI must not update golden files automatically, and reviewers must inspect the semantic diff before accepting an update.
 
@@ -235,7 +260,7 @@ Coverage is a diagnostic, not a target.
 
 Use it to find untested critical branches and unexpectedly weak packages. Do not write low-value tests solely to increase a percentage, and do not infer test quality from coverage alone.
 
-Pure domain logic will often warrant higher coverage than CLI wiring or external adapters. Uneven coverage is acceptable when it reflects risk.
+Dense configuration semantics will often warrant higher coverage than simple input wiring. Uneven coverage is acceptable when it reflects risk.
 
 Increasing coverage is valuable only when the newly covered behavior protects a meaningful risk at an acceptable cost.
 
@@ -303,7 +328,7 @@ A test suite is sufficient when:
 - legitimate internal changes usually do not require test edits; and
 - additional tests would mostly repeat existing protection or preserve inconsequential implementation details.
 
-Sufficiency is a risk judgment, not a coverage percentage or test count. Reassess it as the application, its users, and the consequences of failure evolve.
+Sufficiency is a risk judgment, not a coverage percentage or test count. Reassess it as the library, its consumers, and the consequences of failure evolve.
 
 The governing rule is:
 

docs/release.md

@@ -0,0 +1,241 @@
+# ConfigKit Release Procedure
+
+This is the maintainer runbook for ConfigKit's [release policy](policy/release.md).
+ConfigKit publishes one source-only Go module: the immutable source tag and the
+matching checked-in release note are the deliverables. There is no application
+binary, container image, generated package, or separate registry publication.
+
+## Release identity and support
+
+- Module: `gitea.maximumdirect.net/eric/ConfigKit`
+- Primary branch: `main`
+- Authoritative remote: `origin`, expected to fetch from
+  `https://gitea.maximumdirect.net/eric/ConfigKit.git`
+- Tags: stable Semantic Versions such as `v0.1.0`; prerelease tags are not supported
+- Minimum Go language/toolchain line: Go 1.25, tested with a supported patched
+  Go 1.25 toolchain and one newer supported toolchain
+- Build targets: Linux and macOS on amd64 and arm64
+
+The automated checker cross-builds all four targets. Before release, the maintainer
+must also obtain native test, race, symlink, and permission-fixture evidence on Linux
+and macOS; cross-compilation is not native execution evidence.
+
+## 1. Prepare access and tools
+
+The release maintainer owns this procedure. Work in a dedicated clone with Git, Bash 3.2 or newer,
+and two supported Go toolchains available. The checker supports the stock macOS
+Bash; its filename enumeration is null-delimited, including names with whitespace. Configure private-host Git and Go module
+access through the normal credential manager before starting. Keep tokens, passwords,
+SSH material, and environment dumps outside the repository and command transcript.
+Verify access without displaying credentials:
+
+```sh
+git remote get-url origin
+git ls-remote origin HEAD
+```
+
+The remote URL must equal the authoritative URL above. Select the latest supported
+patch in the Go 1.25 line and a newer supported toolchain. The values below are the
+toolchains used while this runbook was introduced; update them to supported patch
+releases during candidate preparation and record the actual versions in the release
+note or validation record.
+
+```sh
+export CONFIGKIT_BASELINE_TOOLCHAIN=go1.25.5
+export CONFIGKIT_NEWER_TOOLCHAIN=go1.26.7
+GOTOOLCHAIN="$CONFIGKIT_BASELINE_TOOLCHAIN" go version
+GOTOOLCHAIN="$CONFIGKIT_NEWER_TOOLCHAIN" go version
+GOTOOLCHAIN="$CONFIGKIT_BASELINE_TOOLCHAIN" go mod download
+GOTOOLCHAIN="$CONFIGKIT_NEWER_TOOLCHAIN" go mod download
+```
+
+Those commands explicitly acquire toolchains and dependencies. The checker itself
+sets `GOPROXY=off` and fails if its required cache is incomplete.
+
+## 2. Select the version and review compatibility
+
+Choose an unused stable version and compare the candidate with the previous tag.
+For the first release, review the whole public surface and current contracts.
+
+```sh
+release_version=v0.1.0
+printf '%s\n' "$release_version" | grep -Eq '^v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)$'
+git show-ref --verify --quiet "refs/tags/$release_version" && exit 1 || :
+if git ls-remote --exit-code --tags origin "refs/tags/$release_version" >/dev/null; then
+  printf 'remote tag already exists: %s\n' "$release_version" >&2
+  exit 1
+else
+  test $? -eq 2
+fi
+git log --oneline --decorate --no-merges
+```
+
+Review exported Go signatures and ownership, YAML and scalar behavior, precedence,
+diagnostic codes, minimum Go version, and platform support. Before `v1.0.0`, any
+intentional incompatible minor-release change must be explained with consumer action;
+patch releases must remain compatible within their minor line. Review the maintained
+consumer examples and the migration guide even when signatures did not change.
+
+Reassess `go.yaml.in/yaml/v4` rather than using `@latest` implicitly:
+
+```sh
+go list -m -versions go.yaml.in/yaml/v4
+go list -m -json go.yaml.in/yaml/v4@latest
+```
+
+If changing the explicit pin, make that a reviewed dependency change and rerun the
+complete checker, especially parser policy, limits, diagnostics, and rendering tests.
+If retaining a prerelease pin, record that decision in Compatibility.
+
+## 3. Write the release note
+
+Create `docs/releases/$release_version.md` before selecting the candidate. It must
+begin with `# ConfigKit $release_version` and contain `Summary`, `Compatibility`,
+`Upgrade`, and `Changes` sections. Use “No action required” where appropriate. Link
+to current canonical contracts instead of copying their definitions. Do not create a
+release note until there is a real candidate and reviewed content.
+
+## 4. Validate the source candidate
+
+Run the complete checker from the repository root after dependencies have been
+acquired. It validates module manifests in a disposable copy, both supported Go
+toolchains, offline default tests, the native race detector, vet, native builds,
+CGO-disabled cross-builds, formatting and whitespace (including untracked files),
+examples, package isolation, and local documentation links.
+
+```sh
+./scripts/release-check.sh
+```
+
+`go vet` is the repository's static-analysis baseline. No additional linter is
+claimed. Links to current ConfigKit files and headings are strict. Recorded links to
+sibling consumer source are checked when those checkouts exist and are otherwise
+inapplicable in an isolated clone; their immutable revisions remain recorded in the
+extraction inventory.
+
+Complete these manual gates and record their evidence with the candidate:
+
+- The maintainer reviews compatibility and downstream migration impact against
+  `docs/config.md`, Go API comments, and the consumer guide.
+- The maintainer inspects `git status --short`, `git diff`, and the candidate file
+  list for credentials, private data, generated output, unintended files, and
+  unrelated changes.
+- The maintainer records successful native checker runs on Linux and macOS, including
+  filesystem capability tests. If either host is unavailable, release remains
+  blocked unless platform support is formally narrowed in current documentation.
+- The maintainer removes completed, obsolete documents under `docs/roadmap/`, repairs
+  inbound links, and reruns the checker. ADR rationale remains historical.
+- The maintainer reviews the exact version-matched release note for accuracy and
+  confirms it describes only deliverables that will actually be published.
+
+## 5. Commit, push, and record the candidate
+
+Commit all candidate source, contract, example, roadmap-retirement, and release-note
+changes, rerun the checker against that clean commit, then push only `main` and record
+the exact commit in Git metadata local to the clone:
+
+```sh
+git add .
+git commit --message "Prepare ConfigKit $release_version"
+./scripts/release-check.sh
+test -z "$(git status --porcelain=v1 --untracked-files=all)"
+git push origin HEAD:refs/heads/main
+git rev-parse HEAD | tee .git/configkit-release-candidate
+candidate_commit=$(cat .git/configkit-release-candidate)
+```
+
+If review changes the source after this point, create and validate a new candidate;
+do not reuse the recorded commit.
+
+## 6. Run publication guards
+
+Immediately before tagging, fetch the authoritative branch and tags, restore the
+recorded variables if this is a new shell, and execute every guard:
+
+```sh
+release_version=v0.1.0
+candidate_commit=$(cat .git/configkit-release-candidate)
+git fetch --prune origin main
+git fetch --tags origin
+test "$(git branch --show-current)" = main
+test -z "$(git status --porcelain=v1 --untracked-files=all)"
+test "$(git rev-parse HEAD)" = "$candidate_commit"
+test "$(git rev-parse refs/remotes/origin/main)" = "$candidate_commit"
+test "$(git remote get-url origin)" = 'https://gitea.maximumdirect.net/eric/ConfigKit.git'
+printf '%s\n' "$release_version" | grep -Eq '^v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)$'
+test -f "docs/releases/$release_version.md"
+test "$(sed -n '1p' "docs/releases/$release_version.md")" = "# ConfigKit $release_version"
+for heading in Summary Compatibility Upgrade Changes; do
+  grep -Fxq "## $heading" "docs/releases/$release_version.md"
+done
+git show-ref --verify --quiet "refs/tags/$release_version" && exit 1 || :
+if git ls-remote --exit-code --tags origin "refs/tags/$release_version" >/dev/null; then
+  printf 'remote tag already exists: %s\n' "$release_version" >&2
+  exit 1
+else
+  test $? -eq 2
+fi
+```
+
+Confirm the recorded automated and manual approvals apply to exactly
+`$candidate_commit`. A successful older run is not evidence for a changed commit.
+
+## 7. Tag and publish the exact ref
+
+Create one annotated tag at the guarded commit, verify its peeled target, and push
+only that ref. Do not use `git push --tags`.
+
+```sh
+git tag --annotate "$release_version" --message "ConfigKit $release_version" "$candidate_commit"
+test "$(git rev-list -n 1 "$release_version")" = "$candidate_commit"
+git push origin "refs/tags/$release_version:refs/tags/$release_version"
+```
+
+The pushed tag completes the declared source-only publication. No Gitea release
+object or binary upload is required.
+
+## 8. Verify publication and clean consumer retrieval
+
+Verify the authoritative tag and then retrieve that exact version into a new module
+without a local `replace` directive. Private-host credentials must already be
+configured. Use direct origin retrieval unless an explicitly approved internal Go
+proxy is the publication channel.
+
+```sh
+remote_tag_commit=$(git ls-remote origin "refs/tags/$release_version^{}" | awk '{print $1}')
+test "$remote_tag_commit" = "$candidate_commit"
+consumer_dir=$(mktemp -d)
+(
+  cd "$consumer_dir"
+  go mod init example.invalid/configkit-release-check
+  GOPRIVATE=gitea.maximumdirect.net GOPROXY=direct go get "gitea.maximumdirect.net/eric/ConfigKit@$release_version"
+  cat > main.go <<'EOF'
+package main
+
+import configkit "gitea.maximumdirect.net/eric/ConfigKit"
+
+func main() {
+        _ = configkit.NoSource()
+}
+EOF
+  gofmt -w main.go
+  go test ./...
+  go build ./...
+  go list -m -f '{{.Path}} {{.Version}}' gitea.maximumdirect.net/eric/ConfigKit
+)
+rm -rf -- "$consumer_dir"
+```
+
+The final `go list` output must show the exact module path and `$release_version`.
+Record the result with the release evidence.
+
+## 9. Handle failures without replacement
+
+Before publication, fix the candidate, rerun every gate, commit, push, and record a
+new exact candidate. An unpublished local tag may be deleted only after verifying it
+was never pushed or used by another release channel.
+
+After publication, never move, delete, or recreate the tag and never replace its
+release note. Correct the defect on `main`, choose a new semantically appropriate
+version, and repeat this procedure. A supported advisory or deprecation mechanism
+may warn consumers about the old version but cannot change its contents.

docs/releases/v0.1.0.md

@@ -0,0 +1,72 @@
+# ConfigKit v0.1.0
+
+This is the first source release of ConfigKit, an opinionated Go library for strict,
+explainable YAML configuration loading.
+
+## Summary
+
+ConfigKit provides one typed loading lifecycle for file, byte, reader, and
+defaults-only inputs. It supports bounded explicit imports, strict composition,
+selected profiles, explicit CLI overrides, application-owned defaults and recorded
+resolution, semantic validation, provenance, redacted effective output, and safe
+historical explanation.
+
+Typed environment-name secret references and an explicit secrets-directory bootstrap
+keep credential acquisition separate from ordinary loading. Maintained consumer
+examples demonstrate simple and composed application shapes without exposing the
+private YAML parser dependency.
+
+See the [configuration contract](../config.md) and
+[consumer adoption guide](../consumers/README.md) for current behavior and examples.
+
+## Compatibility
+
+This is the initial public release, so there is no earlier ConfigKit release to
+upgrade from. The module requires Go 1.25 or newer and supports Linux and macOS on
+amd64 and arm64. Candidate validation uses patched Go 1.25 and Go 1.26 toolchains;
+native Linux and macOS evidence is required in addition to cross-builds.
+
+Configuration intentionally uses a strict YAML subset. Duplicate keys, aliases,
+anchors, merge keys, unsupported tags and scalar forms, trailing documents, and
+unknown typed fields are rejected. Composition rejects overlapping non-mapping
+definitions; replacement occurs only through selected profiles, explicit CLI
+overrides, or recorded application resolution.
+
+The private parser dependency remains explicitly pinned to
+`go.yaml.in/yaml/v4 v4.0.0-rc.6`, which was still the newest available v4 release when
+the candidate was prepared. No upstream YAML declarations are part of ConfigKit's
+public API.
+
+## Upgrade
+
+No action is required for an earlier ConfigKit version because this is the first
+release. New consumers can add the module with:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.1.0
+```
+
+The authoritative host may require its normal private-module credentials. Consumers
+adapting existing application configuration should review the intentional migration
+differences in the [consumer adoption guide](../consumers/README.md), particularly
+strict YAML, string schema versions, explicit empty/null behavior, canonical module
+objects, `int64` dynamic integers, explicit CLI records, and typed secret references.
+
+## Changes
+
+- Added opaque file, byte, reader, and defaults-only sources with fixed resource
+  limits and safe structured diagnostics.
+- Added an owned strict YAML model, typed schema binding, detached defaults and
+  results, canonical projection, and private parser isolation.
+- Added explicit path/glob imports, deterministic strict composition, canonical
+  source identity and containment, and selected profile overlays.
+- Added layered presence, origins, chronological history, version checks, recorded
+  defaulting and normalization, and read-only semantic validation.
+- Added secret classification, deterministic redacted effective YAML, typed
+  environment references, and bounded explicit secrets-directory bootstrap reports.
+- Added parser-neutral explicit CLI overrides and safe structured/human historical
+  value explanations, including removed and superseded values.
+- Added maintained Narratio-, Notarius-, WeatherReporter-, and Seriatim-shaped
+  examples and migration coverage using only ConfigKit and the standard library.
+- Added race and bounded fuzz coverage plus a two-toolchain, offline, cross-platform
+  source-release checker and maintainer runbook.

docs/releases/v0.1.1.md

@@ -0,0 +1,43 @@
+# ConfigKit v0.1.1
+
+This release corrects the spelling of ConfigKit's reserved configuration key.
+
+## Summary
+
+The reserved directive key is now `configkit` instead of `_configkit`. Import
+declarations continue to use the same `imports` sequence and path/glob syntax.
+See the [configuration contract](../config.md#explicit-imports).
+
+## Compatibility
+
+This is an intentional configuration compatibility change, approved as a one-time
+[patch-release exception](../policy/release.md#policy-exceptions) because there are
+no current consumers. `_configkit` is no longer a directive or a reserved key; it
+is ordinary application data and is subject to normal schema validation. There is
+no legacy alias. `configkit` is now reserved, including in nested mappings, profiles,
+and override paths and values.
+
+The Go API, diagnostic codes, Go minimum version, and supported platforms are
+unchanged. The private YAML dependency remains pinned to
+`go.yaml.in/yaml/v4 v4.0.0-rc.6`, confirmed as the newest available v4 release
+during candidate preparation.
+
+## Upgrade
+
+Rename `_configkit` directive keys to `configkit` in root configuration files and
+imported fragments. Rename any application-owned `configkit` fields to avoid the
+newly reserved key. Update diagnostic-path expectations to the new spelling.
+Files retaining `_configkit` will not load their declared imports.
+
+Consumers can select this release with:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.1.1
+```
+
+## Changes
+
+- Renamed the directive key consistently across imports, profiles, and overrides.
+- Updated current configuration documentation and the composed consumer fixture.
+- Updated behavioral tests and added coverage proving the old spelling is not an
+  import alias.

docs/releases/v0.1.2.md

@@ -0,0 +1,50 @@
+# ConfigKit v0.1.2
+
+This release adds safe enumeration of effective and removed metadata paths.
+
+## Summary
+
+`Metadata.HistoricalPaths` returns the semantic paths observed during a load,
+including paths removed by profiles, CLI overrides, or recorded resolution. This
+allows applications to discover hidden declarations and inspect their ownership
+through the existing `Metadata.History` API without reloading configuration or
+resolving unselected credentials.
+
+See the [layered metadata contract](../config.md#layered-metadata) for current
+behavior.
+
+## Compatibility
+
+This is a backward-compatible API addition. Existing APIs, configuration syntax,
+diagnostic codes, minimum Go version, and supported platforms are unchanged.
+Existing consumers do not need to change their code or configuration.
+
+Returned paths are detached, unique by canonical JSON Pointer spelling, and sorted
+by that spelling. The enumeration includes defaults and the root, exposes no
+configuration values, and performs no I/O or secret resolution.
+
+The private YAML dependency remains pinned to
+`go.yaml.in/yaml/v4 v4.0.0-rc.6`, confirmed as the newest available v4 release during
+candidate preparation.
+
+## Upgrade
+
+No action required. Consumers that want historical path discovery can update with:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.1.2
+```
+
+Then iterate over `result.Metadata().HistoricalPaths()` and use `History` for
+chronological definitions or `Lookup` to determine whether each path remains
+effective.
+
+## Changes
+
+- Added `Metadata.HistoricalPaths` with detached, deterministic path enumeration.
+- Included effective, default, and removed paths while excluding paths that never
+  existed.
+- Added coverage for parent removal, mapping-to-sequence shape replacement, escaped
+  mapping keys, detached results, zero metadata, composite base ownership, and
+  avoidance of superseded secret lookup.
+- Documented historical path enumeration in the current configuration contract.

docs/releases/v0.2.0.md

@@ -0,0 +1,58 @@
+# ConfigKit v0.2.0
+
+This release adds integer schema-version input and consistent canonical sequence
+path handling.
+
+## Summary
+
+Applications can retain YAML such as `version: 4` while storing and checking their
+version as a Go string. The exception applies only to the path designated by
+`VersionRule`; unrelated fields keep strict decoding. See the
+[configuration contract](../config.md#application-schema-versions).
+
+## Compatibility
+
+This minor release intentionally changes integer schema-version input from a
+`version.type` rejection to invoking the application checker with its decimal text.
+Plain string destinations at that path now accept it. Public Go API shapes,
+minimum Go version, and supported platforms are unchanged.
+
+Parsed sequence indices now require canonical nonnegative decimal spelling.
+Signed tokens such as `+0` and `-0`, previously accepted by some traversal paths,
+no longer identify sequence elements. They remain valid literal mapping keys.
+Version checking, decoding, and metadata matching now agree on this boundary.
+
+The private YAML dependency remains pinned to `go.yaml.in/yaml/v4 v4.0.0-rc.6`,
+confirmed as the newest available v4 release during candidate preparation. Linux
+candidate checks use Go 1.25.5 and Go 1.26.7. The maintainer confirmed native macOS
+checks at `244104f` and approved carrying that result across the documentation-only
+candidate changes under the [recorded exception](../policy/release.md#policy-exceptions).
+
+## Upgrade
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.2.0
+```
+
+Existing string schema versions need no configuration changes. Applications using
+integer YAML versions can retain them and use a plain string field plus a
+`VersionRule`. Review the application checker: it now receives integer identifiers
+as text, and remains responsible for rejecting unsupported versions. Fractions,
+exponents, booleans, and containers remain invalid version kinds.
+
+Replace signed sequence-index tokens in configured paths with canonical indices
+(for example, `/versions/+0` becomes `/versions/0`). Do not change signed mapping
+keys. Literal integers elsewhere still do not decode into strings, and numeric
+destinations retain their existing range checks.
+
+## Changes
+
+- Added load-local, path-scoped integer-to-string schema-version decoding without
+  mutating original input or introducing general scalar coercion.
+- Preserved numeric historical input, string effective output, decoding
+  reconciliation, source attribution, and secret redaction.
+- Unified sequence path interpretation and added regression coverage for signed
+  tokens, literal mapping keys, and explicit key/index distinctions.
+- Updated maintained Notarius-shaped fixtures and migration guidance for quoted
+  and integer version forms.
+- Retired completed implementation and remediation roadmaps to Git history.

docs/roadmap/deferred.md

@@ -0,0 +1,28 @@
+# Deferred Scope
+
+This document owns intentionally excluded features, not a committed delivery plan.
+The [architecture policy](../policy/architecture.md) governs changes to these
+boundaries. Implemented behavior belongs in the [configuration contract](../config.md)
+and consumer adoption in the [consumer guide](../consumers/README.md).
+
+## Explicit non-goals and deferred scope
+
+General environment-variable configuration overlays are not supported. Typed secret
+references and explicit directory bootstrap are the environment convention. Literal
+credential configuration and implicit secret-directory discovery are unsupported.
+
+The initial release deliberately excludes:
+
+- automatic `conf.d` discovery;
+- remote imports, arbitrary providers, and additional configuration formats;
+- custom merge strategies, sequence merging, reset operators, and profile inheritance;
+- parser-specific or custom structural-decoding extension APIs;
+- generic dynamic-subtree conversion and module-registry APIs;
+- automatic schema migrations;
+- hot reload, runtime resource replacement, and remote resource validation;
+- a bundled CLI framework or application commands; and
+- configuration diffs and fingerprints.
+
+Applications may build their own validate, effective-value, source, bootstrap, and
+explanation commands around the library. Broadening these boundaries requires
+concrete consumer evidence, compatibility review, and an explicit design change.

document_compare.go

@@ -0,0 +1,110 @@
+package configkit
+
+import (
+	"cmp"
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	"slices"
+)
+
+type documentDeltaKind uint8
+
+const (
+	documentAdded documentDeltaKind = iota + 1
+	documentRemoved
+	documentReplaced
+)
+
+type documentDelta struct {
+	path Path
+	kind documentDeltaKind
+}
+
+// compareConfigurationValues ignores locations and sensitivity metadata. Mapping
+// order is immaterial; sequences are atomic for change reporting.
+func compareConfigurationValues(before, after *document.Node) []documentDelta {
+	var deltas []documentDelta
+	compareConfigurationValue(before, after, Path{}, &deltas)
+	slices.SortFunc(deltas, func(left, right documentDelta) int {
+		if order := cmp.Compare(left.path.String(), right.path.String()); order != 0 {
+			return order
+		}
+		return int(left.kind) - int(right.kind)
+	})
+	return deltas
+}
+
+func compareConfigurationValue(before, after *document.Node, path Path, deltas *[]documentDelta) {
+	if before == nil && after == nil {
+		return
+	}
+	if before == nil {
+		*deltas = append(*deltas, documentDelta{path: path.clone(), kind: documentAdded})
+		return
+	}
+	if after == nil {
+		*deltas = append(*deltas, documentDelta{path: path.clone(), kind: documentRemoved})
+		return
+	}
+	if before.Kind != after.Kind {
+		*deltas = append(*deltas, documentDelta{path: path.clone(), kind: documentReplaced})
+		return
+	}
+	switch before.Kind {
+	case document.Mapping:
+		beforeEntries := make(map[string]*document.Node, len(before.Mapping))
+		afterEntries := make(map[string]*document.Node, len(after.Mapping))
+		keys := make(map[string]bool, len(before.Mapping)+len(after.Mapping))
+		for _, entry := range before.Mapping {
+			beforeEntries[entry.Key] = entry.Value
+			keys[entry.Key] = true
+		}
+		for _, entry := range after.Mapping {
+			afterEntries[entry.Key] = entry.Value
+			keys[entry.Key] = true
+		}
+		ordered := make([]string, 0, len(keys))
+		for key := range keys {
+			ordered = append(ordered, key)
+		}
+		slices.Sort(ordered)
+		for _, key := range ordered {
+			compareConfigurationValue(beforeEntries[key], afterEntries[key], path.Key(key), deltas)
+		}
+	case document.Sequence:
+		if !configurationValuesEqual(before, after) {
+			*deltas = append(*deltas, documentDelta{path: path.clone(), kind: documentReplaced})
+		}
+	default:
+		if before.Text != after.Text {
+			*deltas = append(*deltas, documentDelta{path: path.clone(), kind: documentReplaced})
+		}
+	}
+}
+
+// configurationValuesEqual compares only configuration values, not provenance or
+// sensitivity. Callers auditing validation must check sensitivity separately.
+func configurationValuesEqual(left, right *document.Node) bool {
+	if left == nil || right == nil {
+		return left == right
+	}
+	if left.Kind != right.Kind || left.Text != right.Text || len(left.Mapping) != len(right.Mapping) || len(left.Sequence) != len(right.Sequence) {
+		return false
+	}
+	if left.Kind == document.Mapping {
+		leftMap := make(map[string]*document.Node, len(left.Mapping))
+		for _, entry := range left.Mapping {
+			leftMap[entry.Key] = entry.Value
+		}
+		for _, entry := range right.Mapping {
+			if !configurationValuesEqual(leftMap[entry.Key], entry.Value) {
+				return false
+			}
+		}
+	}
+	for i := range left.Sequence {
+		if !configurationValuesEqual(left.Sequence[i], right.Sequence[i]) {
+			return false
+		}
+	}
+	return true
+}

document_walk.go

@@ -0,0 +1,86 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// walkDocument visits value nodes in preorder, preserving stored mapping order
+// and sequence index order. Mapping keys are not separate nodes in this walk.
+// The callback borrows paths and nodes; clone any mutable state it retains.
+// Specialized origin, pruning, and budget traversals retain their own semantics.
+func walkDocument(node *document.Node, path Path, visit func(Path, *document.Node)) {
+	if node == nil {
+		return
+	}
+	visit(path, node)
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			walkDocument(entry.Value, path.Key(entry.Key), visit)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			walkDocument(child, path.Index(index), visit)
+		}
+	}
+}
+
+// indexDocument borrows nodes from its caller-owned tree.
+func indexDocument(node *document.Node, path Path, index map[string]*document.Node) {
+	walkDocument(node, path, func(path Path, node *document.Node) {
+		index[path.String()] = node
+	})
+}
+
+func documentAtPath(root *document.Node, path Path) (*document.Node, bool) {
+	if root == nil || !path.valid() {
+		return nil, false
+	}
+	current := root
+	for _, segment := range path.segments {
+		switch current.Kind {
+		case document.Mapping:
+			if segment.kind == pathSegmentIndex {
+				return nil, false
+			}
+			found := false
+			for _, entry := range current.Mapping {
+				if entry.Key == segment.key {
+					current = entry.Value
+					found = true
+					break
+				}
+			}
+			if !found {
+				return nil, false
+			}
+		case document.Sequence:
+			index, ok := schemaIndex(segment)
+			if !ok || index >= len(current.Sequence) {
+				return nil, false
+			}
+			current = current.Sequence[index]
+		default:
+			return nil, false
+		}
+	}
+	return current, true
+}
+
+// countInputNodes includes mapping-key nodes, unlike walkDocument's value-only
+// traversal. Aggregate source budgets charge every input occurrence.
+func countInputNodes(node *document.Node) int {
+	if node == nil {
+		return 0
+	}
+	count := 1
+	if node.Kind == document.Mapping {
+		count += len(node.Mapping)
+		for _, entry := range node.Mapping {
+			count += countInputNodes(entry.Value)
+		}
+	} else if node.Kind == document.Sequence {
+		for _, child := range node.Sequence {
+			count += countInputNodes(child)
+		}
+	}
+	return count
+}

documentation_test.go

@@ -0,0 +1,151 @@
+package configkit
+
+import (
+	"bufio"
+	"fmt"
+	"net/url"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+	"unicode"
+)
+
+var markdownLinkPattern = regexp.MustCompile(`(?m)(?:^|[[:space:]])\[[^\]]*\]\(([^)[:space:]]+)`)
+
+func TestLocalDocumentationLinks(t *testing.T) {
+	repository, err := filepath.Abs(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	var failures []string
+	err = filepath.WalkDir(repository, func(path string, entry os.DirEntry, walkErr error) error {
+		if walkErr != nil {
+			return walkErr
+		}
+		if entry.IsDir() {
+			if entry.Name() == ".git" {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if filepath.Ext(path) != ".md" {
+			return nil
+		}
+		content, readErr := os.ReadFile(path)
+		if readErr != nil {
+			return readErr
+		}
+		for _, match := range markdownLinkPattern.FindAllStringSubmatch(string(content), -1) {
+			target := strings.Trim(match[1], "<>")
+			if strings.Contains(target, "://") || strings.HasPrefix(target, "mailto:") {
+				continue
+			}
+			filePart, fragment, _ := strings.Cut(target, "#")
+			decoded, decodeErr := url.PathUnescape(filePart)
+			if decodeErr != nil {
+				failures = append(failures, fmt.Sprintf("%s: invalid escaped link %q", relativePath(repository, path), target))
+				continue
+			}
+			linked := path
+			if decoded != "" {
+				linked = filepath.Clean(filepath.Join(filepath.Dir(path), filepath.FromSlash(decoded)))
+			}
+			_, statErr := os.Stat(linked)
+			outside := linked != repository && !strings.HasPrefix(linked, repository+string(filepath.Separator))
+			if os.IsNotExist(statErr) && outside && optionalSiblingSource(repository, linked) {
+				continue // Recorded sibling-source links are optional in isolated clones.
+			}
+			if statErr != nil {
+				failures = append(failures, fmt.Sprintf("%s: missing local link %q", relativePath(repository, path), target))
+				continue
+			}
+			if fragment != "" && filepath.Ext(linked) == ".md" && !markdownHasFragment(linked, fragment) {
+				failures = append(failures, fmt.Sprintf("%s: missing fragment in %q", relativePath(repository, path), target))
+			}
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(failures) != 0 {
+		t.Fatalf("documentation link failures:\n%s", strings.Join(failures, "\n"))
+	}
+}
+
+func optionalSiblingSource(repository, linked string) bool {
+	relative, err := filepath.Rel(filepath.Dir(repository), linked)
+	if err != nil || relative == "." || filepath.IsAbs(relative) {
+		return false
+	}
+	parts := strings.Split(filepath.ToSlash(relative), "/")
+	if len(parts) == 0 {
+		return false
+	}
+	switch strings.ToLower(parts[0]) {
+	case "narratio", "notarius", "weatherreporter", "seriatim":
+		return true
+	default:
+		return false
+	}
+}
+
+func markdownHasFragment(path, wanted string) bool {
+	file, err := os.Open(path)
+	if err != nil {
+		return false
+	}
+	defer file.Close()
+	wanted, err = url.PathUnescape(wanted)
+	if err != nil {
+		return false
+	}
+	seen := make(map[string]int)
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if !strings.HasPrefix(line, "#") {
+			continue
+		}
+		heading := strings.TrimSpace(strings.TrimLeft(line, "#"))
+		base := markdownFragment(heading)
+		count := seen[base]
+		seen[base]++
+		fragment := base
+		if count > 0 {
+			fragment = fmt.Sprintf("%s-%d", base, count)
+		}
+		if fragment == wanted {
+			return true
+		}
+	}
+	return false
+}
+
+func markdownFragment(heading string) string {
+	var result strings.Builder
+	previousSpace := false
+	for _, character := range strings.ToLower(heading) {
+		switch {
+		case unicode.IsLetter(character), unicode.IsDigit(character), character == '_', character == '-':
+			result.WriteRune(character)
+			previousSpace = false
+		case unicode.IsSpace(character):
+			if result.Len() > 0 && !previousSpace {
+				result.WriteByte('-')
+				previousSpace = true
+			}
+		}
+	}
+	return strings.TrimRight(result.String(), "-")
+}
+
+func relativePath(root, path string) string {
+	relative, err := filepath.Rel(root, path)
+	if err != nil {
+		return path
+	}
+	return filepath.ToSlash(relative)
+}

example_test.go

@@ -0,0 +1,150 @@
+package configkit_test
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func ExampleLoad() {
+	type Config struct {
+		Enabled bool `yaml:"enabled"`
+		Workers int  `yaml:"workers"`
+	}
+	result, err := configkit.Load[Config](configkit.Bytes("example.yml", []byte("enabled: true\nworkers: 3\n")))
+	fmt.Println(result.Config, err)
+	// Output: {true 3} <nil>
+}
+
+func ExampleNoSource() {
+	type Config struct {
+		Workers int `yaml:"workers"`
+	}
+	result, err := configkit.Load(configkit.NoSource(), configkit.Options[Config]{
+		Definition: configkit.Definition[Config]{Defaults: func() Config { return Config{Workers: 4} }},
+	})
+	fmt.Println(result.Config.Workers, len(result.Sources()), err)
+	// Output: 4 0 <nil>
+}
+
+func ExampleValidation_Error() {
+	type Config struct {
+		First  string `yaml:"first"`
+		Second string `yaml:"second"`
+	}
+	_, err := configkit.Load(configkit.Bytes("duplicates.yml", []byte("first: same\nsecond: same\n")), configkit.Options[Config]{
+		Definition: configkit.Definition[Config]{Validate: func(configuration *Config, validation *configkit.Validation) {
+			if configuration.First == configuration.Second {
+				validation.Error(configkit.Keys("second"), "must differ from first", configkit.Keys("first"))
+			}
+		}},
+	})
+	var loadError *configkit.LoadError
+	errors.As(err, &loadError)
+	diagnostic := loadError.Diagnostics()[0]
+	fmt.Println(diagnostic.Code, diagnostic.Path, len(diagnostic.Related))
+	// Output: validate.error /second 1
+}
+
+func ExampleVersionRule() {
+	type Config struct {
+		Version string `yaml:"version"`
+	}
+	_, err := configkit.Load(configkit.Bytes("version.yml", []byte("version: old\n")), configkit.Options[Config]{
+		Definition: configkit.Definition[Config]{Version: &configkit.VersionRule{
+			Path: configkit.Keys("version"),
+			Check: func(version configkit.Version, validation *configkit.Validation) error {
+				if version.Text != "v1" {
+					validation.Error(configkit.Keys("version"), "unsupported version; migrate using the application guide")
+				}
+				return nil
+			},
+		}},
+	})
+	var loadError *configkit.LoadError
+	errors.As(err, &loadError)
+	fmt.Println(loadError.Diagnostics()[0].Message)
+	// Output: unsupported version; migrate using the application guide
+}
+
+func ExampleResolution() {
+	type Config struct {
+		Input   int `yaml:"input"`
+		Derived int `yaml:"derived"`
+	}
+	var order []string
+	result, err := configkit.Load(configkit.Bytes("resolved.yml", []byte("input: 3\n")), configkit.Options[Config]{
+		Definition: configkit.Definition[Config]{
+			Resolve: func(configuration *Config, resolution *configkit.Resolution) error {
+				order = append(order, "resolve")
+				configkit.Default(resolution, configkit.Keys("derived"), &configuration.Derived,
+					configuration.Input*2, "derived from input", configkit.Keys("input"))
+				return nil
+			},
+			Validate: func(configuration *Config, validation *configkit.Validation) {
+				order = append(order, "validate")
+				if configuration.Derived < configuration.Input {
+					validation.Error(configkit.Keys("derived"), "must not be less than input")
+				}
+			},
+		},
+	})
+	fmt.Println(strings.Join(order, " then "), result.Config.Derived, err)
+	// Output: resolve then validate 6 <nil>
+}
+
+func ExampleSecretEnv() {
+	const name = "CONFIGKIT_EXAMPLE_API_KEY"
+	previous, present := os.LookupEnv(name)
+	_ = os.Setenv(name, "disposable-example-value")
+	defer func() {
+		if present {
+			_ = os.Setenv(name, previous)
+		} else {
+			_ = os.Unsetenv(name)
+		}
+	}()
+
+	type Config struct {
+		APIKey configkit.Secret `yaml:"api_key"`
+	}
+	result, err := configkit.Load[Config](configkit.Bytes("secret.yml", []byte("api_key: "+name+"\n")))
+	fmt.Println(result.Config.APIKey.EnvName(), result.Config.APIKey.IsResolved(), err)
+	// Output: CONFIGKIT_EXAMPLE_API_KEY true <nil>
+}
+
+func ExampleOverride() {
+	type Config struct {
+		Workers int `yaml:"workers"`
+	}
+	workers, workersWasSet := 0, true // Supplied by the application's CLI parser.
+	var overrides []configkit.Override
+	if workersWasSet {
+		overrides = append(overrides, configkit.Override{
+			Path: configkit.Keys("workers"), Value: workers, Flag: "--workers",
+		})
+	}
+	result, err := configkit.Load(configkit.NoSource(), configkit.Options[Config]{
+		Definition: configkit.Definition[Config]{Defaults: func() Config { return Config{Workers: 4} }},
+		Overrides:  overrides,
+	})
+	fmt.Println(result.Config.Workers, result.Sources()[0].Label, err)
+	// Output: 0 --workers <nil>
+}
+
+func ExampleResult_Explain() {
+	type Config struct {
+		Workers int `yaml:"workers"`
+	}
+	result, err := configkit.Load(configkit.Bytes("config.yml", []byte("workers: 8\n")), configkit.Options[Config]{
+		Definition: configkit.Definition[Config]{Defaults: func() Config { return Config{Workers: 4} }},
+	})
+	explanation, ok := result.Explain(configkit.Keys("workers"))
+	fmt.Printf("%s -> %s known=%t error=%v\n",
+		strings.TrimSpace(explanation.History[0].After.YAML),
+		strings.TrimSpace(explanation.Effective.YAML), ok, err)
+	// Output: 4 -> 8 known=true error=<nil>
+}

examples/consumers/consumers.go

@@ -0,0 +1,331 @@
+// Package consumers contains small, maintained application shapes that exercise
+// ConfigKit without making consumer domain policy part of the library API.
+package consumers
+
+import (
+	"math"
+	"strconv"
+	"strings"
+	"time"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// Artifact is a Narratio-shaped generated output declaration.
+type Artifact struct {
+	Output  string        `yaml:"output"`
+	Timeout time.Duration `yaml:"timeout"`
+}
+
+// NarratioConfig demonstrates application defaults, partial imports, and
+// application-owned model-profile and artifact semantics.
+type NarratioConfig struct {
+	PollInterval time.Duration       `yaml:"poll_interval"`
+	Timeout      time.Duration       `yaml:"timeout"`
+	ModelProfile string              `yaml:"model_profile"`
+	Artifacts    map[string]Artifact `yaml:"artifacts"`
+}
+
+// LoadNarratio loads the maintained import fixture with Narratio-shaped rules.
+func LoadNarratio(path string) (*configkit.Result[NarratioConfig], error) {
+	return LoadNarratioProfile(path, "")
+}
+
+// LoadNarratioProfile selects a ConfigKit configuration overlay independently
+// from the application's ModelProfile field.
+func LoadNarratioProfile(path, profile string) (*configkit.Result[NarratioConfig], error) {
+	return configkit.Load(configkit.File(path), configkit.Options[NarratioConfig]{
+		Imports: true, Profile: profile,
+		Definition: configkit.Definition[NarratioConfig]{
+			Defaults: func() NarratioConfig { return NarratioConfig{PollInterval: time.Minute} },
+			Resolve:  resolveNarratio,
+			Validate: validateNarratio,
+		},
+	})
+}
+
+func resolveNarratio(configuration *NarratioConfig, resolution *configkit.Resolution) error {
+	configkit.Default(resolution, configkit.Keys("timeout"), &configuration.Timeout,
+		configuration.PollInterval/10, "derived from poll interval", configkit.Keys("poll_interval"))
+	for name, artifact := range configuration.Artifacts {
+		path := configkit.Keys("artifacts", name)
+		configkit.Default(resolution, path.Key("timeout"), &artifact.Timeout,
+			configuration.Timeout, "inherited pipeline timeout", configkit.Keys("timeout"))
+		configkit.Normalize(resolution, path.Key("output"), &artifact.Output,
+			strings.TrimSpace(artifact.Output), "trimmed artifact output")
+		configuration.Artifacts[name] = artifact
+	}
+	return nil
+}
+
+func validateNarratio(configuration *NarratioConfig, validation *configkit.Validation) {
+	if configuration.PollInterval <= 0 {
+		validation.Error(configkit.Keys("poll_interval"), "must be positive")
+	}
+	if configuration.Timeout <= 0 {
+		validation.Error(configkit.Keys("timeout"), "must be positive")
+	}
+	for name, artifact := range configuration.Artifacts {
+		path := configkit.Keys("artifacts", name)
+		if artifact.Output == "" {
+			validation.Error(path.Key("output"), "must be nonempty")
+		}
+		if artifact.Timeout <= 0 {
+			validation.Error(path.Key("timeout"), "must be positive")
+		}
+	}
+}
+
+// ObjectBinding is a Notarius-shaped canonical object binding. Options remain
+// dynamic because the application catalog selects their schema at runtime.
+type ObjectBinding struct {
+	Object  string         `yaml:"object"`
+	Options map[string]any `yaml:"options"`
+}
+
+// NotariusConfig demonstrates string-backed versions and dynamic catalog options.
+type NotariusConfig struct {
+	Version  string          `yaml:"version"`
+	Bindings []ObjectBinding `yaml:"bindings"`
+}
+
+// LoadNotarius loads a Notarius-shaped configuration.
+func LoadNotarius(path string) (*configkit.Result[NotariusConfig], error) {
+	return configkit.Load(configkit.File(path), configkit.Options[NotariusConfig]{
+		Definition: configkit.Definition[NotariusConfig]{
+			Version:  &configkit.VersionRule{Path: configkit.Keys("version"), Check: requireVersion("4")},
+			Validate: validateNotarius,
+		},
+	})
+}
+
+func requireVersion(expected string) func(configkit.Version, *configkit.Validation) error {
+	return func(version configkit.Version, validation *configkit.Validation) error {
+		if !version.Present || version.Null || version.Text != expected {
+			validation.Error(configkit.Keys("version"), "unsupported configuration version")
+		}
+		return nil
+	}
+}
+
+func validateNotarius(configuration *NotariusConfig, validation *configkit.Validation) {
+	for index, binding := range configuration.Bindings {
+		path := configkit.Keys("bindings").Index(index)
+		if binding.Object != "summary" && binding.Object != "registry" {
+			validation.Error(path.Key("object"), "unsupported object binding")
+		}
+		for key, value := range binding.Options {
+			optionPath := path.Key("options").Key(key)
+			switch key {
+			case "window_units":
+				units, ok := value.(int64)
+				if !ok || units < 1 {
+					validation.Error(optionPath, "must be a positive integer")
+				}
+			case "format":
+				if _, ok := value.(string); !ok {
+					validation.Error(optionPath, "must be a string")
+				}
+			default:
+				validation.Error(optionPath, "unsupported object option")
+			}
+		}
+	}
+}
+
+// NotariusWindow converts ConfigKit's portable dynamic integer only at the
+// application runtime boundary.
+func NotariusWindow(configuration NotariusConfig, binding int) (int, bool) {
+	if binding < 0 || binding >= len(configuration.Bindings) {
+		return 0, false
+	}
+	value, ok := configuration.Bindings[binding].Options["window_units"].(int64)
+	if !ok || value < 1 || (strconv.IntSize == 32 && value > math.MaxInt32) {
+		return 0, false
+	}
+	return int(value), true
+}
+
+// ModuleOptions is a WeatherReporter-shaped union of known module option fields.
+type ModuleOptions struct {
+	WindowUnits *int     `yaml:"window_units"`
+	Sections    []string `yaml:"sections"`
+}
+
+// ModuleSpec uses the canonical object form instead of a string shorthand.
+type ModuleSpec struct {
+	Module  string        `yaml:"module"`
+	Options ModuleOptions `yaml:"options"`
+}
+
+// ReportConfig keeps report-specific lists application-owned.
+type ReportConfig struct {
+	Modules       []ModuleSpec `yaml:"modules"`
+	PathTemplates []string     `yaml:"path_templates"`
+}
+
+// WeatherConfig demonstrates simple defaults, reports, module options, and an
+// optional typed secret reference.
+type WeatherConfig struct {
+	Units   string                  `yaml:"units"`
+	Reports map[string]ReportConfig `yaml:"reports"`
+	Token   *configkit.Secret       `yaml:"token"`
+}
+
+// LoadWeather loads a WeatherReporter-shaped source and explicit CLI records.
+func LoadWeather(source configkit.Source, overrides ...configkit.Override) (*configkit.Result[WeatherConfig], error) {
+	return configkit.Load(source, configkit.Options[WeatherConfig]{
+		Overrides: overrides,
+		Definition: weatherDefinition(func() WeatherConfig {
+			return WeatherConfig{Units: "metric"}
+		}),
+	})
+}
+
+// LoadConstructedWeather runs the same rules over a detached constructed value.
+func LoadConstructedWeather(configuration WeatherConfig) (*configkit.Result[WeatherConfig], error) {
+	return configkit.Load(configkit.NoSource(), configkit.Options[WeatherConfig]{
+		Definition: weatherDefinition(func() WeatherConfig { return configuration }),
+	})
+}
+
+func weatherDefinition(defaults func() WeatherConfig) configkit.Definition[WeatherConfig] {
+	return configkit.Definition[WeatherConfig]{
+		Defaults: defaults,
+		Resolve: func(configuration *WeatherConfig, resolution *configkit.Resolution) error {
+			for reportName, report := range configuration.Reports {
+				var modules []ModuleSpec
+				if report.Modules != nil {
+					modules = append(make([]ModuleSpec, 0, len(report.Modules)), report.Modules...)
+				}
+				for index, module := range modules {
+					if module.Options.Sections != nil {
+						sections := append([]string(nil), module.Options.Sections...)
+						for position := range sections {
+							sections[position] = strings.TrimSpace(sections[position])
+						}
+						module.Options.Sections = sections
+					}
+					modules[index] = module
+				}
+				configkit.Normalize(resolution, configkit.Keys("reports", reportName, "modules"),
+					&report.Modules, modules, "normalized module options")
+				configuration.Reports[reportName] = report
+			}
+			return nil
+		},
+		Validate: validateWeather,
+	}
+}
+
+func validateWeather(configuration *WeatherConfig, validation *configkit.Validation) {
+	if configuration.Units != "metric" && configuration.Units != "us" {
+		validation.Error(configkit.Keys("units"), "must be metric or us")
+	}
+	for reportName, report := range configuration.Reports {
+		path := configkit.Keys("reports", reportName)
+		if report.Modules != nil && len(report.Modules) == 0 {
+			validation.Error(path.Key("modules"), "must contain at least one module")
+		}
+		if report.PathTemplates != nil && len(report.PathTemplates) == 0 {
+			validation.Error(path.Key("path_templates"), "must contain at least one template")
+		}
+		for index, module := range report.Modules {
+			modulePath := path.Key("modules").Index(index)
+			switch module.Module {
+			case "chunker":
+				if module.Options.WindowUnits == nil || *module.Options.WindowUnits < 1 {
+					validation.Error(modulePath.Key("options").Key("window_units"), "must be a positive integer")
+				}
+				if module.Options.Sections != nil {
+					validation.Error(modulePath.Key("options").Key("sections"), "not supported by this module")
+				}
+			case "discussion":
+				if module.Options.WindowUnits != nil {
+					validation.Error(modulePath.Key("options").Key("window_units"), "not supported by this module")
+				}
+				if len(module.Options.Sections) == 0 {
+					validation.Error(modulePath.Key("options").Key("sections"), "must contain at least one section")
+				}
+			default:
+				validation.Error(modulePath.Key("module"), "unsupported module")
+			}
+		}
+	}
+}
+
+// SpeakerRule and SpeakerRules preserve declaration order for application matching.
+type SpeakerRule struct {
+	Prefix string `yaml:"prefix"`
+	Name   string `yaml:"name"`
+}
+
+type SpeakerRules struct {
+	Rules []SpeakerRule `yaml:"rules"`
+}
+
+// AutocorrectRule and AutocorrectRules preserve replacement order.
+type AutocorrectRule struct {
+	Find    string `yaml:"find"`
+	Replace string `yaml:"replace"`
+}
+
+type AutocorrectRules struct {
+	Rules []AutocorrectRule `yaml:"rules"`
+}
+
+// LoadSpeakerRules loads one independent Seriatim-shaped rule file.
+func LoadSpeakerRules(path string) (*configkit.Result[SpeakerRules], error) {
+	return configkit.Load(configkit.File(path), configkit.Options[SpeakerRules]{
+		Definition: configkit.Definition[SpeakerRules]{Validate: validateSpeakerRules},
+	})
+}
+
+// LoadAutocorrectRules loads another independent ordered rule file.
+func LoadAutocorrectRules(path string) (*configkit.Result[AutocorrectRules], error) {
+	return configkit.Load(configkit.File(path), configkit.Options[AutocorrectRules]{
+		Definition: configkit.Definition[AutocorrectRules]{Validate: validateAutocorrectRules},
+	})
+}
+
+func validateSpeakerRules(configuration *SpeakerRules, validation *configkit.Validation) {
+	seen := make(map[string]int)
+	for index, rule := range configuration.Rules {
+		path := configkit.Keys("rules").Index(index)
+		if rule.Prefix == "" {
+			validation.Error(path.Key("prefix"), "must be nonempty")
+		}
+		if previous, ok := seen[rule.Prefix]; ok {
+			validation.Error(path.Key("prefix"), "prefix must be unique",
+				configkit.Keys("rules").Index(previous).Key("prefix"))
+		}
+		seen[rule.Prefix] = index
+	}
+}
+
+func validateAutocorrectRules(configuration *AutocorrectRules, validation *configkit.Validation) {
+	for index, rule := range configuration.Rules {
+		path := configkit.Keys("rules").Index(index)
+		if rule.Find == "" {
+			validation.Error(path.Key("find"), "must be nonempty")
+		}
+	}
+}
+
+// SpeakerName is application runtime matching, not configuration infrastructure.
+func SpeakerName(rules SpeakerRules, input string) (string, bool) {
+	for _, rule := range rules.Rules {
+		if strings.HasPrefix(input, rule.Prefix) {
+			return rule.Name, true
+		}
+	}
+	return "", false
+}
+
+// ApplyAutocorrect is application runtime replacement in declared order.
+func ApplyAutocorrect(rules AutocorrectRules, input string) string {
+	for _, rule := range rules.Rules {
+		input = strings.ReplaceAll(input, rule.Find, rule.Replace)
+	}
+	return input
+}

examples/consumers/consumers_test.go

@@ -0,0 +1,267 @@
+package consumers_test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func Example_narratio() {
+	result, err := consumers.LoadNarratio(fixture("narratio", "pipeline.yml"))
+	artifact := result.Config.Artifacts["transcript"]
+	fmt.Println(result.Config.ModelProfile, artifact.Output, result.Config.Timeout, artifact.Timeout, err)
+	// Output: concise transcript.txt 6s 6s <nil>
+}
+
+func Example_notarius() {
+	result, err := consumers.LoadNotarius(fixture("notarius", "config.yml"))
+	window, ok := consumers.NotariusWindow(result.Config, 0)
+	fmt.Println(result.Config.Version, result.Config.Bindings[0].Object, window, ok, err)
+	// Output: 4 summary 12 true <nil>
+}
+
+func Example_weatherReporter() {
+	defaults, defaultsErr := consumers.LoadWeather(configkit.NoSource())
+	result, err := consumers.LoadWeather(configkit.File(fixture("weather", "config.yml")))
+	discussion := result.Config.Reports["daily"].Modules[1]
+	fmt.Println(defaults.Config.Units, result.Config.Units, discussion.Options.Sections[0], defaultsErr, err)
+	// Output: metric us short_term <nil> <nil>
+}
+
+func Example_seriatim() {
+	speakers, speakerErr := consumers.LoadSpeakerRules(fixture("seriatim", "speakers.yml"))
+	autocorrect, autocorrectErr := consumers.LoadAutocorrectRules(fixture("seriatim", "autocorrect.yml"))
+	name, found := consumers.SpeakerName(speakers.Config, "Guest: teh event occured")
+	text := consumers.ApplyAutocorrect(autocorrect.Config, "teh event occured")
+	fmt.Println(name, found, text, speakerErr, autocorrectErr)
+	// Output: visitor true the event occurred <nil> <nil>
+}
+
+func Example_weatherStartup() {
+	const blockedName = "CONFIGKIT_WEATHER_EXAMPLE_BLOCKED"
+	const tokenName = "CONFIGKIT_WEATHER_EXAMPLE_TOKEN"
+	restoreBlocked := preserveEnvironment(blockedName)
+	restoreToken := preserveEnvironment(tokenName)
+	defer restoreBlocked()
+	defer restoreToken()
+	_ = os.Setenv(blockedName, "")
+	_ = os.Unsetenv(tokenName)
+
+	directory, _ := os.MkdirTemp("", "configkit-weather-example-")
+	defer os.RemoveAll(directory)
+	_ = os.WriteFile(filepath.Join(directory, blockedName), []byte("unused-example-value\n"), 0o600)
+	_ = os.WriteFile(filepath.Join(directory, tokenName), []byte("disposable-example-credential\n"), 0o600)
+	report, bootstrapErr := configkit.LoadSecretsDir(directory)
+	var reportOutput bytes.Buffer
+	_, reportWriteErr := report.WriteTo(&reportOutput)
+
+	result, loadErr := consumers.LoadWeather(configkit.Bytes("weather.yml", []byte("units: metric\ntoken: "+tokenName+"\n")),
+		configkit.Override{Path: configkit.Keys("units"), Value: "us", Flag: "--units"})
+	var explanation bytes.Buffer
+	explainErr := result.WriteExplanation(&explanation, configkit.Keys("units"))
+	blocked, loaded := secretStatus(report, blockedName), secretStatus(report, tokenName)
+	value, present := os.LookupEnv(tokenName)
+	fmt.Printf("blocked=%s loaded=%s report=%t reference=%s explained=%t unchanged=%t errors=%t\n",
+		blocked, loaded, reportOutput.Len() > 0, result.Config.Token.EnvName(),
+		strings.Contains(explanation.String(), "--units"), present && value == "disposable-example-credential",
+		bootstrapErr != nil || reportWriteErr != nil || loadErr != nil || explainErr != nil)
+	// Output: blocked=skipped_existing loaded=loaded report=true reference=CONFIGKIT_WEATHER_EXAMPLE_TOKEN explained=true unchanged=true errors=false
+}
+
+func TestMaintainedConsumerFixturesPreserveDomainOutcomes(t *testing.T) {
+	narratio, err := consumers.LoadNarratio(fixture("narratio", "pipeline.yml"))
+	if err != nil || narratio.Config.Artifacts["transcript"].Output != "transcript.txt" ||
+		narratio.Config.ModelProfile != "concise" || len(narratio.Sources()) != 2 {
+		t.Fatalf("Narratio shape = %#v, %v", narratio, err)
+	}
+	if origin, ok := narratio.Metadata().Lookup(configkit.Keys("artifacts", "transcript", "output")); !ok || origin.Origin.Layer != configkit.LayerResolution {
+		t.Fatalf("artifact origin = %#v, %v", origin, ok)
+	}
+	profiled, err := consumers.LoadNarratioProfile(fixture("narratio", "pipeline.yml"), "testing")
+	if err != nil || profiled.Config.PollInterval.String() != "2m0s" ||
+		profiled.Config.ModelProfile != "concise" || len(profiled.Sources()) != 3 {
+		t.Fatalf("configuration/application profiles = %#v, %v", profiled, err)
+	}
+
+	notarius, err := consumers.LoadNotarius(fixture("notarius", "config.yml"))
+	window, converted := consumers.NotariusWindow(notarius.Config, 0)
+	if err != nil || !converted || window != 12 {
+		t.Fatalf("Notarius shape = %#v, %v, %v", notarius, converted, err)
+	}
+	if _, ok := notarius.Config.Bindings[0].Options["window_units"].(int64); !ok {
+		t.Fatalf("dynamic integer type = %T", notarius.Config.Bindings[0].Options["window_units"])
+	}
+	integerNotarius, err := consumers.LoadNotarius(fixture("notarius", "integer-version.yml"))
+	if err != nil || integerNotarius.Config.Version != notarius.Config.Version || integerNotarius.Config.Version != "4" {
+		t.Fatalf("Notarius version forms = %#v / %#v, %v", notarius.Config, integerNotarius, err)
+	}
+
+	weather, err := consumers.LoadWeather(configkit.File(fixture("weather", "config.yml")))
+	if err != nil || weather.Config.Reports["daily"].Modules[1].Options.Sections[0] != "short_term" {
+		t.Fatalf("WeatherReporter shape = %#v, %v", weather, err)
+	}
+
+	speakers, err := consumers.LoadSpeakerRules(fixture("seriatim", "speakers.yml"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	name, ok := consumers.SpeakerName(speakers.Config, "Host: introduction")
+	if !ok || name != "narrator" || speakers.Config.Rules[0].Prefix != "Host:" {
+		t.Fatalf("ordered speaker rules = %#v, %q, %v", speakers.Config.Rules, name, ok)
+	}
+}
+
+func TestMigrationFixturesFailWithStableCodesAndPaths(t *testing.T) {
+	tests := []struct {
+		name string
+		load func() error
+		code configkit.Code
+		path string
+	}{
+		{
+			name: "unsupported application version",
+			load: func() error {
+				_, err := consumers.LoadNotarius(fixture("notarius", "unsupported-version.yml"))
+				return err
+			},
+			code: configkit.CodeVersionError, path: "/version",
+		},
+		{
+			name: "non-integer numeric version",
+			load: func() error {
+				_, err := consumers.LoadNotarius(fixture("notarius", "noninteger-version.yml"))
+				return err
+			},
+			code: configkit.CodeVersionType, path: "/version",
+		},
+		{
+			name: "module shorthand",
+			load: func() error {
+				_, err := consumers.LoadWeather(configkit.File(fixture("weather", "shorthand-module.yml")))
+				return err
+			},
+			code: configkit.CodeDecodeType, path: "/reports/daily/modules/0",
+		},
+		{
+			name: "explicit empty modules",
+			load: func() error {
+				_, err := consumers.LoadWeather(configkit.File(fixture("weather", "empty-modules.yml")))
+				return err
+			},
+			code: configkit.CodeValidateError, path: "/reports/daily/modules",
+		},
+		{
+			name: "explicit empty path templates",
+			load: func() error {
+				_, err := consumers.LoadWeather(configkit.File(fixture("weather", "empty-paths.yml")))
+				return err
+			},
+			code: configkit.CodeValidateError, path: "/reports/daily/path_templates",
+		},
+		{
+			name: "explicit null required option",
+			load: func() error {
+				_, err := consumers.LoadWeather(configkit.File(fixture("weather", "null-window.yml")))
+				return err
+			},
+			code: configkit.CodeValidateError, path: "/reports/daily/modules/0/options/window_units",
+		},
+		{
+			name: "strict unknown rule field",
+			load: func() error {
+				_, err := consumers.LoadSpeakerRules(fixture("seriatim", "unknown-field.yml"))
+				return err
+			},
+			code: configkit.CodeDecodeUnknownField, path: "/rules/0/color",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			var loadError *configkit.LoadError
+			if err := test.load(); !errors.As(err, &loadError) {
+				t.Fatalf("error = %T %v", err, err)
+			}
+			diagnostics := loadError.Diagnostics()
+			if len(diagnostics) == 0 || diagnostics[0].Code != test.code || diagnostics[0].Path.String() != test.path {
+				t.Fatalf("diagnostics = %#v", diagnostics)
+			}
+			if strings.Contains(diagnostics[0].Message, "wrong-before-profile") {
+				t.Fatalf("diagnostic exposed raw parser input: %#v", diagnostics[0])
+			}
+		})
+	}
+}
+
+func TestWeatherLoadedAndConstructedRulesAgreeWithoutMutation(t *testing.T) {
+	loaded, err := consumers.LoadWeather(configkit.File(fixture("weather", "config.yml")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	reconstructed, err := consumers.LoadConstructedWeather(loaded.Config)
+	if err != nil || !reflect.DeepEqual(reconstructed.Config, loaded.Config) {
+		t.Fatalf("loaded/constructed outcomes differ: %#v, %v", reconstructed, err)
+	}
+	constructed := consumers.WeatherConfig{Units: loaded.Config.Units, Reports: map[string]consumers.ReportConfig{}}
+	constructed.Reports["daily"] = consumers.ReportConfig{
+		Modules:       []consumers.ModuleSpec{{Module: "discussion", Options: consumers.ModuleOptions{Sections: []string{" short_term "}}}},
+		PathTemplates: []string{"daily/{date}.md"},
+	}
+	wantInput := constructed.Reports["daily"].Modules[0].Options.Sections[0]
+	result, err := consumers.LoadConstructedWeather(constructed)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := constructed.Reports["daily"].Modules[0].Options.Sections[0]; got != wantInput {
+		t.Fatalf("constructed options mutated from %q to %q", wantInput, got)
+	}
+	if result.Config.Reports["daily"].Modules[0].Options.Sections[0] != "short_term" {
+		t.Fatalf("normalized result = %#v", result.Config.Reports["daily"])
+	}
+
+	empty := consumers.WeatherConfig{Units: "metric", Reports: map[string]consumers.ReportConfig{
+		"daily": {Modules: []consumers.ModuleSpec{}},
+	}}
+	_, err = consumers.LoadConstructedWeather(empty)
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || loadError.Diagnostics()[0].Code != configkit.CodeValidateError {
+		t.Fatalf("constructed empty modules error = %v", err)
+	}
+	if !reflect.DeepEqual(empty.Reports["daily"].Modules, []consumers.ModuleSpec{}) {
+		t.Fatal("constructed empty module input was mutated")
+	}
+}
+
+func fixture(parts ...string) string {
+	_, file, _, _ := runtime.Caller(0)
+	values := append([]string{filepath.Dir(file), "testdata"}, parts...)
+	return filepath.Join(values...)
+}
+
+func preserveEnvironment(name string) func() {
+	value, present := os.LookupEnv(name)
+	return func() {
+		if present {
+			_ = os.Setenv(name, value)
+		} else {
+			_ = os.Unsetenv(name)
+		}
+	}
+}
+
+func secretStatus(report *configkit.SecretsReport, name string) configkit.SecretFileStatus {
+	for _, entry := range report.Entries() {
+		if entry.Name == name {
+			return entry.Status
+		}
+	}
+	return "missing"
+}

examples/consumers/testdata/narratio/artifacts.yml

@@ -0,0 +1,3 @@
+artifacts:
+  transcript:
+    output: ' transcript.txt '

examples/consumers/testdata/narratio/pipeline.yml

@@ -0,0 +1,5 @@
+configkit:
+  imports:
+    - path: artifacts.yml
+poll_interval: 1m
+model_profile: concise

examples/consumers/testdata/narratio/profiles/testing.yml

@@ -0,0 +1 @@
+poll_interval: 2m

examples/consumers/testdata/notarius/config.yml

@@ -0,0 +1,6 @@
+version: "4"
+bindings:
+  - object: summary
+    options:
+      window_units: 12
+      format: markdown

examples/consumers/testdata/notarius/integer-version.yml

@@ -0,0 +1,6 @@
+version: 4
+bindings:
+  - object: summary
+    options:
+      window_units: 12
+      format: markdown

examples/consumers/testdata/notarius/noninteger-version.yml

@@ -0,0 +1,2 @@
+version: 4.0
+bindings: []

examples/consumers/testdata/notarius/unsupported-version.yml

@@ -0,0 +1,2 @@
+version: 5
+bindings: []

examples/consumers/testdata/seriatim/autocorrect.yml

@@ -0,0 +1,5 @@
+rules:
+  - find: teh
+    replace: the
+  - find: occured
+    replace: occurred

examples/consumers/testdata/seriatim/speakers.yml

@@ -0,0 +1,5 @@
+rules:
+  - prefix: 'Host:'
+    name: narrator
+  - prefix: 'Guest:'
+    name: visitor

examples/consumers/testdata/seriatim/unknown-field.yml

@@ -0,0 +1,4 @@
+rules:
+  - prefix: 'Host:'
+    name: narrator
+    color: blue

examples/consumers/testdata/weather/config.yml

@@ -0,0 +1,11 @@
+units: us
+reports:
+  daily:
+    modules:
+      - module: chunker
+        options:
+          window_units: 12
+      - module: discussion
+        options:
+          sections: [' short_term ']
+    path_templates: ['daily/{date}.md']

examples/consumers/testdata/weather/empty-modules.yml

@@ -0,0 +1,3 @@
+reports:
+  daily:
+    modules: []

examples/consumers/testdata/weather/empty-paths.yml

@@ -0,0 +1,3 @@
+reports:
+  daily:
+    path_templates: []

examples/consumers/testdata/weather/null-window.yml

@@ -0,0 +1,6 @@
+reports:
+  daily:
+    modules:
+      - module: chunker
+        options:
+          window_units: null

examples/consumers/testdata/weather/shorthand-module.yml

@@ -0,0 +1,3 @@
+reports:
+  daily:
+    modules: [chunker]

explain.go

@@ -0,0 +1,122 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// ValueSnapshot is a detached, deterministic YAML representation of one value.
+// Present distinguishes absence or deletion from an explicit null value.
+type ValueSnapshot struct {
+	Present  bool
+	Redacted bool
+	YAML     string
+}
+
+// ValueChange describes one explicit definition or audited domain operation.
+type ValueChange struct {
+	Before       ValueSnapshot
+	After        ValueSnapshot
+	Origin       Origin
+	Reasons      []string
+	Dependencies []Path
+}
+
+// SecretReferenceInfo describes a successfully resolved effective reference.
+// Name is the environment-variable name, never its value.
+type SecretReferenceInfo struct {
+	Name string
+}
+
+// Explanation is a detached safe account of one effective or historical path.
+type Explanation struct {
+	Path            Path
+	Effective       ValueSnapshot
+	Origin          Origin
+	History         []ValueChange
+	SecretReference *SecretReferenceInfo
+}
+
+// Explain returns safe effective and chronological values for a known final or
+// historical path. Unknown and malformed paths return false.
+func (r *Result[T]) Explain(path Path) (Explanation, bool) {
+	if r == nil || r.final == nil || r.baseline == nil || !r.metadata.hasHistoricalPath(path) {
+		return Explanation{}, false
+	}
+	explanation := Explanation{Path: path.clone(), Effective: valueSnapshot(r.final, path, r.secrets)}
+	if info, ok := r.metadata.Lookup(path); ok {
+		explanation.Origin = cloneOrigin(info.Origin)
+	}
+
+	working := document.Clone(r.baseline)
+	if _, present := documentAtPath(working, path); present {
+		explanation.History = append(explanation.History, ValueChange{
+			After: valueSnapshot(working, path, r.secrets), Origin: Origin{Layer: LayerDefault},
+		})
+	}
+	for _, application := range r.applications {
+		operationPath, relevant := applicationOperationPath(application, path)
+		var before ValueSnapshot
+		if relevant {
+			before = valueSnapshot(working, path, r.secrets)
+		}
+		applyHistoryApplication(&working, application)
+		if !relevant {
+			continue
+		}
+		after := valueSnapshot(working, path, r.secrets)
+		if !before.Present && !after.Present {
+			continue
+		}
+		explanation.History = append(explanation.History, ValueChange{
+			Before: before,
+			After:  after,
+			Origin: historyApplicationOrigin(application, operationPath),
+		})
+	}
+
+	// Intermediate input replacements do not permanently suppress defaults:
+	// decoding applies them to the final input tree. Show that reconciliation
+	// explicitly, including canonical scalar conversions, before domain changes.
+	replayed, _ := documentAtPath(working, path)
+	decoded, _ := documentAtPath(r.input, path)
+	if !configurationValuesEqual(replayed, decoded) {
+		origin := cloneOrigin(r.inputOrigins[path.String()])
+		origin.Operation = ChangeDecoding
+		explanation.History = append(explanation.History, ValueChange{
+			Before:  valueSnapshot(working, path, r.secrets),
+			After:   valueSnapshot(r.input, path, r.secrets),
+			Origin:  origin,
+			Reasons: []string{"applied retained defaults and canonical typed decoding"},
+		})
+	}
+
+	working = document.Clone(r.input)
+	for _, snapshot := range coalescedResolutionSnapshots(r.resolution) {
+		relevant := pathsIntersect(snapshot.change.Path, path)
+		var before ValueSnapshot
+		if relevant {
+			before = valueSnapshot(working, path, r.secrets)
+		}
+		replaceHistoryPath(&working, snapshot.change.Path, snapshot.after)
+		if !relevant {
+			continue
+		}
+		after := valueSnapshot(working, path, r.secrets)
+		if !before.Present && !after.Present {
+			continue
+		}
+		explanation.History = append(explanation.History, ValueChange{
+			Before:       before,
+			After:        after,
+			Origin:       cloneOrigin(snapshot.change.Origin),
+			Reasons:      append([]string(nil), snapshot.change.Reasons...),
+			Dependencies: clonePaths(snapshot.change.Dependencies),
+		})
+	}
+
+	if !explanation.Effective.Present && len(explanation.History) == 0 {
+		return Explanation{}, false
+	}
+	if name, ok := r.secretReferences[path.String()]; ok && !pathIsSecret(r.secrets, path) {
+		explanation.SecretReference = &SecretReferenceInfo{Name: name}
+	}
+	return explanation, true
+}

explain_render.go

@@ -0,0 +1,123 @@
+package configkit
+
+import (
+	"errors"
+	"io"
+	"strconv"
+	"strings"
+)
+
+var (
+	errExplanationUnavailable = errors.New("configkit: configuration explanation is unavailable")
+	errExplanationUnknownPath = errors.New("configkit: configuration explanation path is unknown")
+)
+
+// WriteExplanation writes a deterministic human-readable rendering of Explain.
+// Unknown paths produce no output. Writer failures may leave partial output.
+func (r *Result[T]) WriteExplanation(output io.Writer, path Path) error {
+	if r == nil || output == nil {
+		return errExplanationUnavailable
+	}
+	explanation, ok := r.Explain(path)
+	if !ok {
+		return errExplanationUnknownPath
+	}
+	var rendered strings.Builder
+	rendered.WriteString("path: ")
+	rendered.WriteString(strconv.Quote(explanation.Path.String()))
+	rendered.WriteByte('\n')
+	writeExplanationOrigin(&rendered, "effective source", explanation.Origin, r.sources)
+	writeExplanationSnapshot(&rendered, "effective (canonical typed)", explanation.Effective, 0)
+	if explanation.SecretReference != nil {
+		rendered.WriteString("secret reference: ")
+		rendered.WriteString(strconv.Quote(explanation.SecretReference.Name))
+		rendered.WriteString(" (resolved from process environment)\n")
+	}
+	rendered.WriteString("history:\n")
+	for _, change := range explanation.History {
+		rendered.WriteString("  - representation: ")
+		if change.Origin.Operation == ChangeDecoding {
+			rendered.WriteString("input to canonical typed\n")
+		} else if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerProfile || change.Origin.Layer == LayerCLI {
+			rendered.WriteString("policy-validated input\n")
+		} else {
+			rendered.WriteString("canonical typed\n")
+		}
+		writeExplanationOrigin(&rendered, "    source", change.Origin, r.sources)
+		writeExplanationSnapshot(&rendered, "    before", change.Before, 4)
+		writeExplanationSnapshot(&rendered, "    after", change.After, 4)
+		for _, reason := range change.Reasons {
+			rendered.WriteString("    reason: ")
+			rendered.WriteString(strconv.Quote(reason))
+			rendered.WriteByte('\n')
+		}
+		for _, dependency := range change.Dependencies {
+			rendered.WriteString("    dependency: ")
+			rendered.WriteString(strconv.Quote(dependency.String()))
+			rendered.WriteByte('\n')
+		}
+	}
+	data := []byte(rendered.String())
+	written, err := output.Write(data)
+	if err == nil && written != len(data) {
+		return io.ErrShortWrite
+	}
+	return err
+}
+
+func writeExplanationOrigin(output *strings.Builder, label string, origin Origin, sources []SourceInfo) {
+	output.WriteString(label)
+	output.WriteString(": layer=")
+	output.WriteString(strconv.Quote(string(origin.Layer)))
+	if origin.Operation != "" {
+		output.WriteString(" operation=")
+		output.WriteString(strconv.Quote(string(origin.Operation)))
+	}
+	output.WriteByte('\n')
+	for _, location := range origin.Locations {
+		output.WriteString("      location: source=")
+		output.WriteString(strconv.Quote(location.SourceID))
+		if label := sourceLabel(sources, location.SourceID); label != "" {
+			output.WriteString(" label=")
+			output.WriteString(strconv.Quote(label))
+		}
+		output.WriteString(" line=")
+		output.WriteString(strconv.Itoa(location.Line))
+		output.WriteString(" column=")
+		output.WriteString(strconv.Itoa(location.Column))
+		output.WriteByte('\n')
+	}
+}
+
+func sourceLabel(sources []SourceInfo, sourceID string) string {
+	for _, source := range sources {
+		if source.ID == sourceID {
+			return source.Label
+		}
+	}
+	return ""
+}
+
+func writeExplanationSnapshot(output *strings.Builder, label string, snapshot ValueSnapshot, indent int) {
+	if !snapshot.Present {
+		output.WriteString(label)
+		output.WriteString(": absent\n")
+		return
+	}
+	output.WriteString(label)
+	if snapshot.Redacted {
+		output.WriteString(" (redacted)")
+	}
+	output.WriteString(":\n")
+	prefix := strings.Repeat(" ", indent+2)
+	for _, line := range strings.SplitAfter(snapshot.YAML, "\n") {
+		if line == "" {
+			continue
+		}
+		output.WriteString(prefix)
+		output.WriteString(line)
+		if !strings.HasSuffix(line, "\n") {
+			output.WriteByte('\n')
+		}
+	}
+}

explain_test.go

@@ -0,0 +1,516 @@
+package configkit
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type explanationConcurrency struct {
+	Transcription struct {
+		Concurrency int `yaml:"concurrency"`
+	} `yaml:"transcription"`
+}
+
+func TestExplainShowsDefaultBaseAndEffectiveProfileValue(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "config.yml", "configkit:\n  imports: [{path: transcription.yml}]\n")
+	writeImportFile(t, directory, "transcription.yml", "transcription:\n  concurrency: 8\n")
+	writeImportFile(t, directory, filepath.Join("profiles", "testing.yml"), "transcription:\n  concurrency: 1\n")
+	result, err := Load[explanationConcurrency](File(mainPath), Options[explanationConcurrency]{
+		Imports: true, Profile: "testing",
+		Definition: Definition[explanationConcurrency]{Defaults: func() explanationConcurrency {
+			value := explanationConcurrency{}
+			value.Transcription.Concurrency = 4
+			return value
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	explanation, ok := result.Explain(Keys("transcription", "concurrency"))
+	if !ok {
+		t.Fatal("known path was not explained")
+	}
+	if got := explanationValues(explanation); !reflect.DeepEqual(got, []string{"4\n", "8\n", "1\n"}) {
+		t.Fatalf("history values = %#v", got)
+	}
+	if explanation.Effective.YAML != "1\n" || explanation.Origin.Layer != LayerProfile ||
+		len(explanation.Origin.Locations) != 1 || explanation.Origin.Locations[0].SourceID != "source-3" ||
+		explanation.Origin.Locations[0].Line != 2 {
+		t.Fatalf("effective explanation = %#v", explanation)
+	}
+}
+
+type explanationConfig struct {
+	Concurrency int               `yaml:"concurrency"`
+	Equal       string            `yaml:"equal"`
+	Limit       int               `yaml:"limit"`
+	Items       []string          `yaml:"items"`
+	Labels      map[string]string `yaml:"labels"`
+	Dynamic     any               `yaml:"dynamic"`
+	Maybe       *string           `yaml:"maybe"`
+	Empty       []string          `yaml:"empty"`
+	Credential  Secret            `yaml:"credential"`
+	Classified  Secret            `yaml:"classified"`
+	DerivedA    string            `yaml:"derived_a"`
+	DerivedB    string            `yaml:"derived_b"`
+}
+
+func TestExplainReplaysCLIResolutionContainersAndSafeHistoricalValues(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "config.yml", `concurrency: 8
+equal: same
+limit: wrong-before-profile
+items: [base, removed]
+labels: {base: retained}
+dynamic: {retired: HISTORICAL_EXPLAIN_CANARY, public: visible}
+credential: APP_SECRET
+classified: APP_SECRET
+`)
+	writeImportFile(t, directory, filepath.Join("profiles", "testing.yml"), `concurrency: 1
+equal: same
+limit: 7
+items: [profile]
+labels: {profile: retained}
+dynamic: selected
+`)
+	result, err := loadWithSecretLookup(File(mainPath), func(name string) (string, bool) {
+		if name != "APP_SECRET" {
+			t.Fatalf("secret lookup = %q", name)
+		}
+		return "RESOLVED_EXPLAIN_CANARY", true
+	}, Options[explanationConfig]{
+		Profile: "testing",
+		Overrides: []Override{
+			{Path: Keys("concurrency"), Value: 2, Flag: "--concurrency"},
+			{Path: Keys("equal"), Value: "same", Flag: "--equal"},
+			{Path: Keys("items"), Value: []any{"cli", "restored"}, Flag: "--items"},
+			{Path: Keys("labels"), Value: map[string]any{"cli": "retained"}, Flag: "--label"},
+		},
+		Definition: Definition[explanationConfig]{
+			Defaults: func() explanationConfig {
+				return explanationConfig{
+					Concurrency: 4, Equal: "same", Labels: map[string]string{"default": "retained"},
+					Maybe: nil, Empty: []string{}, Credential: SecretEnv("APP_SECRET"), Classified: SecretEnv("APP_SECRET"),
+				}
+			},
+			Secret: func(path Path) bool {
+				return path.String() == "/dynamic/retired" || path.String() == "/classified"
+			},
+			Resolve: func(configuration *explanationConfig, resolution *Resolution) error {
+				Normalize(resolution, Keys("concurrency"), &configuration.Concurrency, 3,
+					"normalized\nconcurrency", Keys("labels"))
+				Normalize(resolution, Keys("equal"), &configuration.Equal, "same", "record equal value")
+				Normalize(resolution, Keys("derived_a"), &configuration.DerivedA, "DERIVED_EXPLAIN_A",
+					"derived cycle a", Keys("derived_b"))
+				Normalize(resolution, Keys("derived_b"), &configuration.DerivedB, "DERIVED_EXPLAIN_B",
+					"derived cycle b", Keys("derived_a"), Keys("dynamic"))
+				return nil
+			},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	concurrency, ok := result.Explain(Keys("concurrency"))
+	if !ok {
+		t.Fatal("concurrency was not explained")
+	}
+	if got := explanationValues(concurrency); !reflect.DeepEqual(got, []string{"4\n", "8\n", "1\n", "2\n", "3\n"}) {
+		t.Fatalf("concurrency history = %#v", got)
+	}
+	if got := historyValueLayers(concurrency); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerBase, LayerProfile, LayerCLI, LayerResolution}) {
+		t.Fatalf("concurrency layers = %#v", got)
+	}
+	if concurrency.History[4].Before.YAML != "2\n" || concurrency.History[4].Reasons[0] != "normalized\nconcurrency" ||
+		concurrency.History[4].Dependencies[0].String() != "/labels" {
+		t.Fatalf("resolution history = %#v", concurrency.History[4])
+	}
+
+	equal, _ := result.Explain(Keys("equal"))
+	if len(equal.History) != 5 {
+		t.Fatalf("equal writes were lost: %#v", equal.History)
+	}
+	for _, change := range equal.History[1:] {
+		if change.Before.YAML != "same\n" || change.After.YAML != "same\n" {
+			t.Fatalf("equal replacement = %#v", change)
+		}
+	}
+
+	limit, _ := result.Explain(Keys("limit"))
+	if got := explanationValues(limit); !reflect.DeepEqual(got, []string{"0\n", "wrong-before-profile\n", "7\n"}) {
+		t.Fatalf("overridden invalid input was not retained: %#v", got)
+	}
+	removed, ok := result.Explain(Keys("dynamic", "retired"))
+	if !ok || removed.Effective.Present || len(removed.History) < 2 {
+		t.Fatalf("removed path = %#v, %v", removed, ok)
+	}
+	for _, change := range removed.History {
+		if strings.Contains(change.Before.YAML+change.After.YAML, "HISTORICAL_EXPLAIN_CANARY") {
+			t.Fatalf("historical secret escaped redaction: %#v", change)
+		}
+	}
+	if !removed.History[0].After.Redacted || removed.History[len(removed.History)-1].After.Present {
+		t.Fatalf("removed secret history = %#v", removed.History)
+	}
+	dynamic, _ := result.Explain(Keys("dynamic"))
+	if len(dynamic.History) < 3 || !dynamic.History[1].After.Redacted ||
+		strings.Contains(dynamic.History[1].After.YAML, "HISTORICAL_EXPLAIN_CANARY") {
+		t.Fatalf("container redaction history = %#v", dynamic.History)
+	}
+
+	items, _ := result.Explain(Keys("items"))
+	if len(items.History) != 4 || !strings.Contains(items.History[1].After.YAML, "removed") ||
+		strings.Contains(items.History[2].After.YAML, "removed") || !strings.Contains(items.History[3].After.YAML, "restored") {
+		t.Fatalf("atomic sequence history = %#v", items.History)
+	}
+	removedIndex, ok := result.Explain(Keys("items").Index(1))
+	if !ok || !removedIndex.History[1].Before.Present || removedIndex.History[1].After.Present ||
+		!removedIndex.History[2].After.Present {
+		t.Fatalf("sequence descendant history = %#v", removedIndex.History)
+	}
+	labels, _ := result.Explain(Keys("labels"))
+	if len(labels.History) != 4 || !strings.Contains(labels.Effective.YAML, "default: retained") ||
+		!strings.Contains(labels.Effective.YAML, "profile: retained") || !strings.Contains(labels.Effective.YAML, "cli: retained") {
+		t.Fatalf("composite mapping history = %#v", labels)
+	}
+	if labels.Origin.Layer != LayerCLI || len(labels.Origin.Locations) != 3 {
+		t.Fatalf("composite effective origin = %#v", labels.Origin)
+	}
+
+	maybe, _ := result.Explain(Keys("maybe"))
+	empty, _ := result.Explain(Keys("empty"))
+	if !maybe.Effective.Present || maybe.Effective.YAML != "null\n" || !empty.Effective.Present || empty.Effective.YAML != "[]\n" {
+		t.Fatalf("null/empty snapshots = maybe %#v empty %#v", maybe.Effective, empty.Effective)
+	}
+	credential, _ := result.Explain(Keys("credential"))
+	if credential.SecretReference == nil || credential.SecretReference.Name != "APP_SECRET" || credential.Effective.YAML != "APP_SECRET\n" {
+		t.Fatalf("secret reference = %#v", credential)
+	}
+	classified, _ := result.Explain(Keys("classified"))
+	if classified.SecretReference != nil || !classified.Effective.Redacted ||
+		strings.Contains(classified.Effective.YAML, "APP_SECRET") {
+		t.Fatalf("classified secret reference = %#v", classified)
+	}
+	var derivedExplanations []Explanation
+	for _, path := range []Path{Keys("derived_a"), Keys("derived_b")} {
+		derived, _ := result.Explain(path)
+		if !derived.Effective.Redacted || strings.Contains(derived.Effective.YAML, "DERIVED_EXPLAIN") {
+			t.Fatalf("dependency closure at %s = %#v", path.String(), derived)
+		}
+		derivedExplanations = append(derivedExplanations, derived)
+	}
+
+	formatted := fmt.Sprintf("%v %#v %v", removed, credential, derivedExplanations)
+	encoded, marshalErr := json.Marshal(append([]Explanation{removed, credential, classified}, derivedExplanations...))
+	if marshalErr != nil || strings.Contains(formatted+string(encoded), "RESOLVED_EXPLAIN_CANARY") ||
+		strings.Contains(formatted+string(encoded), "HISTORICAL_EXPLAIN_CANARY") ||
+		strings.Contains(formatted+string(encoded), "DERIVED_EXPLAIN") {
+		t.Fatalf("public records disclosed a canary: %s %s", formatted, encoded)
+	}
+
+	concurrency.Path.segments[0].key = "mutated"
+	concurrency.History[4].Reasons[0] = "mutated"
+	concurrency.History[4].Dependencies[0].segments[0].key = "mutated"
+	result.Config.Concurrency = 99
+	again, _ := result.Explain(Keys("concurrency"))
+	if again.Path.String() != "/concurrency" || again.Effective.YAML != "3\n" ||
+		again.History[4].Reasons[0] != "normalized\nconcurrency" ||
+		again.History[4].Dependencies[0].String() != "/labels" {
+		t.Fatalf("returned explanation aliased internal history: %#v", again)
+	}
+
+	var first, second bytes.Buffer
+	if err := result.WriteExplanation(&first, Keys("concurrency")); err != nil {
+		t.Fatal(err)
+	}
+	if err := result.WriteExplanation(&second, Keys("concurrency")); err != nil || first.String() != second.String() {
+		t.Fatalf("nondeterministic rendering: %v\n%s\n%s", err, first.String(), second.String())
+	}
+	if strings.Contains(first.String(), "\nconcurrency\"") || !strings.Contains(first.String(), `"normalized\nconcurrency"`) {
+		t.Fatalf("reason controls were not escaped: %q", first.String())
+	}
+	unknownOutput := bytes.NewBufferString("unchanged")
+	if err := result.WriteExplanation(unknownOutput, Keys("unknown")); !errors.Is(err, errExplanationUnknownPath) || unknownOutput.String() != "unchanged" {
+		t.Fatalf("unknown writer = %q, %v", unknownOutput.String(), err)
+	}
+	if _, ok := result.Explain(Keys("items").Index(-1)); ok {
+		t.Fatal("malformed path was explained")
+	}
+	wantWriteError := errors.New("write failed")
+	if err := result.WriteExplanation(errorWriter{err: wantWriteError}, Keys("concurrency")); !errors.Is(err, wantWriteError) {
+		t.Fatalf("writer error = %v", err)
+	}
+	if err := result.WriteExplanation(shortWriter{}, Keys("concurrency")); !errors.Is(err, io.ErrShortWrite) {
+		t.Fatalf("short writer error = %v", err)
+	}
+	var invalidInput bytes.Buffer
+	if err := result.WriteExplanation(&invalidInput, Keys("limit")); err != nil ||
+		!strings.Contains(invalidInput.String(), "policy-validated input") ||
+		strings.Contains(invalidInput.String(), "wrong-before-profile\n    source: layer=\"base\"\n    before (canonical typed)") {
+		t.Fatalf("historical representation rendering = %v\n%s", err, invalidInput.String())
+	}
+}
+
+func TestSecretsBootstrapLoadAndExplanationRemainSeparateAndSafe(t *testing.T) {
+	const name = "CONFIGKIT_EXPLAIN_BOOTSTRAP_TEST"
+	previous, present := os.LookupEnv(name)
+	if err := os.Unsetenv(name); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if present {
+			_ = os.Setenv(name, previous)
+		} else {
+			_ = os.Unsetenv(name)
+		}
+	})
+	directory := t.TempDir()
+	writeSecretTestFile(t, directory, name, "BOOTSTRAP_EXPLAIN_CANARY\n")
+	report, err := LoadSecretsDir(directory)
+	if err != nil || len(report.Entries()) != 1 || report.Entries()[0].Status != SecretFileLoaded {
+		t.Fatalf("bootstrap = %#v, %v", report, err)
+	}
+	type config struct {
+		Credential Secret `yaml:"credential"`
+	}
+	result, err := Load[config](Bytes("config.yml", []byte("credential: "+name+"\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var output bytes.Buffer
+	if err := result.WriteExplanation(&output, Keys("credential")); err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(output.String(), name) || strings.Contains(output.String(), "BOOTSTRAP_EXPLAIN_CANARY") {
+		t.Fatalf("explanation mixed bootstrap value into reference report: %s", output.String())
+	}
+}
+
+func TestHistoryNodeBudgetCountsSharedTreesOnce(t *testing.T) {
+	leaf := &document.Node{Kind: document.String, Text: "value"}
+	root := &document.Node{Kind: document.Mapping, Mapping: []document.Entry{{Key: "value", Value: leaf}}}
+	applications := []historyApplication{{layer: LayerBase, value: root}}
+	if !historyNodesWithin(2, root, root, root, applications, nil) {
+		t.Fatal("structurally shared history was counted more than once")
+	}
+	other := document.Clone(root)
+	if historyNodesWithin(3, root, other, root, applications, nil) {
+		t.Fatal("distinct retained nodes did not consume the budget")
+	}
+}
+
+func explanationValues(explanation Explanation) []string {
+	values := make([]string, len(explanation.History))
+	for index, change := range explanation.History {
+		values[index] = change.After.YAML
+	}
+	return values
+}
+
+func historyValueLayers(explanation Explanation) []Layer {
+	layers := make([]Layer, len(explanation.History))
+	for index, change := range explanation.History {
+		layers[index] = change.Origin.Layer
+	}
+	return layers
+}
+
+func TestExplanationReconcilesDefaultsAfterInputReplacement(t *testing.T) {
+	type cfg struct {
+		M map[string]int `yaml:"m"`
+	}
+	for _, base := range []string{"null", "replaced-text"} {
+		for _, cli := range []bool{false, true} {
+			t.Run(fmt.Sprintf("%s/cli=%v", base, cli), func(t *testing.T) {
+				root := t.TempDir()
+				main := writeImportFile(t, root, "main.yml", "m: "+base+"\n")
+				opts := Options[cfg]{Definition: Definition[cfg]{Defaults: func() cfg { return cfg{M: map[string]int{"a": 1}} }}}
+				if cli {
+					opts.Overrides = []Override{{Path: Keys("m"), Value: map[string]any{"b": 2}, Flag: "--m"}}
+				} else {
+					writeImportFile(t, root, "profiles/p.yml", "m: {b: 2}\n")
+					opts.Profile = "p"
+				}
+				result, err := Load(File(main), opts)
+				if err != nil {
+					t.Fatal(err)
+				}
+				for _, path := range []Path{Keys("m"), Keys("m", "a")} {
+					explanation, ok := result.Explain(path)
+					if !ok {
+						t.Fatal("missing explanation")
+					}
+					last := explanation.History[len(explanation.History)-1]
+					if last.After != explanation.Effective || last.Origin.Operation != ChangeDecoding {
+						t.Fatalf("history does not reconcile final value: %#v", explanation)
+					}
+					info, _ := result.Metadata().Lookup(path)
+					if last.Origin.Layer != info.Origin.Layer {
+						t.Fatal("decoding lost effective origin")
+					}
+				}
+			})
+		}
+	}
+}
+
+func TestIntegerVersionInspectionRetainsInputAndReconcilesTypedValue(t *testing.T) {
+	type config struct {
+		Version string `yaml:"version"`
+	}
+	rule := &VersionRule{Path: Keys("version"), Check: func(version Version, _ *Validation) error {
+		if version.Text != "4" {
+			t.Errorf("version = %#v", version)
+		}
+		return nil
+	}}
+	options := Options[config]{Definition: Definition[config]{
+		Defaults: func() config { return config{Version: "4"} },
+		Version:  rule,
+	}}
+	first, err := Load(Bytes("version.yml", []byte("version: 4\n")), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := Load(Bytes("version.yml", []byte("version: 4\n")), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if first.Config.Version != "4" || second.Config.Version != "4" {
+		t.Fatalf("configurations = %#v / %#v", first.Config, second.Config)
+	}
+	inputVersion, inputOK := documentAtPath(first.applications[0].value, Keys("version"))
+	finalVersion, finalOK := documentAtPath(first.final, Keys("version"))
+	if !inputOK || inputVersion.Kind != document.Number || inputVersion.Text != "4" ||
+		!finalOK || finalVersion.Kind != document.String || finalVersion.Text != "4" {
+		t.Fatalf("input/final = %#v / %#v", inputVersion, finalVersion)
+	}
+
+	info, ok := first.Metadata().Lookup(Keys("version"))
+	if !ok || !info.Explicit || info.Null || info.Origin.Layer != LayerBase || len(info.Origin.Locations) != 1 ||
+		info.Origin.Locations[0].SourceID != "source-1" || info.Origin.Locations[0].Line != 1 {
+		t.Fatalf("version metadata = %#v, %v", info, ok)
+	}
+	explanation, ok := first.Explain(Keys("version"))
+	if !ok || explanation.Effective.YAML != "'4'\n" {
+		t.Fatalf("version explanation = %#v, %v", explanation, ok)
+	}
+	foundInput, foundDecoding := false, false
+	for _, change := range explanation.History {
+		if change.After.YAML == "4\n" && change.Origin.Layer == LayerBase {
+			foundInput = true
+		}
+		if change.Before.YAML == "4\n" && change.After.YAML == "'4'\n" && change.Origin.Operation == ChangeDecoding {
+			foundDecoding = true
+		}
+		if change.Origin.Layer == LayerResolution {
+			t.Fatalf("conversion recorded as resolution: %#v", change)
+		}
+	}
+	if !foundInput || !foundDecoding {
+		t.Fatalf("version history = %#v", explanation.History)
+	}
+	paths := first.Metadata().HistoricalPaths()
+	foundPath := false
+	for _, path := range paths {
+		if path.String() == "/version" {
+			foundPath = true
+		}
+	}
+	if !foundPath || len(first.Metadata().History(Keys("version"))) < 2 {
+		t.Fatalf("historical version paths/history = %#v / %#v", paths, first.Metadata().History(Keys("version")))
+	}
+
+	var effective strings.Builder
+	if err := first.WriteEffective(&effective); err != nil || effective.String() != "version: '4'\n" {
+		t.Fatalf("effective output = %q, %v", effective.String(), err)
+	}
+	first.Config.Version = "mutated"
+	effective.Reset()
+	if err := first.WriteEffective(&effective); err != nil || effective.String() != "version: '4'\n" || second.Config.Version != "4" {
+		t.Fatalf("detached results = %q / %#v, %v", effective.String(), second.Config, err)
+	}
+}
+
+func TestSensitiveIntegerVersionStaysRedactedWithoutSecretLookup(t *testing.T) {
+	type config struct {
+		Version string `yaml:"version"`
+	}
+	lookups := 0
+	result, err := loadWithSecretLookup(Bytes("version.yml", []byte("version: 987654321\n")), func(string) (string, bool) {
+		lookups++
+		return "CREDENTIAL_CANARY", true
+	}, Options[config]{Definition: Definition[config]{
+		Version: &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error { return nil }},
+		Secret:  func(path Path) bool { return path.String() == "/version" },
+	}})
+	if err != nil || result.Config.Version != "987654321" || lookups != 0 {
+		t.Fatalf("result = %#v, lookups = %d, err = %v", result, lookups, err)
+	}
+	var effective strings.Builder
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	explanation, ok := result.Explain(Keys("version"))
+	formatted := effective.String() + fmt.Sprintf("%#v %#v", explanation, result.Metadata().History(Keys("version")))
+	if !ok || !explanation.Effective.Redacted || strings.Contains(formatted, "987654321") || strings.Contains(formatted, "CREDENTIAL_CANARY") {
+		t.Fatalf("sensitive inspection = %q / %#v", effective.String(), explanation)
+	}
+}
+
+func TestExplanationReplaysResolutionAdditionsAndOverlappingIntent(t *testing.T) {
+	type cfg struct {
+		M map[string]any `yaml:"m"`
+	}
+	for _, overlap := range []bool{false, true} {
+		t.Run(fmt.Sprint(overlap), func(t *testing.T) {
+			result, err := Load(Bytes("base", []byte("m: {old: 1}\n")), Options[cfg]{Definition: Definition[cfg]{Resolve: func(c *cfg, r *Resolution) error {
+				c.M["new"] = map[string]any{"child": int64(7)}
+				delete(c.M, "old")
+				r.Record(Keys("m", "new"), ChangeDerivedDefault, "add nested", Keys("m", "old"))
+				r.Record(Keys("m", "old"), ChangeNormalization, "remove old")
+				if overlap {
+					r.Record(Keys("m"), ChangeNormalization, "update container")
+				}
+				return nil
+			}}})
+			if err != nil {
+				t.Fatal(err)
+			}
+			for _, path := range []Path{Keys("m"), Keys("m", "new"), Keys("m", "new", "child"), Keys("m", "old")} {
+				x, ok := result.Explain(path)
+				if !ok {
+					t.Fatal("missing known history")
+				}
+				var count int
+				for _, v := range x.History {
+					if v.Origin.Layer == LayerResolution {
+						count++
+						if overlap && len(v.Reasons) != 3 {
+							t.Fatal("lost overlapping intent")
+						}
+					}
+				}
+				if overlap && count != 1 {
+					t.Fatalf("overlapping records applied %d times", count)
+				}
+				last := x.History[len(x.History)-1]
+				if last.After != x.Effective {
+					t.Fatalf("incorrect replay for %s: %#v", path, x)
+				}
+			}
+		})
+	}
+}

filesystem.go

@@ -0,0 +1,121 @@
+package configkit
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func validateFilesystemContext(source Source, context *FilesystemContext, main parsedSource) *Diagnostic {
+	rootPath, _, _, diagnostic := importFilesystem(source, context, main)
+	if diagnostic != nil {
+		return diagnostic
+	}
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		value := globFilesystemDiagnostic(err, "configuration root could not be opened", Location{})
+		value.Primary = nil
+		return &value
+	}
+	if err := root.Close(); err != nil {
+		value := importDiagnostic(CodeImportRead, "configuration root could not be closed", nil)
+		return &value
+	}
+	return nil
+}
+
+func filesystemOptionDiagnostics(source Source, context *FilesystemContext) []Diagnostic {
+	switch source.kind {
+	case sourceNone:
+		return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "NoSource cannot use filesystem context")}
+	case sourceFile:
+		if context.BaseDir != "" {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "file sources determine their own import base directory")}
+		}
+	case sourceBytes, sourceReader:
+		if context.BaseDir == "" || context.Root == "" {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "byte and reader filesystem context requires base and root directories")}
+		}
+		if !filepath.IsAbs(context.BaseDir) || !filepath.IsAbs(context.Root) {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "byte and reader filesystem base and root directories must be absolute")}
+		}
+	}
+	return nil
+}
+
+func importFilesystem(source Source, context *FilesystemContext, main parsedSource) (rootPath, basePath, mainPath string, diagnostic *Diagnostic) {
+	if main.source == nil || main.document == nil {
+		value := importDiagnostic(CodeImportInvalid, "imports require a selected configuration source", nil)
+		return "", "", "", &value
+	}
+
+	if source.kind == sourceFile {
+		if context != nil && context.BaseDir != "" {
+			value := optionDiagnostic(CodeOptionsInvalid, "file sources determine their own import base directory")
+			return "", "", "", &value
+		}
+		canonical, err := canonicalExistingPath(source.path)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "selected source path could not be canonicalized", nil)
+			return "", "", "", &value
+		}
+		mainPath = canonical
+		basePath = filepath.Dir(canonical)
+		rootPath = basePath
+		if context != nil && context.Root != "" {
+			rootPath, err = canonicalExistingPath(context.Root)
+			if err != nil {
+				value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
+				return "", "", "", &value
+			}
+		}
+	} else {
+		if context == nil || context.BaseDir == "" || context.Root == "" {
+			value := optionDiagnostic(CodeOptionsInvalid, "byte and reader imports require absolute base and root directories")
+			return "", "", "", &value
+		}
+		if !filepath.IsAbs(context.BaseDir) || !filepath.IsAbs(context.Root) {
+			value := optionDiagnostic(CodeOptionsInvalid, "byte and reader import base and root directories must be absolute")
+			return "", "", "", &value
+		}
+		var err error
+		basePath, err = canonicalExistingPath(context.BaseDir)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "import base directory could not be canonicalized", nil)
+			return "", "", "", &value
+		}
+		rootPath, err = canonicalExistingPath(context.Root)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
+			return "", "", "", &value
+		}
+	}
+	if !containedPath(rootPath, basePath) || (mainPath != "" && !containedPath(rootPath, mainPath)) {
+		value := importDiagnostic(CodeImportOutsideRoot, "selected source and import base must stay inside the configuration root", nil)
+		return "", "", "", &value
+	}
+	for _, candidate := range []string{rootPath, basePath} {
+		info, err := os.Stat(candidate)
+		if err != nil || !info.IsDir() {
+			value := importDiagnostic(CodeImportInvalid, "configuration root and import base must be directories", nil)
+			return "", "", "", &value
+		}
+	}
+	return rootPath, basePath, mainPath, nil
+}
+
+func canonicalExistingPath(name string) (string, error) {
+	absolute, err := filepath.Abs(name)
+	if err != nil {
+		return "", err
+	}
+	return filepath.EvalSymlinks(filepath.Clean(absolute))
+}
+
+func containedPath(root, target string) bool {
+	relative, err := filepath.Rel(root, target)
+	if err != nil || filepath.IsAbs(relative) {
+		return false
+	}
+	return relative != ".." && !strings.HasPrefix(relative, ".."+string(filepath.Separator))
+}

glob.go

@@ -0,0 +1,205 @@
+package configkit
+
+import (
+	"errors"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"slices"
+	"strings"
+)
+
+const globReadBatch = 128
+
+type globCandidate struct {
+	absolute string
+	relative string
+}
+
+func (s *importTraversal) expandImportGlob(basePath string, declaration importDeclaration) ([]string, []Diagnostic) {
+	location := declaration.location
+	segments, valid := validGlobPattern(declaration.glob)
+	if !valid {
+		return nil, []Diagnostic{importDiagnostic(CodeImportInvalid, "import glob pattern is invalid or uses recursive ** syntax", &location)}
+	}
+
+	candidates := []globCandidate{{absolute: basePath}}
+	for index, segment := range segments {
+		last := index == len(segments)-1
+		literal, patterned := globLiteralSegment(segment)
+		var next []globCandidate
+		for _, candidate := range candidates {
+			var names []string
+			if patterned {
+				matched, diagnostics := s.readGlobMatches(candidate.absolute, segment, location)
+				if len(diagnostics) != 0 {
+					return nil, diagnostics
+				}
+				names = matched
+			} else {
+				names = []string{literal}
+			}
+			for _, name := range names {
+				absolute := filepath.Join(candidate.absolute, filepath.FromSlash(name))
+				relative := path.Join(candidate.relative, name)
+				if !containedPath(s.rootPath, filepath.Clean(absolute)) {
+					return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob traversal is outside the configuration root", &location)}
+				}
+				canonical, err := canonicalExistingPath(absolute)
+				if err != nil {
+					if errors.Is(err, fs.ErrNotExist) && !patterned {
+						continue
+					}
+					return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob match could not be canonicalized", location)}
+				}
+				if !containedPath(s.rootPath, canonical) {
+					return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob traversal resolves outside the configuration root", &location)}
+				}
+				if last {
+					next = append(next, globCandidate{absolute: canonical, relative: relative})
+					if len(s.sources)+len(next) > s.limits.sources {
+						return nil, []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "glob matches exceed the remaining source-count limit", &location)}
+					}
+					continue
+				}
+				info, err := os.Stat(canonical)
+				if err != nil {
+					return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob traversal target could not be inspected", location)}
+				}
+				if info.IsDir() {
+					next = append(next, globCandidate{absolute: canonical, relative: relative})
+				}
+			}
+		}
+		candidates = next
+		if len(candidates) == 0 {
+			return nil, nil
+		}
+	}
+
+	matches := make([]string, len(candidates))
+	for index, candidate := range candidates {
+		matches[index] = candidate.relative
+	}
+	slices.Sort(matches)
+	return matches, nil
+}
+
+func (s *importTraversal) readGlobMatches(directory, pattern string, location Location) ([]string, []Diagnostic) {
+	canonical, err := canonicalExistingPath(directory)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return nil, nil
+		}
+		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be canonicalized", location)}
+	}
+	if !containedPath(s.rootPath, canonical) {
+		return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob directory resolves outside the configuration root", &location)}
+	}
+	relative, err := filepath.Rel(s.rootPath, canonical)
+	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+		return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob directory could not be addressed inside the configuration root", &location)}
+	}
+	directoryFile, err := s.root.Open(relative)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return nil, nil
+		}
+		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be opened", location)}
+	}
+
+	var matches []string
+	for {
+		entries, readErr := directoryFile.ReadDir(globReadBatch)
+		for _, entry := range entries {
+			s.usedEntries++
+			if s.usedEntries > s.limits.entries {
+				_ = directoryFile.Close()
+				return nil, []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "glob traversal exceeds the directory-entry visit limit", &location)}
+			}
+			matched, matchErr := path.Match(pattern, entry.Name())
+			if matchErr != nil {
+				_ = directoryFile.Close()
+				return nil, []Diagnostic{importDiagnostic(CodeImportInvalid, "import glob pattern is invalid", &location)}
+			}
+			if matched {
+				matches = append(matches, entry.Name())
+			}
+		}
+		if errors.Is(readErr, io.EOF) {
+			break
+		}
+		if readErr != nil {
+			_ = directoryFile.Close()
+			return nil, []Diagnostic{globFilesystemDiagnostic(readErr, "glob directory could not be read", location)}
+		}
+	}
+	if err := directoryFile.Close(); err != nil {
+		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be closed", location)}
+	}
+	slices.Sort(matches)
+	return matches, nil
+}
+
+func validGlobPattern(pattern string) ([]string, bool) {
+	if !validImportPath(pattern) {
+		return nil, false
+	}
+	segments := strings.Split(pattern, "/")
+	for _, segment := range segments {
+		if segment == "" || hasRecursiveStars(segment) {
+			return nil, false
+		}
+		if _, err := path.Match(segment, ""); err != nil {
+			return nil, false
+		}
+	}
+	return segments, true
+}
+
+func hasRecursiveStars(segment string) bool {
+	inClass := false
+	for index := 0; index < len(segment); index++ {
+		switch segment[index] {
+		case '\\':
+			index++
+		case '[':
+			inClass = true
+		case ']':
+			inClass = false
+		case '*':
+			if !inClass && index+1 < len(segment) && segment[index+1] == '*' {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func globLiteralSegment(segment string) (string, bool) {
+	var literal strings.Builder
+	for index := 0; index < len(segment); index++ {
+		switch segment[index] {
+		case '*', '?', '[':
+			return "", true
+		case '\\':
+			index++
+			if index < len(segment) {
+				literal.WriteByte(segment[index])
+			}
+		default:
+			literal.WriteByte(segment[index])
+		}
+	}
+	return literal.String(), false
+}
+
+func globFilesystemDiagnostic(err error, message string, location Location) Diagnostic {
+	code := CodeImportRead
+	if errors.Is(err, fs.ErrPermission) {
+		code = CodeImportPermission
+	}
+	return importDiagnostic(code, message, &location)
+}

glob_test.go

@@ -0,0 +1,273 @@
+package configkit
+
+import (
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"testing"
+)
+
+func TestLoadGlobImportsUseLexicalDeclarationOrderAndNestedDFS(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - glob: fragments/*.yml
+    - path: after.yml
+items: {root: root}
+`)
+	aPath := writeImportFile(t, directory, "fragments/a.yml", `configkit:
+  imports: [{path: nested/child.yml}]
+items: {alpha: alpha}
+`)
+	childPath := writeImportFile(t, directory, "fragments/nested/child.yml", "items: {child: child}\n")
+	bPath := writeImportFile(t, directory, "fragments/b.yml", "items: {beta: beta}\n")
+	afterPath := writeImportFile(t, directory, "after.yml", "items: {after: after}\n")
+
+	var previous []SourceInfo
+	for run := 0; run < 2; run++ {
+		result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{Imports: true})
+		if err != nil {
+			t.Fatal(err)
+		}
+		wantItems := map[string]string{"root": "root", "alpha": "alpha", "child": "child", "beta": "beta", "after": "after"}
+		if !reflect.DeepEqual(result.Config.Items, wantItems) {
+			t.Fatalf("items = %#v, want %#v", result.Config.Items, wantItems)
+		}
+		sources := result.Sources()
+		wantPaths := []string{
+			canonicalTestPath(t, mainPath), canonicalTestPath(t, aPath),
+			canonicalTestPath(t, childPath), canonicalTestPath(t, bPath),
+			canonicalTestPath(t, afterPath),
+		}
+		gotPaths := make([]string, len(sources))
+		for index, source := range sources {
+			gotPaths[index] = source.Path
+		}
+		if !reflect.DeepEqual(gotPaths, wantPaths) {
+			t.Fatalf("source paths = %#v, want %#v", gotPaths, wantPaths)
+		}
+		if run != 0 && !reflect.DeepEqual(sources, previous) {
+			t.Fatalf("repeated traversal changed sources: %#v != %#v", sources, previous)
+		}
+		previous = sources
+	}
+}
+
+func TestLoadGlobImportsSupportEscapesClassesAndExplicitConfD(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - glob: 'conf.d/literal\*.yml'
+    - glob: 'conf.d/item-[ab].yml'
+    - glob: 'conf.d/question?.yml'
+items: {root: root}
+`)
+	writeImportFile(t, directory, "conf.d/literal*.yml", "items: {literal: literal}\n")
+	writeImportFile(t, directory, "conf.d/item-b.yml", "items: {b: b}\n")
+	writeImportFile(t, directory, "conf.d/item-a.yml", "items: {a: a}\n")
+	writeImportFile(t, directory, "conf.d/question1.yml", "items: {question: question}\n")
+
+	result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{Imports: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := map[string]string{"root": "root", "literal": "literal", "a": "a", "b": "b", "question": "question"}; !reflect.DeepEqual(result.Config.Items, want) {
+		t.Fatalf("items = %#v, want %#v", result.Config.Items, want)
+	}
+
+	ignoredPath := writeImportFile(t, directory, "conf.d/not-discovered.yml", "unknown: value\n")
+	_ = ignoredPath
+	plainPath := writeImportFile(t, directory, "plain.yml", "items: {root: only}\n")
+	plain, err := Load[importTestConfig](File(plainPath), Options[importTestConfig]{Imports: true})
+	if err != nil || len(plain.Sources()) != 1 || plain.Config.Items["root"] != "only" {
+		t.Fatalf("implicit discovery result = %#v, %v", plain, err)
+	}
+}
+
+func TestLoadGlobImportsRejectInvalidAndUnmatchedPatterns(t *testing.T) {
+	tests := []struct {
+		name    string
+		pattern string
+		code    Code
+	}{
+		{name: "bad class", pattern: "empty/[", code: CodeImportInvalid},
+		{name: "recursive star segment", pattern: "empty/**/value.yml", code: CodeImportInvalid},
+		{name: "recursive adjacent stars", pattern: "empty/value**.yml", code: CodeImportInvalid},
+		{name: "trailing separator", pattern: "empty/", code: CodeImportInvalid},
+		{name: "absolute", pattern: "/tmp/*.yml", code: CodeImportInvalid},
+		{name: "URL", pattern: "https://example.invalid/*.yml", code: CodeImportInvalid},
+		{name: "missing parent escape", pattern: "../missing/*.yml", code: CodeImportOutsideRoot},
+		{name: "unmatched", pattern: "empty/*.yml", code: CodeImportMissing},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			if err := os.Mkdir(filepath.Join(directory, "empty"), 0o755); err != nil {
+				t.Fatal(err)
+			}
+			mainPath := writeGlobRoot(t, directory, test.pattern, false)
+			requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, test.code)
+		})
+	}
+
+	directory := t.TempDir()
+	if err := os.Mkdir(filepath.Join(directory, "empty"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	mainPath := writeGlobRoot(t, directory, "empty/*.yml", true)
+	result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{Imports: true})
+	if err != nil || len(result.Sources()) != 1 {
+		t.Fatalf("optional unmatched glob = %#v, %v", result, err)
+	}
+}
+
+func TestLoadGlobImportsPreserveIdentityAndFileRules(t *testing.T) {
+	t.Run("overlapping declarations", func(t *testing.T) {
+		directory := t.TempDir()
+		writeImportFile(t, directory, "conf/value.yml", "items: {value: value}\n")
+		mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - glob: conf/*.yml
+    - glob: conf/value.*
+`)
+		requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportRepeated)
+	})
+
+	t.Run("matched directory", func(t *testing.T) {
+		directory := t.TempDir()
+		if err := os.MkdirAll(filepath.Join(directory, "conf/matched.yml"), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		mainPath := writeGlobRoot(t, directory, "conf/*.yml", true)
+		requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeSourceNotRegular)
+	})
+
+	t.Run("hard links stay distinct", func(t *testing.T) {
+		directory := t.TempDir()
+		one := writeImportFile(t, directory, "conf/one.yml", "items: {same: value}\n")
+		if err := os.Link(one, filepath.Join(directory, "conf/two.yml")); err != nil {
+			t.Skipf("hard links unavailable: %v", err)
+		}
+		mainPath := writeGlobRoot(t, directory, "conf/*.yml", false)
+		requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeComposeConflict)
+	})
+}
+
+func TestLoadGlobImportsConfineSymlinkDirectories(t *testing.T) {
+	if runtime.GOOS != "linux" && runtime.GOOS != "darwin" {
+		t.Skip("initial filesystem contract covers Linux and macOS")
+	}
+	t.Run("contained absolute target", func(t *testing.T) {
+		root := t.TempDir()
+		target := filepath.Join(root, "target")
+		writeImportFile(t, target, "value.yml", "items: {linked: linked}\n")
+		if err := os.Symlink(target, filepath.Join(root, "linked")); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+		mainPath := writeGlobRoot(t, root, "linked/*.yml", false)
+		result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{Imports: true})
+		if err != nil || result.Config.Items["linked"] != "linked" {
+			t.Fatalf("contained directory symlink = %#v, %v", result, err)
+		}
+	})
+
+	t.Run("escaping target", func(t *testing.T) {
+		root := t.TempDir()
+		outside := t.TempDir()
+		writeImportFile(t, outside, "value.yml", "items: {escaped: escaped}\n")
+		if err := os.Symlink(outside, filepath.Join(root, "linked")); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+		mainPath := writeGlobRoot(t, root, "linked/*.yml", true)
+		requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportOutsideRoot)
+	})
+}
+
+func TestOptionalGlobDoesNotSuppressDirectoryPermissionFailure(t *testing.T) {
+	directory := t.TempDir()
+	locked := filepath.Join(directory, "locked")
+	if err := os.Mkdir(locked, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chmod(locked, 0); err != nil {
+		t.Skipf("cannot create unreadable directory fixture: %v", err)
+	}
+	defer func() { _ = os.Chmod(locked, 0o700) }()
+	if opened, err := os.Open(locked); err == nil {
+		_ = opened.Close()
+		t.Skip("filesystem does not enforce the unreadable-directory fixture")
+	}
+	mainPath := writeGlobRoot(t, directory, "locked/*.yml", true)
+	requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportPermission)
+}
+
+func TestGlobTraversalBoundsNonmatchesAndAcceptedCandidates(t *testing.T) {
+	directory := t.TempDir()
+	for _, name := range []string{"one.txt", "two.txt", "three.txt"} {
+		writeImportFile(t, directory, "many/"+name, "ignored\n")
+	}
+	mainPath := writeGlobRoot(t, directory, "many/*.yml", true)
+	main := parseImportMain(t, mainPath)
+	limits := defaultImportLimits()
+	limits.entries = 2
+	_, _, diagnostics := loadImportedDocuments(File(mainPath), nil, main, nil, limits)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeImportLimitExceeded {
+		t.Fatalf("entry-budget diagnostics = %#v", diagnostics)
+	}
+
+	for _, name := range []string{"a.yml", "b.yml"} {
+		writeImportFile(t, directory, "matches/"+name, "items: {"+name[:1]+": value}\n")
+	}
+	mainPath = writeGlobRoot(t, directory, "matches/*.yml", false)
+	main = parseImportMain(t, mainPath)
+	limits = defaultImportLimits()
+	limits.sources = 2
+	_, _, diagnostics = loadImportedDocuments(File(mainPath), nil, main, nil, limits)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeImportLimitExceeded {
+		t.Fatalf("candidate-budget diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestFilesystemContextIsValidatedWithoutImportsOrActiveProfile(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", "items: {root: root}\n")
+	result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{
+		Profile: "default", Filesystem: &FilesystemContext{Root: directory},
+	})
+	if err != nil || result.Config.Items["root"] != "root" {
+		t.Fatalf("file filesystem context = %#v, %v", result, err)
+	}
+	requireLoadCode(t, File(mainPath), Options[importTestConfig]{Filesystem: &FilesystemContext{BaseDir: directory, Root: directory}}, CodeOptionsInvalid)
+	requireLoadCode(t, File(mainPath), Options[importTestConfig]{Filesystem: &FilesystemContext{Root: filepath.Join(directory, "missing")}}, CodeImportInvalid)
+
+	input := Bytes("memory.yml", []byte("items: {root: memory}\n"))
+	result, err = Load[importTestConfig](input, Options[importTestConfig]{Filesystem: &FilesystemContext{BaseDir: directory, Root: directory}})
+	if err != nil || result.Config.Items["root"] != "memory" {
+		t.Fatalf("byte filesystem context = %#v, %v", result, err)
+	}
+	requireLoadCode(t, Bytes("memory.yml", []byte("{}\n")), Options[importTestConfig]{Filesystem: &FilesystemContext{BaseDir: directory}}, CodeOptionsInvalid)
+	defaultsOnly, err := Load[importTestConfig](NoSource(), Options[importTestConfig]{Profile: "default"})
+	if err != nil || defaultsOnly == nil {
+		t.Fatalf("default profile with NoSource = %#v, %v", defaultsOnly, err)
+	}
+
+	if runtime.GOOS == "linux" || runtime.GOOS == "darwin" {
+		selectedLink := filepath.Join(directory, "selected-link.yml")
+		if err := os.Symlink(mainPath, selectedLink); err == nil {
+			linked, err := Load[importTestConfig](File(selectedLink), Options[importTestConfig]{Filesystem: &FilesystemContext{Root: directory}})
+			if err != nil || linked.Sources()[0].Path != canonicalTestPath(t, mainPath) {
+				t.Fatalf("selected symlink identity = %#v, %v", linked, err)
+			}
+		}
+	}
+}
+
+func writeGlobRoot(t *testing.T, directory, pattern string, optional bool) string {
+	t.Helper()
+	optionalLine := ""
+	if optional {
+		optionalLine = "\n      optional: true"
+	}
+	return writeImportFile(t, directory, "main.yml", "configkit:\n  imports:\n    - glob: '"+pattern+"'"+optionalLine+"\n")
+}

go.mod

@@ -0,0 +1,5 @@
+module gitea.maximumdirect.net/eric/ConfigKit
+
+go 1.25.0
+
+require go.yaml.in/yaml/v4 v4.0.0-rc.6

go.sum

@@ -0,0 +1,2 @@
+go.yaml.in/yaml/v4 v4.0.0-rc.6 h1:1h7H1ohdUh93/FyE4YaDa1Zh64K6VVbjF4K6WUxMtH4=
+go.yaml.in/yaml/v4 v4.0.0-rc.6/go.mod h1:aZqd9kCMsGL7AuUv/m/PvWLdg5sjJsZ4oHDEnfPPfY0=

history.go

@@ -0,0 +1,276 @@
+package configkit
+
+import (
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+const historyNodeLimit = 4_000_000
+
+// History applications borrow immutable policy-validated input trees. Metadata
+// records answer presence and origin queries; replay reconstructs value changes.
+// Replay reconciles with canonical decoding before domain resolution so both
+// contracts agree on the effective result without retaining a tree per field.
+type historyApplication struct {
+	layer  Layer
+	path   Path
+	value  *document.Node
+	origin Origin
+}
+
+func cloneHistoryApplications(values []historyApplication) []historyApplication {
+	if values == nil {
+		return nil
+	}
+	cloned := make([]historyApplication, len(values))
+	for index, value := range values {
+		cloned[index] = historyApplication{
+			layer: value.layer, path: value.path.clone(), value: value.value,
+			origin: cloneOrigin(value.origin),
+		}
+	}
+	return cloned
+}
+
+func applicationOperationPath(application historyApplication, query Path) (Path, bool) {
+	if !application.path.valid() || application.value == nil {
+		return Path{}, false
+	}
+	if application.path.String() != "" {
+		if pathsIntersect(application.path, query) {
+			return application.path.clone(), true
+		}
+		return Path{}, false
+	}
+	if len(query.segments) == 0 {
+		return Path{}, true
+	}
+	for length := 1; length <= len(query.segments); length++ {
+		prefix := Path{segments: clonePathSegments(query.segments[:length], 0)}
+		node, present := documentAtPath(application.value, prefix)
+		if !present {
+			return Path{}, false
+		}
+		if length == len(query.segments) || node.Kind != document.Mapping {
+			return prefix, true
+		}
+	}
+	return Path{}, false
+}
+
+func historyApplicationOrigin(application historyApplication, operationPath Path) Origin {
+	if application.origin.Layer != "" {
+		return cloneOrigin(application.origin)
+	}
+	origin := Origin{Layer: application.layer}
+	relative, ok := relativeHistoryPath(application.path, operationPath)
+	if !ok {
+		return origin
+	}
+	if node, present := documentAtPath(application.value, relative); present {
+		origin.Locations = []Location{publicLocation(node.Location)}
+	}
+	return origin
+}
+
+func relativeHistoryPath(root, path Path) (Path, bool) {
+	if !pathContains(root, path) {
+		return Path{}, false
+	}
+	return Path{segments: clonePathSegments(path.segments[len(root.segments):], 0)}, true
+}
+
+func pathsIntersect(left, right Path) bool {
+	return pathContains(left, right) || pathContains(right, left)
+}
+
+func applyHistoryApplication(root **document.Node, application historyApplication) {
+	if application.path.String() == "" {
+		mergeHistoryDocument(root, application.value)
+		return
+	}
+	mergeHistoryAtPath(root, application.path, application.value)
+}
+
+func mergeHistoryDocument(target **document.Node, incoming *document.Node) {
+	if incoming == nil {
+		return
+	}
+	if *target == nil || (*target).Kind != document.Mapping || incoming.Kind != document.Mapping {
+		*target = document.Clone(incoming)
+		return
+	}
+	for _, entry := range incoming.Mapping {
+		position := documentEntryIndex((*target).Mapping, entry.Key)
+		if position < 0 {
+			cloned := entry
+			cloned.Value = document.Clone(entry.Value)
+			(*target).Mapping = append((*target).Mapping, cloned)
+			continue
+		}
+		mergeHistoryDocument(&(*target).Mapping[position].Value, entry.Value)
+	}
+}
+
+func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node) {
+	if root == nil || *root == nil || len(path.segments) == 0 {
+		mergeHistoryDocument(root, incoming)
+		return
+	}
+	current := *root
+	for index, segment := range path.segments {
+		if current.Kind != document.Mapping || segment.kind == pathSegmentIndex {
+			return
+		}
+		position := documentEntryIndex(current.Mapping, segment.key)
+		if index == len(path.segments)-1 {
+			if position < 0 {
+				current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: document.Clone(incoming)})
+			} else {
+				mergeHistoryDocument(&current.Mapping[position].Value, incoming)
+			}
+			return
+		}
+		if position < 0 {
+			child := &document.Node{Kind: document.Mapping}
+			current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: child})
+			current = child
+		} else {
+			current = current.Mapping[position].Value
+		}
+	}
+}
+
+// Resolution records describe one before/final transformation, not assignment
+// order. A parent already carries its descendants' final values; merge overlapping
+// intent into that event rather than replay a child assignment a second time.
+func coalescedResolutionSnapshots(snapshots []resolutionSnapshot) []resolutionSnapshot {
+	groups := make([]resolutionSnapshot, 0, len(snapshots))
+	for _, snapshot := range snapshots {
+		merged := false
+		for i := range groups {
+			if !pathContains(groups[i].change.Path, snapshot.change.Path) {
+				continue
+			}
+			for _, reason := range snapshot.change.Reasons {
+				groups[i].change.Reasons = appendUniqueString(groups[i].change.Reasons, reason)
+			}
+			for _, dependency := range snapshot.change.Dependencies {
+				groups[i].change.Dependencies = appendUniquePath(groups[i].change.Dependencies, dependency)
+			}
+			if snapshot.change.Origin.Operation == ChangeNormalization {
+				groups[i].change.Origin.Operation = ChangeNormalization
+			}
+			merged = true
+			break
+		}
+		if !merged {
+			snapshot.change = cloneChange(snapshot.change)
+			groups = append(groups, snapshot)
+		}
+	}
+	return groups
+}
+
+func replaceHistoryPath(root **document.Node, path Path, replacement *document.Node) {
+	if root == nil || *root == nil {
+		return
+	}
+	if len(path.segments) == 0 {
+		*root = document.Clone(replacement)
+		return
+	}
+	current := *root
+	for index, segment := range path.segments {
+		last := index == len(path.segments)-1
+		switch current.Kind {
+		case document.Mapping:
+			if segment.kind == pathSegmentIndex {
+				return
+			}
+			position := documentEntryIndex(current.Mapping, segment.key)
+			if position < 0 {
+				if replacement == nil {
+					return
+				}
+				child := &document.Node{Kind: document.Mapping}
+				if last {
+					child = document.Clone(replacement)
+				}
+				current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: child})
+				if last {
+					return
+				}
+				current = child
+				continue
+			}
+			if last {
+				if replacement == nil {
+					current.Mapping = append(current.Mapping[:position], current.Mapping[position+1:]...)
+				} else {
+					current.Mapping[position].Value = document.Clone(replacement)
+				}
+				return
+			}
+			current = current.Mapping[position].Value
+		case document.Sequence:
+			position, ok := schemaIndex(segment)
+			if !ok || position >= len(current.Sequence) {
+				return
+			}
+			if last {
+				if replacement != nil {
+					current.Sequence[position] = document.Clone(replacement)
+				}
+				return
+			}
+			current = current.Sequence[position]
+		default:
+			return
+		}
+	}
+}
+
+func historyNodesWithinLimit(baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
+	return historyNodesWithin(historyNodeLimit, baseline, input, final, applications, resolutions)
+}
+
+func historyNodesWithin(limit int, baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
+	seen := make(map[*document.Node]bool)
+	count := 0
+	var visit func(*document.Node) bool
+	visit = func(node *document.Node) bool {
+		if node == nil || seen[node] {
+			return true
+		}
+		seen[node] = true
+		count++
+		if count > limit {
+			return false
+		}
+		for _, entry := range node.Mapping {
+			if !visit(entry.Value) {
+				return false
+			}
+		}
+		for _, child := range node.Sequence {
+			if !visit(child) {
+				return false
+			}
+		}
+		return true
+	}
+	if !visit(baseline) || !visit(input) || !visit(final) {
+		return false
+	}
+	for _, application := range applications {
+		if !visit(application.value) {
+			return false
+		}
+	}
+	for _, resolution := range resolutions {
+		if !visit(resolution.before) || !visit(resolution.after) {
+			return false
+		}
+	}
+	return true
+}

import_boundary_test.go

@@ -0,0 +1,50 @@
+package configkit
+
+import (
+	"go/parser"
+	"go/token"
+	"io/fs"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestDependencyImportBoundary(t *testing.T) {
+	t.Parallel()
+
+	err := filepath.WalkDir(".", func(path string, entry fs.DirEntry, walkErr error) error {
+		if walkErr != nil {
+			return walkErr
+		}
+		if entry.IsDir() {
+			if path == ".git" {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if filepath.Ext(path) != ".go" {
+			return nil
+		}
+		file, err := parser.ParseFile(token.NewFileSet(), path, nil, parser.ImportsOnly)
+		if err != nil {
+			return err
+		}
+		for _, imported := range file.Imports {
+			importPath, err := strconv.Unquote(imported.Path.Value)
+			if err != nil {
+				return err
+			}
+			if strings.HasPrefix(importPath, "go.yaml.in/yaml/") && !strings.HasPrefix(filepath.ToSlash(path), "internal/yaml/") {
+				t.Errorf("%s imports private YAML dependency %s", path, importPath)
+			}
+			if strings.HasPrefix(importPath, "github.com/google/go-cmp/") && !strings.HasSuffix(path, "_test.go") {
+				t.Errorf("%s imports test-only dependency %s", path, importPath)
+			}
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+}

imports.go

@@ -0,0 +1,324 @@
+package configkit
+
+import (
+	"bytes"
+	"errors"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+const (
+	maxImportDepth   = 64
+	maxImportSources = 1024
+	maxGlobEntries   = 1_000_000
+)
+
+type importLimits struct {
+	depth   int
+	sources int
+	bytes   int64
+	nodes   int
+	entries int
+}
+
+func defaultImportLimits() importLimits {
+	return importLimits{
+		depth: maxImportDepth, sources: maxImportSources,
+		bytes: maxTotalBytes, nodes: maxDocumentNodes, entries: maxGlobEntries,
+	}
+}
+
+type importRoute struct {
+	canonical string
+	locations []Location
+}
+
+type importTraversal struct {
+	rootPath string
+	root     *os.Root
+	limits   importLimits
+	rule     *VersionRule
+
+	documents   []*document.Node
+	sources     []SourceInfo
+	active      map[string]int
+	visited     map[string]importRoute
+	stack       []importRoute
+	usedBytes   int64
+	usedNodes   int
+	usedEntries int
+}
+
+func loadImportedDocuments(source Source, context *FilesystemContext, main parsedSource, rule *VersionRule, limits importLimits) (*composedDocument, []SourceInfo, []Diagnostic) {
+	rootPath, basePath, mainPath, diagnostic := importFilesystem(source, context, main)
+	if diagnostic != nil {
+		return nil, sourceInfos(main.source), []Diagnostic{*diagnostic}
+	}
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		diagnostic := globFilesystemDiagnostic(err, "configuration root could not be opened", Location{})
+		diagnostic.Primary = nil
+		return nil, sourceInfos(main.source), []Diagnostic{diagnostic}
+	}
+
+	mainInfo := cloneSourceInfo(*main.source)
+	if mainPath != "" {
+		mainInfo.Path = mainPath
+	}
+	state := importTraversal{
+		rootPath: rootPath, root: root, limits: limits, rule: rule,
+		documents: make([]*document.Node, 0, 1),
+		sources:   []SourceInfo{mainInfo},
+		active:    make(map[string]int), visited: make(map[string]importRoute),
+		usedBytes: main.bytes, usedNodes: countInputNodes(main.document),
+	}
+	if diagnostic := state.checkInitialLimits(); diagnostic != nil {
+		_ = root.Close()
+		return nil, cloneSourceInfos(state.sources), []Diagnostic{*diagnostic}
+	}
+
+	mainIdentity := mainPath
+	if mainIdentity == "" {
+		mainIdentity = "<selected-source>"
+	}
+	mainRoute := importRoute{canonical: mainIdentity}
+	state.active[mainIdentity] = 0
+	state.visited[mainIdentity] = mainRoute
+	state.stack = append(state.stack, mainRoute)
+	body, declarations, diagnostics := splitImportDirectives(main.document)
+	if len(diagnostics) == 0 {
+		state.documents = append(state.documents, body)
+		diagnostics = state.walkDeclarations(basePath, declarations, 0)
+	}
+
+	closeErr := root.Close()
+	if len(diagnostics) != 0 {
+		return nil, cloneSourceInfos(state.sources), diagnostics
+	}
+	if closeErr != nil {
+		return nil, cloneSourceInfos(state.sources), []Diagnostic{importDiagnostic(CodeImportRead, "configuration root could not be closed", nil)}
+	}
+	composed, diagnostics := composeDocuments(state.documents...)
+	if composed != nil {
+		composed.inputBytes = state.usedBytes
+		composed.inputNodes = state.usedNodes
+	}
+	return composed, cloneSourceInfos(state.sources), diagnostics
+}
+
+func (s *importTraversal) checkInitialLimits() *Diagnostic {
+	if s.limits.sources < 1 {
+		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the source-count limit", nil)
+		return &value
+	}
+	if s.usedBytes > s.limits.bytes {
+		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate byte limit", nil)
+		return &value
+	}
+	if s.usedNodes > s.limits.nodes {
+		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", nil)
+		return &value
+	}
+	return nil
+}
+
+func (s *importTraversal) walkDeclarations(basePath string, declarations []importDeclaration, depth int) []Diagnostic {
+	for _, declaration := range declarations {
+		if declaration.glob != "" {
+			matches, diagnostics := s.expandImportGlob(basePath, declaration)
+			if len(diagnostics) != 0 {
+				return diagnostics
+			}
+			if len(matches) == 0 {
+				if declaration.optional {
+					continue
+				}
+				location := declaration.location
+				return []Diagnostic{importDiagnostic(CodeImportMissing, "required import glob matched no files", &location)}
+			}
+			for _, match := range matches {
+				matched := declaration
+				matched.path = match
+				matched.glob = ""
+				if diagnostics := s.visitPath(basePath, matched, depth+1); len(diagnostics) != 0 {
+					return diagnostics
+				}
+			}
+			continue
+		}
+		if diagnostics := s.visitPath(basePath, declaration, depth+1); len(diagnostics) != 0 {
+			return diagnostics
+		}
+	}
+	return nil
+}
+
+func (s *importTraversal) visitPath(basePath string, declaration importDeclaration, depth int) []Diagnostic {
+	location := declaration.location
+	if depth > s.limits.depth {
+		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the active import-depth limit", &location)}
+	}
+	if !validImportPath(declaration.path) {
+		return []Diagnostic{importDiagnostic(CodeImportInvalid, "import path must be a nonempty slash-relative path without a network scheme", &location)}
+	}
+
+	candidate := filepath.Join(basePath, filepath.FromSlash(path.Clean(declaration.path)))
+	if !containedPath(s.rootPath, filepath.Clean(candidate)) {
+		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target is outside the configuration root", &location)}
+	}
+	canonical, err := canonicalExistingPath(candidate)
+	if err != nil {
+		if declaration.optional && errors.Is(err, fs.ErrNotExist) {
+			return nil
+		}
+		code := CodeImportInvalid
+		message := "import target could not be canonicalized"
+		if errors.Is(err, fs.ErrNotExist) {
+			code, message = CodeImportMissing, "required import target does not exist"
+		} else if errors.Is(err, fs.ErrPermission) {
+			message = "import target cannot be accessed"
+		}
+		return []Diagnostic{importDiagnostic(code, message, &location)}
+	}
+	if !containedPath(s.rootPath, canonical) {
+		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target resolves outside the configuration root", &location)}
+	}
+
+	currentRoute := s.currentRoute(location)
+	if activeIndex, ok := s.active[canonical]; ok {
+		cycle := append([]importRoute(nil), s.stack[activeIndex:]...)
+		return []Diagnostic{importCycleDiagnostic(canonical, location, cycle)}
+	}
+	if previous, ok := s.visited[canonical]; ok {
+		return []Diagnostic{importRepeatedDiagnostic(location, previous, currentRoute)}
+	}
+	if len(s.sources)+1 > s.limits.sources {
+		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the source-count limit", &location)}
+	}
+
+	relative, err := filepath.Rel(s.rootPath, canonical)
+	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target could not be addressed inside the configuration root", &location)}
+	}
+	sourceID := "source-" + strconv.Itoa(len(s.sources)+1)
+	info := SourceInfo{ID: sourceID, Label: declaration.path, Path: canonical, Layer: LayerBase, ImportedFrom: &location}
+	s.sources = append(s.sources, info)
+	data, skipped, diagnostic := s.readImport(relative, sourceID, declaration.optional)
+	if diagnostic != nil {
+		return []Diagnostic{*diagnostic}
+	}
+	if skipped {
+		s.sources = s.sources[:len(s.sources)-1]
+		return nil
+	}
+
+	root, issues := yamladapter.Parse(bytes.NewReader(data), sourceID, yamladapter.Limits{
+		Depth: maxDocumentDepth, Nodes: maxDocumentNodes,
+	})
+	if len(issues) != 0 {
+		return parserDiagnostics(sourceID, issues)
+	}
+	nodeCount := countInputNodes(root)
+	if s.usedNodes+nodeCount > s.limits.nodes {
+		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", &location)}
+	}
+	s.usedNodes += nodeCount
+
+	body, declarations, diagnostics := splitImportDirectives(root)
+	if len(diagnostics) != 0 {
+		return diagnostics
+	}
+	if versionNode := fragmentVersionClaim(body, s.rule); versionNode != nil {
+		versionLocation := publicLocation(versionNode.Location)
+		return []Diagnostic{{
+			Code: CodeImportVersion, Severity: SeverityError, Path: s.rule.Path.clone(),
+			Message: "imported fragments cannot contribute to the schema-version path", Primary: &versionLocation,
+		}}
+	}
+
+	route := importRoute{canonical: canonical, locations: currentRoute.locations}
+	s.documents = append(s.documents, body)
+	s.active[canonical] = len(s.stack)
+	s.visited[canonical] = route
+	s.stack = append(s.stack, route)
+	diagnostics = s.walkDeclarations(filepath.Dir(canonical), declarations, depth)
+	delete(s.active, canonical)
+	s.stack = s.stack[:len(s.stack)-1]
+	return diagnostics
+}
+
+func (s *importTraversal) readImport(relative, sourceID string, optional bool) ([]byte, bool, *Diagnostic) {
+	remaining := s.limits.bytes - s.usedBytes
+	limit := min(int64(maxSourceBytes), remaining)
+	if limit < 0 {
+		limit = 0
+	}
+	data, failure := readRegularFile(relative, s.root.Stat, s.root.Open, limit)
+	if failure != nil {
+		if optional && failure.missingTarget {
+			return nil, true, nil
+		}
+		value := sourceReadDiagnostic(failure, sourceID, CodeImportLimitExceeded, "configuration exceeds a source or aggregate byte limit")
+		return nil, false, &value
+	}
+	s.usedBytes += int64(len(data))
+	return data, false, nil
+}
+
+func (s *importTraversal) currentRoute(location Location) importRoute {
+	locations := make([]Location, 0, len(s.stack)+1)
+	for _, route := range s.stack {
+		if len(route.locations) != 0 {
+			locations = append(locations, route.locations[len(route.locations)-1])
+		}
+	}
+	locations = append(locations, location)
+	return importRoute{locations: locations}
+}
+
+func sourceInfos(source *SourceInfo) []SourceInfo {
+	if source == nil {
+		return nil
+	}
+	return []SourceInfo{cloneSourceInfo(*source)}
+}
+
+func importDiagnostic(code Code, message string, location *Location) Diagnostic {
+	return Diagnostic{Code: code, Severity: SeverityError, Message: message, Primary: cloneLocation(location)}
+}
+
+func importCycleDiagnostic(target string, location Location, cycle []importRoute) Diagnostic {
+	paths := make([]string, 0, len(cycle)+1)
+	related := make([]Location, 0, len(cycle))
+	for _, route := range cycle {
+		paths = append(paths, route.canonical)
+		if len(route.locations) != 0 {
+			related = append(related, route.locations[len(route.locations)-1])
+		}
+	}
+	paths = append(paths, target)
+	return Diagnostic{
+		Code: CodeImportCycle, Severity: SeverityError,
+		Message: "import cycle detected: " + strings.Join(paths, " -> "),
+		Primary: &location, Related: related,
+	}
+}
+
+func importRepeatedDiagnostic(location Location, previous, current importRoute) Diagnostic {
+	related := append([]Location(nil), previous.locations...)
+	if len(current.locations) > 1 {
+		related = append(related, current.locations[:len(current.locations)-1]...)
+	}
+	return Diagnostic{
+		Code: CodeImportRepeated, Severity: SeverityError,
+		Message: "configuration source is included by more than one import route",
+		Primary: &location, Related: related,
+	}
+}

imports_test.go

@@ -0,0 +1,476 @@
+package configkit
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+type importTestConfig struct {
+	Settings struct {
+		Root   string `yaml:"root"`
+		First  string `yaml:"first"`
+		Nested string `yaml:"nested"`
+		Second string `yaml:"second"`
+	} `yaml:"settings"`
+	Version string            `yaml:"version"`
+	Items   map[string]string `yaml:"items"`
+}
+
+func TestLoadFormerDirectiveKeyIsApplicationData(t *testing.T) {
+	type config struct {
+		Legacy struct {
+			Imports []struct {
+				Path string `yaml:"path"`
+			} `yaml:"imports"`
+		} `yaml:"_configkit"`
+	}
+	for _, imports := range []bool{false, true} {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "_configkit:\n  imports:\n    - path: nonexistent.yml\n")
+		result, err := Load[config](File(mainPath), Options[config]{Imports: imports})
+		if err != nil {
+			t.Fatalf("Imports=%v: %v", imports, err)
+		}
+		if len(result.Config.Legacy.Imports) != 1 || result.Config.Legacy.Imports[0].Path != "nonexistent.yml" || len(result.Sources()) != 1 {
+			t.Fatalf("Imports=%v: former directive was not retained as application data: %#v", imports, result)
+		}
+	}
+}
+
+func TestLoadTraversesPathImportsDepthFirstAndComposesOrigins(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: first.yml
+    - path: second.yml
+settings:
+  root: root
+`)
+	firstPath := writeImportFile(t, directory, "first.yml", `configkit:
+  imports:
+    - path: nested/third.yml
+settings:
+  first: first
+`)
+	thirdPath := writeImportFile(t, directory, "nested/third.yml", "settings:\n  nested: nested\n")
+	secondPath := writeImportFile(t, directory, "second.yml", "settings:\n  second: second\n")
+
+	result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{Imports: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := result.Config.Settings; got.Root != "root" || got.First != "first" || got.Nested != "nested" || got.Second != "second" {
+		t.Fatalf("composed settings = %#v", got)
+	}
+	sources := result.Sources()
+	wantPaths := []string{canonicalTestPath(t, mainPath), canonicalTestPath(t, firstPath), canonicalTestPath(t, thirdPath), canonicalTestPath(t, secondPath)}
+	gotPaths := make([]string, len(sources))
+	for index, source := range sources {
+		gotPaths[index] = source.Path
+		if source.ID != "source-"+strconv.Itoa(index+1) {
+			t.Errorf("source %d ID = %q", index, source.ID)
+		}
+		if index > 0 && source.ImportedFrom == nil {
+			t.Errorf("source %d has no import location", index)
+		}
+	}
+	if !reflect.DeepEqual(gotPaths, wantPaths) {
+		t.Fatalf("source paths = %#v, want depth-first %#v", gotPaths, wantPaths)
+	}
+	info, ok := result.Metadata().Lookup(Keys("settings"))
+	if !ok || !info.Explicit || len(info.Origin.Locations) != 4 {
+		t.Fatalf("settings metadata = %#v, %v", info, ok)
+	}
+}
+
+func TestLoadRejectsImportConflictsAndFragmentVersions(t *testing.T) {
+	t.Run("all duplicate claimants", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: one.yml
+    - path: two.yml
+    - path: three.yml
+`)
+		for index, name := range []string{"one.yml", "two.yml", "three.yml"} {
+			writeImportFile(t, directory, name, "settings:\n  root: "+string(rune('a'+index))+"\n")
+		}
+		loadError := requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeComposeConflict)
+		diagnostic := loadError.Diagnostics()[0]
+		if diagnostic.Path.String() != "/settings/root" || diagnostic.Primary == nil || len(diagnostic.Related) != 2 {
+			t.Fatalf("conflict diagnostic = %#v", diagnostic)
+		}
+	})
+
+	versionRule := &VersionRule{Path: Keys("settings", "version"), Check: func(Version, *Validation) error { return nil }}
+	for _, test := range []struct {
+		name     string
+		fragment string
+	}{
+		{name: "exact integer", fragment: "settings:\n  version: 4\n"},
+		{name: "ancestor scalar", fragment: "settings: blocked\n"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", "configkit:\n  imports:\n    - path: fragment.yml\n")
+			writeImportFile(t, directory, "fragment.yml", test.fragment)
+			loadError := requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true, Definition: Definition[importTestConfig]{Version: versionRule}}, CodeImportVersion)
+			diagnostic := loadError.Diagnostics()[0]
+			if diagnostic.Path.String() != "/settings/version" || diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-2" {
+				t.Fatalf("version placement diagnostic = %#v", diagnostic)
+			}
+		})
+	}
+}
+
+func TestLoadImportDirectiveValidationAndTemporaryGlobGate(t *testing.T) {
+	tests := []struct {
+		name string
+		body string
+		code Code
+	}{
+		{name: "directive kind", body: "configkit: []\n", code: CodeImportInvalid},
+		{name: "unknown directive field", body: "configkit: {unknown: true}\n", code: CodeImportInvalid},
+		{name: "imports kind", body: "configkit: {imports: {}}\n", code: CodeImportInvalid},
+		{name: "declaration kind", body: "configkit: {imports: [file.yml]}\n", code: CodeImportInvalid},
+		{name: "missing target", body: "configkit: {imports: [{optional: true}]}\n", code: CodeImportInvalid},
+		{name: "two targets", body: "configkit: {imports: [{path: a.yml, glob: '*.yml'}]}\n", code: CodeImportInvalid},
+		{name: "unknown declaration field", body: "configkit: {imports: [{path: a.yml, extra: true}]}\n", code: CodeImportInvalid},
+		{name: "optional kind", body: "configkit: {imports: [{path: a.yml, optional: 'yes'}]}\n", code: CodeImportInvalid},
+		{name: "nested directive", body: "nested: {configkit: {}}\n", code: CodeImportInvalid},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", test.body)
+			requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, test.code)
+		})
+	}
+}
+
+func TestLoadImportMissingContainmentAndRegularFileRules(t *testing.T) {
+	t.Run("optional missing", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: missing.yml
+      optional: true
+settings: {root: retained}
+`)
+		result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{Imports: true})
+		if err != nil || result.Config.Settings.Root != "retained" || len(result.Sources()) != 1 {
+			t.Fatalf("optional result = %#v, %v", result, err)
+		}
+	})
+
+	tests := []struct {
+		name  string
+		path  string
+		setup func(*testing.T, string)
+		code  Code
+	}{
+		{name: "required missing", path: "missing.yml", code: CodeImportMissing},
+		{name: "parent traversal", path: "../outside.yml", setup: func(t *testing.T, directory string) {
+			writeImportFile(t, filepath.Dir(directory), "outside.yml", "settings: {root: outside}\n")
+		}, code: CodeImportOutsideRoot},
+		{name: "directory", path: "directory", setup: func(t *testing.T, directory string) {
+			if err := os.Mkdir(filepath.Join(directory, "directory"), 0o755); err != nil {
+				t.Fatal(err)
+			}
+		}, code: CodeSourceNotRegular},
+		{name: "absolute", path: "/tmp/configkit.yml", code: CodeImportInvalid},
+		{name: "network URL", path: "https://example.invalid/config.yml", code: CodeImportInvalid},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			if test.setup != nil {
+				test.setup(t, directory)
+			}
+			mainPath := writeImportFile(t, directory, "main.yml", "configkit:\n  imports:\n    - path: \""+test.path+"\"\n")
+			requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, test.code)
+		})
+	}
+}
+
+func TestLoadImportIdentityReportsCyclesAndRepeatedRoutes(t *testing.T) {
+	t.Run("cycle", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit:\n  imports: [{path: a.yml}]\n")
+		writeImportFile(t, directory, "a.yml", "configkit:\n  imports: [{path: b.yml}]\n")
+		writeImportFile(t, directory, "b.yml", "configkit:\n  imports: [{path: main.yml}]\n")
+		loadError := requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportCycle)
+		diagnostic := loadError.Diagnostics()[0]
+		if diagnostic.Primary == nil || len(diagnostic.Related) != 2 || !strings.Contains(diagnostic.Message, "main.yml") || !strings.Contains(diagnostic.Message, "a.yml") || !strings.Contains(diagnostic.Message, "b.yml") {
+			t.Fatalf("cycle diagnostic = %#v", diagnostic)
+		}
+	})
+
+	t.Run("diamond", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit:\n  imports: [{path: left.yml}, {path: right.yml}]\n")
+		writeImportFile(t, directory, "left.yml", "configkit:\n  imports: [{path: shared.yml}]\n")
+		writeImportFile(t, directory, "right.yml", "configkit:\n  imports: [{path: shared.yml}]\n")
+		writeImportFile(t, directory, "shared.yml", "settings: {root: shared}\n")
+		loadError := requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportRepeated)
+		diagnostic := loadError.Diagnostics()[0]
+		if diagnostic.Primary == nil || len(diagnostic.Related) < 3 {
+			t.Fatalf("repeated-route diagnostic = %#v", diagnostic)
+		}
+	})
+
+	t.Run("symlink aliases", func(t *testing.T) {
+		if runtime.GOOS != "linux" && runtime.GOOS != "darwin" {
+			t.Skip("initial filesystem contract covers Linux and macOS")
+		}
+		directory := t.TempDir()
+		writeImportFile(t, directory, "target.yml", "settings: {root: target}\n")
+		if err := os.Symlink("target.yml", filepath.Join(directory, "first-link.yml")); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+		if err := os.Symlink("target.yml", filepath.Join(directory, "second-link.yml")); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: first-link.yml}, {path: second-link.yml}]}\n")
+		requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportRepeated)
+	})
+}
+
+func TestLoadImportsCanonicalSymlinksAndKeepsHardLinksDistinct(t *testing.T) {
+	if runtime.GOOS != "linux" && runtime.GOOS != "darwin" {
+		t.Skip("initial filesystem contract covers Linux and macOS")
+	}
+	t.Run("contained absolute symlink", func(t *testing.T) {
+		directory := t.TempDir()
+		target := writeImportFile(t, directory, "target.yml", "settings: {first: linked}\n")
+		if err := os.Symlink(target, filepath.Join(directory, "link.yml")); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: link.yml}]}\n")
+		result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{Imports: true})
+		if err != nil || result.Config.Settings.First != "linked" || result.Sources()[1].Path != canonicalTestPath(t, target) {
+			t.Fatalf("symlink result = %#v, %v", result, err)
+		}
+	})
+
+	t.Run("symlink escape", func(t *testing.T) {
+		root := t.TempDir()
+		outside := t.TempDir()
+		target := writeImportFile(t, outside, "outside.yml", "settings: {root: outside}\n")
+		if err := os.Symlink(target, filepath.Join(root, "escape.yml")); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+		mainPath := writeImportFile(t, root, "main.yml", "configkit: {imports: [{path: escape.yml}]}\n")
+		requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportOutsideRoot)
+	})
+
+	t.Run("hard links are path identities", func(t *testing.T) {
+		directory := t.TempDir()
+		one := writeImportFile(t, directory, "one.yml", "settings: {first: one}\n")
+		if err := os.Link(one, filepath.Join(directory, "two.yml")); err != nil {
+			t.Skipf("hard links unavailable: %v", err)
+		}
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: one.yml}, {path: two.yml}]}\n")
+		requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeComposeConflict)
+	})
+
+	t.Run("canonicalization failure", func(t *testing.T) {
+		directory := t.TempDir()
+		if err := os.Symlink("loop.yml", filepath.Join(directory, "loop.yml")); err != nil {
+			t.Skipf("symlinks unavailable: %v", err)
+		}
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: loop.yml}]}\n")
+		requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportInvalid)
+	})
+}
+
+func TestLoadByteAndReaderImportsRequireExplicitFilesystemContext(t *testing.T) {
+	directory := t.TempDir()
+	writeImportFile(t, directory, "fragment.yml", "settings: {first: imported}\n")
+	input := []byte("configkit: {imports: [{path: fragment.yml}]}\n")
+	context := &FilesystemContext{BaseDir: directory, Root: directory}
+	for _, test := range []struct {
+		name   string
+		source func() Source
+	}{
+		{name: "bytes URI label", source: func() Source { return Bytes("s3://bucket/main.yml", input) }},
+		{name: "reader URI label", source: func() Source { return Reader("https://example.invalid/main.yml", strings.NewReader(string(input))) }},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := Load[importTestConfig](test.source(), Options[importTestConfig]{Imports: true, Filesystem: context})
+			if err != nil || result.Config.Settings.First != "imported" {
+				t.Fatalf("explicit filesystem result = %#v, %v", result, err)
+			}
+		})
+	}
+	requireLoadCode(t, Bytes("memory.yml", input), Options[importTestConfig]{Imports: true}, CodeOptionsInvalid)
+	requireLoadCode(t, Bytes("memory.yml", input), Options[importTestConfig]{Imports: true, Filesystem: &FilesystemContext{BaseDir: "relative", Root: directory}}, CodeOptionsInvalid)
+	requireLoadCode(t, Bytes("memory.yml", input), Options[importTestConfig]{Imports: true, Filesystem: &FilesystemContext{BaseDir: directory, Root: t.TempDir()}}, CodeImportOutsideRoot)
+}
+
+func TestFileImportRootCanContainTheSelectedFileDirectory(t *testing.T) {
+	root := t.TempDir()
+	mainPath := writeImportFile(t, root, "configs/main.yml", "configkit: {imports: [{path: ../shared.yml}]}\n")
+	writeImportFile(t, root, "shared.yml", "settings: {root: shared}\n")
+	result, err := Load[importTestConfig](File(mainPath), Options[importTestConfig]{
+		Imports: true, Filesystem: &FilesystemContext{Root: root},
+	})
+	if err != nil || result.Config.Settings.Root != "shared" {
+		t.Fatalf("explicit-root result = %#v, %v", result, err)
+	}
+	requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true, Filesystem: &FilesystemContext{BaseDir: filepath.Dir(mainPath), Root: root}}, CodeOptionsInvalid)
+}
+
+func TestImportedParseAndDecodeDiagnosticsKeepTheirSource(t *testing.T) {
+	t.Run("parse", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: broken.yml}]}\n")
+		writeImportFile(t, directory, "broken.yml", "settings: [\n")
+		loadError := requireAnyLoadError(t, File(mainPath), Options[importTestConfig]{Imports: true})
+		diagnostics := loadError.Diagnostics()
+		if diagnostics[0].Code != CodeYAMLInvalid || diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" || len(loadError.Sources()) != 2 {
+			t.Fatalf("parse failure = diagnostics %#v, sources %#v", diagnostics, loadError.Sources())
+		}
+	})
+
+	t.Run("decode", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: unknown.yml}]}\n")
+		writeImportFile(t, directory, "unknown.yml", "unknown: value\n")
+		loadError := requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeDecodeUnknownField)
+		diagnostic := loadError.Diagnostics()[0]
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-2" {
+			t.Fatalf("decode diagnostic = %#v", diagnostic)
+		}
+	})
+}
+
+func TestOptionalImportDoesNotSuppressUnsafeOrUnreadableTargets(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "outside.yml", "configkit: {imports: [{path: ../missing.yml, optional: true}]}\n")
+	requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeImportOutsideRoot)
+
+	locked := writeImportFile(t, directory, "locked.yml", "settings: {root: locked}\n")
+	if err := os.Chmod(locked, 0); err != nil {
+		t.Skipf("cannot create unreadable fixture: %v", err)
+	}
+	defer func() { _ = os.Chmod(locked, 0o600) }()
+	if file, err := os.Open(locked); err == nil {
+		_ = file.Close()
+		t.Skip("filesystem does not enforce the unreadable-file fixture")
+	}
+	mainPath = writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: locked.yml, optional: true}]}\n")
+	requireLoadCode(t, File(mainPath), Options[importTestConfig]{Imports: true}, CodeSourcePermission)
+}
+
+func TestImportTraversalEnforcesPrivateBudgets(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: one.yml}, {path: two.yml}]}\n")
+	writeImportFile(t, directory, "one.yml", "settings: {first: one}\n")
+	writeImportFile(t, directory, "two.yml", "settings: {second: two}\n")
+	main := parseImportMain(t, mainPath)
+	for _, test := range []struct {
+		name   string
+		limits importLimits
+	}{
+		{name: "source count", limits: importLimits{depth: 8, sources: 2, bytes: maxTotalBytes, nodes: maxDocumentNodes}},
+		{name: "aggregate bytes", limits: importLimits{depth: 8, sources: 8, bytes: main.bytes + 1, nodes: maxDocumentNodes}},
+		{name: "aggregate nodes", limits: importLimits{depth: 8, sources: 8, bytes: maxTotalBytes, nodes: countInputNodes(main.document) + 1}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			_, _, diagnostics := loadImportedDocuments(File(mainPath), nil, main, nil, test.limits)
+			if len(diagnostics) != 1 || diagnostics[0].Code != CodeImportLimitExceeded {
+				t.Fatalf("budget diagnostics = %#v", diagnostics)
+			}
+		})
+	}
+
+	writeImportFile(t, directory, "one.yml", "configkit: {imports: [{path: nested.yml}]}\n")
+	writeImportFile(t, directory, "nested.yml", "settings: {nested: nested}\n")
+	main = parseImportMain(t, mainPath)
+	_, _, diagnostics := loadImportedDocuments(File(mainPath), nil, main, nil, importLimits{depth: 1, sources: 8, bytes: maxTotalBytes, nodes: maxDocumentNodes})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeImportLimitExceeded {
+		t.Fatalf("depth diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestImportTraversalAcceptsExactPrivateBudgets(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: imported.yml}]}\n")
+	importedPath := writeImportFile(t, directory, "imported.yml", "settings: {first: imported}\n")
+	main := parseImportMain(t, mainPath)
+	imported := parseImportMain(t, importedPath)
+	data, err := os.ReadFile(importedPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	limits := importLimits{
+		depth:   1,
+		sources: 2,
+		bytes:   main.bytes + int64(len(data)),
+		nodes:   countInputNodes(main.document) + countInputNodes(imported.document),
+	}
+	composed, sources, diagnostics := loadImportedDocuments(File(mainPath), nil, main, nil, limits)
+	if len(diagnostics) != 0 || composed == nil || len(sources) != 2 {
+		t.Fatalf("exact-limit composition = (%#v, %#v, %#v)", composed, sources, diagnostics)
+	}
+}
+
+func writeImportFile(t *testing.T, root, relative, contents string) string {
+	t.Helper()
+	filename := filepath.Join(root, filepath.FromSlash(relative))
+	if err := os.MkdirAll(filepath.Dir(filename), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filename, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	return filename
+}
+
+func canonicalTestPath(t *testing.T, filename string) string {
+	t.Helper()
+	canonical, err := canonicalExistingPath(filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return canonical
+}
+
+func requireLoadCode(t *testing.T, source Source, options Options[importTestConfig], code Code) *LoadError {
+	t.Helper()
+	loadError := requireAnyLoadError(t, source, options)
+	diagnostics := loadError.Diagnostics()
+	if len(diagnostics) == 0 || diagnostics[0].Code != code {
+		t.Fatalf("diagnostics = %#v, want %s", diagnostics, code)
+	}
+	return loadError
+}
+
+func requireAnyLoadError(t *testing.T, source Source, options Options[importTestConfig]) *LoadError {
+	t.Helper()
+	result, err := Load[importTestConfig](source, options)
+	if result != nil || err == nil {
+		t.Fatalf("result, error = %#v, %v", result, err)
+	}
+	var loadError *LoadError
+	if !errors.As(err, &loadError) {
+		t.Fatalf("error type = %T", err)
+	}
+	return loadError
+}
+
+func parseImportMain(t *testing.T, filename string) parsedSource {
+	t.Helper()
+	parsed, diagnostics := parseSource(File(filename), "source-1", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatalf("parse diagnostics = %#v", diagnostics)
+	}
+	return parsed
+}

independence_test.go

@@ -0,0 +1,116 @@
+package configkit
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"strings"
+	"sync"
+	"testing"
+)
+
+type independentText string
+
+func (value independentText) MarshalText() ([]byte, error) {
+	return []byte(value), nil
+}
+
+func (value *independentText) UnmarshalText(text []byte) error {
+	*value = independentText(text)
+	return nil
+}
+
+type independentConfig struct {
+	Labels map[string]string `yaml:"labels"`
+	Items  []string          `yaml:"items"`
+	Custom independentText   `yaml:"custom"`
+}
+
+func TestParallelLoadsShareRulesButNotMutableValues(t *testing.T) {
+	definition := Definition[independentConfig]{
+		Defaults: func() independentConfig {
+			return independentConfig{
+				Labels: map[string]string{"default": "kept"},
+				Items:  []string{"default"},
+				Custom: "default",
+			}
+		},
+	}
+
+	const count = 32
+	results := make([]*Result[independentConfig], count)
+	errorsByIndex := make([]error, count)
+	var group sync.WaitGroup
+	for index := range count {
+		group.Add(1)
+		go func(index int) {
+			defer group.Done()
+			input := fmt.Sprintf("custom: value-%d\n", index)
+			results[index], errorsByIndex[index] = Load(Bytes("parallel.yml", []byte(input)), Options[independentConfig]{Definition: definition})
+		}(index)
+	}
+	group.Wait()
+
+	for index, err := range errorsByIndex {
+		if err != nil {
+			t.Fatalf("Load(%d) failed: %v", index, err)
+		}
+	}
+	results[0].Config.Labels["default"] = "mutated"
+	results[0].Config.Items[0] = "mutated"
+	results[0].Config.Custom = "mutated"
+	for index := 1; index < count; index++ {
+		if results[index].Config.Labels["default"] != "kept" || results[index].Config.Items[0] != "default" || results[index].Config.Custom != independentText(fmt.Sprintf("value-%d", index)) {
+			t.Fatalf("result %d shares mutable state: %#v", index, results[index].Config)
+		}
+	}
+	var effective bytes.Buffer
+	if err := results[0].WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String(), "mutated") || !strings.Contains(effective.String(), "default: kept") || !strings.Contains(effective.String(), "value-0") {
+		t.Fatalf("frozen effective snapshot = %q", effective.String())
+	}
+}
+
+func TestConcurrentLoadsOfCopiedReaderSourceHaveOneClaim(t *testing.T) {
+	source := Reader("shared", strings.NewReader("value: one\n"))
+	copied := source
+	type config struct {
+		Value string `yaml:"value"`
+	}
+
+	start := make(chan struct{})
+	errorsFound := make(chan error, 2)
+	var group sync.WaitGroup
+	for _, candidate := range []Source{source, copied} {
+		group.Add(1)
+		go func(candidate Source) {
+			defer group.Done()
+			<-start
+			_, err := Load[config](candidate)
+			errorsFound <- err
+		}(candidate)
+	}
+	close(start)
+	group.Wait()
+	close(errorsFound)
+
+	successes, used := 0, 0
+	for err := range errorsFound {
+		if err == nil {
+			successes++
+			continue
+		}
+		var loadError *LoadError
+		if errors.As(err, &loadError) {
+			diagnostics := loadError.Diagnostics()
+			if len(diagnostics) == 1 && diagnostics[0].Code == CodeSourceReaderUsed {
+				used++
+			}
+		}
+	}
+	if successes != 1 || used != 1 {
+		t.Fatalf("successes=%d reader-used=%d", successes, used)
+	}
+}

internal/document/document.go

@@ -0,0 +1,65 @@
+// Package document defines ConfigKit's parser-independent configuration tree.
+package document
+
+// Kind identifies a configuration node category.
+type Kind uint8
+
+const (
+	Mapping Kind = iota + 1
+	Sequence
+	String
+	Boolean
+	Number
+	Null
+)
+
+// Location identifies a source coordinate. Root depth and unavailable coordinates
+// are represented independently by callers; line and column are one-based when set.
+type Location struct {
+	SourceID string
+	Line     int
+	Column   int
+}
+
+// Entry is an ordered mapping entry with separate key and value locations.
+type Entry struct {
+	Key           string
+	KeyLocation   Location
+	ValueLocation Location
+	Value         *Node
+}
+
+// Node is ConfigKit's owned configuration representation. Number Text retains its
+// exact validated decimal spelling; Boolean Text is "true" or "false".
+type Node struct {
+	// Sensitive preserves field classification from concrete typed projections.
+	// It is metadata, not part of the configuration value or emitted YAML.
+	Sensitive bool
+	Kind      Kind
+	Location  Location
+	Text      string
+	Mapping   []Entry
+	Sequence  []*Node
+}
+
+// Clone returns a detached copy of a node tree.
+func Clone(node *Node) *Node {
+	if node == nil {
+		return nil
+	}
+	cloned := *node
+	if node.Mapping != nil {
+		cloned.Mapping = make([]Entry, len(node.Mapping))
+		for i, entry := range node.Mapping {
+			cloned.Mapping[i] = entry
+			cloned.Mapping[i].Value = Clone(entry.Value)
+		}
+	}
+	if node.Sequence != nil {
+		cloned.Sequence = make([]*Node, len(node.Sequence))
+		for i, child := range node.Sequence {
+			cloned.Sequence[i] = Clone(child)
+		}
+	}
+	return &cloned
+}

internal/yaml/fuzz_test.go

@@ -0,0 +1,61 @@
+package yaml
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func FuzzParseDocument(f *testing.F) {
+	for _, seed := range []string{
+		"{}\n",
+		"service:\n  enabled: true\n  ports: [8080, 8081]\n",
+		"values: {empty: '', nothing: null, number: -1.5e2}\n",
+		"duplicate: one\nduplicate: two\n",
+		"alias: &value one\nother: *value\n",
+	} {
+		f.Add([]byte(seed))
+	}
+
+	limits := Limits{Depth: 64, Nodes: 4096, ParserDepth: 128}
+	f.Fuzz(func(t *testing.T, input []byte) {
+		if len(input) > 64<<10 {
+			return
+		}
+		parsed, issues := Parse(bytes.NewReader(input), "fuzz-input", limits)
+		if len(issues) != 0 {
+			return
+		}
+
+		var output bytes.Buffer
+		if err := Write(&output, parsed); err != nil {
+			t.Fatalf("Write() failed for accepted document: %v", err)
+		}
+		reparsed, issues := Parse(bytes.NewReader(output.Bytes()), "round-trip", limits)
+		if len(issues) != 0 {
+			t.Fatalf("rendered accepted document was rejected: %#v", issues)
+		}
+		clearLocations(parsed)
+		clearLocations(reparsed)
+		if !reflect.DeepEqual(parsed, reparsed) {
+			t.Fatalf("document changed across parse/write/parse: before=%#v after=%#v", parsed, reparsed)
+		}
+	})
+}
+
+func clearLocations(node *document.Node) {
+	if node == nil {
+		return
+	}
+	node.Location = document.Location{}
+	for index := range node.Mapping {
+		node.Mapping[index].KeyLocation = document.Location{}
+		node.Mapping[index].ValueLocation = document.Location{}
+		clearLocations(node.Mapping[index].Value)
+	}
+	for _, child := range node.Sequence {
+		clearLocations(child)
+	}
+}

internal/yaml/parse.go

@@ -0,0 +1,274 @@
+// Package yaml adapts the private upstream parser to ConfigKit's document model.
+package yaml
+
+import (
+	"errors"
+	"io"
+	"regexp"
+	"strings"
+
+	"go.yaml.in/yaml/v4"
+	"go.yaml.in/yaml/v4/plugin/limit"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+const (
+	codeInvalid           = "yaml.invalid"
+	codeEmpty             = "yaml.empty"
+	codeMultipleDocuments = "yaml.multiple_documents"
+	codeRootType          = "yaml.root_type"
+	codeDuplicateKey      = "yaml.duplicate_key"
+	codeTag               = "yaml.tag"
+	codeAnchor            = "yaml.anchor"
+	codeAlias             = "yaml.alias"
+	codeMergeKey          = "yaml.merge_key"
+	codeScalar            = "yaml.scalar"
+	codeDepthLimit        = "yaml.depth_limit"
+	codeNodeLimit         = "yaml.node_limit"
+)
+
+var (
+	jsonNumberPattern      = regexp.MustCompile(`^-?(0|[1-9][0-9]*)(\.[0-9]+)?([eE][+-]?[0-9]+)?$`)
+	decimalLikePattern     = regexp.MustCompile(`^[+-]?[0-9][0-9_]*(\.[0-9_]*)?([eE][+-]?[0-9_]*)?$`)
+	prefixedNumberPattern  = regexp.MustCompile(`^[+-]?0[xXoO][0-9A-Fa-f_]+$`)
+	leadingDecimalPattern  = regexp.MustCompile(`^[+-]?\.[0-9_]+$`)
+	nonFiniteNumberPattern = regexp.MustCompile(`(?i)^[+-]?\.(inf|nan)$`)
+	errParserDepth         = errors.New("configkit yaml parser depth limit")
+)
+
+// Limits controls private parser and translated-tree resource bounds.
+type Limits struct {
+	Depth       int
+	Nodes       int
+	ParserDepth int
+}
+
+// Issue is a parser-independent, value-free YAML diagnostic.
+type Issue struct {
+	Code    string
+	Message string
+	Line    int
+	Column  int
+	Related []document.Location
+}
+
+// Parse loads exactly one policy-valid mapping document.
+func Parse(input io.Reader, sourceID string, limits Limits) (*document.Node, []Issue) {
+	parserDepth := limits.ParserDepth
+	if parserDepth <= 0 {
+		parserDepth = limits.Depth + 2
+	}
+	depthExceeded := false
+	plugin := limit.New(limit.DepthFunc(func(depth int, _ *limit.DepthContext) error {
+		if depth > parserDepth {
+			depthExceeded = true
+			return errParserDepth
+		}
+		return nil
+	}))
+	loader, err := yaml.NewLoader(input, yaml.WithPlugin(plugin))
+	if err != nil {
+		return nil, []Issue{{Code: codeInvalid, Message: "YAML parser could not be initialized"}}
+	}
+
+	var parsed yaml.Node
+	if err := loader.Load(&parsed); err != nil {
+		if errors.Is(err, io.EOF) {
+			return nil, []Issue{{Code: codeEmpty, Message: "configuration input is empty"}}
+		}
+		return nil, []Issue{parserIssue(err, depthExceeded)}
+	}
+	var trailing yaml.Node
+	if err := loader.Load(&trailing); err == nil {
+		return nil, []Issue{{Code: codeMultipleDocuments, Message: "configuration input must contain exactly one document", Line: trailing.Line, Column: trailing.Column}}
+	} else if !errors.Is(err, io.EOF) {
+		return nil, []Issue{parserIssue(err, depthExceeded)}
+	}
+
+	root := documentRoot(&parsed)
+	if root == nil || (root.Kind == yaml.ScalarNode && root.Value == "" && root.ShortTag() == "!!null") {
+		return nil, []Issue{{Code: codeEmpty, Message: "configuration document is empty", Line: parsed.Line, Column: parsed.Column}}
+	}
+	if root.Kind != yaml.MappingNode {
+		return nil, []Issue{{Code: codeRootType, Message: "configuration document root must be a mapping", Line: root.Line, Column: root.Column}}
+	}
+
+	state := translationState{sourceID: sourceID, limits: limits}
+	translated, issue := state.translate(root, 0)
+	if issue != nil {
+		return nil, []Issue{*issue}
+	}
+	return translated, nil
+}
+
+type translationState struct {
+	sourceID string
+	limits   Limits
+	nodes    int
+}
+
+func (s *translationState) translate(node *yaml.Node, depth int) (*document.Node, *Issue) {
+	if depth > s.limits.Depth {
+		return nil, nodeIssue(codeDepthLimit, "configuration exceeds the nesting depth limit", node)
+	}
+	if node.Style&yaml.TaggedStyle != 0 {
+		return nil, nodeIssue(codeTag, "explicit YAML tags are not supported", node)
+	}
+	if node.Anchor != "" {
+		return nil, nodeIssue(codeAnchor, "YAML anchors are not supported", node)
+	}
+	if node.Kind == yaml.AliasNode || node.Alias != nil {
+		return nil, nodeIssue(codeAlias, "YAML aliases are not supported", node)
+	}
+	if issue := s.chargeNode(node); issue != nil {
+		return nil, issue
+	}
+
+	location := document.Location{SourceID: s.sourceID, Line: node.Line, Column: node.Column}
+	switch node.Kind {
+	case yaml.MappingNode:
+		if len(node.Content)%2 != 0 {
+			return nil, nodeIssue(codeInvalid, "mapping structure is invalid", node)
+		}
+		result := &document.Node{Kind: document.Mapping, Location: location, Mapping: make([]document.Entry, 0, len(node.Content)/2)}
+		seen := make(map[string]document.Location, len(node.Content)/2)
+		for i := 0; i < len(node.Content); i += 2 {
+			keyNode, valueNode := node.Content[i], node.Content[i+1]
+			if issue := s.chargeNode(keyNode); issue != nil {
+				return nil, issue
+			}
+			key, issue := mappingKey(keyNode)
+			if issue != nil {
+				return nil, issue
+			}
+			if key == "<<" {
+				return nil, nodeIssue(codeMergeKey, "YAML merge keys are not supported", keyNode)
+			}
+			keyLocation := document.Location{SourceID: s.sourceID, Line: keyNode.Line, Column: keyNode.Column}
+			if previous, ok := seen[key]; ok {
+				issue := nodeIssue(codeDuplicateKey, "mapping key is defined more than once", keyNode)
+				issue.Related = []document.Location{previous}
+				return nil, issue
+			}
+			seen[key] = keyLocation
+			value, issue := s.translate(valueNode, depth+1)
+			if issue != nil {
+				return nil, issue
+			}
+			result.Mapping = append(result.Mapping, document.Entry{
+				Key:           key,
+				KeyLocation:   keyLocation,
+				ValueLocation: value.Location,
+				Value:         value,
+			})
+		}
+		return result, nil
+	case yaml.SequenceNode:
+		result := &document.Node{Kind: document.Sequence, Location: location, Sequence: make([]*document.Node, 0, len(node.Content))}
+		for _, child := range node.Content {
+			translated, issue := s.translate(child, depth+1)
+			if issue != nil {
+				return nil, issue
+			}
+			result.Sequence = append(result.Sequence, translated)
+		}
+		return result, nil
+	case yaml.ScalarNode:
+		kind, text, issue := scalar(node)
+		if issue != nil {
+			return nil, issue
+		}
+		return &document.Node{Kind: kind, Location: location, Text: text}, nil
+	default:
+		return nil, nodeIssue(codeInvalid, "YAML node kind is not supported", node)
+	}
+}
+
+func (s *translationState) chargeNode(node *yaml.Node) *Issue {
+	s.nodes++
+	if s.limits.Nodes >= 0 && s.nodes > s.limits.Nodes {
+		return nodeIssue(codeNodeLimit, "configuration exceeds the node limit", node)
+	}
+	return nil
+}
+
+func mappingKey(node *yaml.Node) (string, *Issue) {
+	if node.Kind == yaml.AliasNode || node.Alias != nil {
+		return "", nodeIssue(codeAlias, "YAML aliases are not supported", node)
+	}
+	if node.Kind != yaml.ScalarNode {
+		return "", nodeIssue(codeScalar, "mapping keys must be strings", node)
+	}
+	if node.Style&yaml.TaggedStyle != 0 {
+		return "", nodeIssue(codeTag, "explicit YAML tags are not supported", node)
+	}
+	if node.Anchor != "" {
+		return "", nodeIssue(codeAnchor, "YAML anchors are not supported", node)
+	}
+	kind, text, issue := scalar(node)
+	if issue != nil {
+		return "", issue
+	}
+	if kind != document.String {
+		return "", nodeIssue(codeScalar, "mapping keys must be strings", node)
+	}
+	return text, nil
+}
+
+func scalar(node *yaml.Node) (document.Kind, string, *Issue) {
+	if node.Style&(yaml.DoubleQuotedStyle|yaml.SingleQuotedStyle|yaml.LiteralStyle|yaml.FoldedStyle) != 0 {
+		return document.String, node.Value, nil
+	}
+	value := node.Value
+	switch value {
+	case "":
+		if node.ShortTag() == "!!null" {
+			return document.Null, "", nil
+		}
+	case "null", "~":
+		return document.Null, "", nil
+	case "true", "false":
+		return document.Boolean, value, nil
+	}
+	lower := strings.ToLower(value)
+	if lower == "true" || lower == "false" || lower == "null" || lower == "yes" || lower == "no" || lower == "on" || lower == "off" {
+		return 0, "", nodeIssue(codeScalar, "plain scalar uses an unsupported reserved form", node)
+	}
+	if jsonNumberPattern.MatchString(value) {
+		return document.Number, value, nil
+	}
+	if decimalLikePattern.MatchString(value) || prefixedNumberPattern.MatchString(value) ||
+		leadingDecimalPattern.MatchString(value) || nonFiniteNumberPattern.MatchString(value) {
+		return 0, "", nodeIssue(codeScalar, "plain scalar uses an unsupported numeric form", node)
+	}
+	if tag := node.ShortTag(); tag == "!!int" || tag == "!!float" {
+		return 0, "", nodeIssue(codeScalar, "plain scalar uses an unsupported numeric form", node)
+	}
+	return document.String, value, nil
+}
+
+func documentRoot(node *yaml.Node) *yaml.Node {
+	if node.Kind == yaml.DocumentNode && len(node.Content) == 1 {
+		return node.Content[0]
+	}
+	return node
+}
+
+func parserIssue(err error, depthExceeded bool) Issue {
+	code := codeInvalid
+	if depthExceeded || errors.Is(err, errParserDepth) {
+		code = codeDepthLimit
+	}
+	issue := Issue{Code: code, Message: "configuration contains invalid YAML"}
+	var loadError *yaml.LoadError
+	if errors.As(err, &loadError) {
+		issue.Line = loadError.Mark.Line
+		issue.Column = loadError.Mark.Column
+	}
+	return issue
+}
+
+func nodeIssue(code, message string, node *yaml.Node) *Issue {
+	return &Issue{Code: code, Message: message, Line: node.Line, Column: node.Column}
+}

internal/yaml/parse_test.go

@@ -0,0 +1,200 @@
+package yaml
+
+import (
+	"io"
+	"strings"
+	"testing"
+
+	"go.yaml.in/yaml/v4"
+	"go.yaml.in/yaml/v4/plugin/limit"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestParsePreservesDocumentValuesAndLocations(t *testing.T) {
+	t.Parallel()
+
+	input := "# comment\nwide: 9223372036854775808\nwhen: 2026-09-06\nquoted: \"yes\"\nflow: [true, null, ~, false]\nempty:\n"
+	root, issues := Parse(strings.NewReader(input), "source-1", generousLimits())
+	if len(issues) != 0 {
+		t.Fatalf("Parse() issues = %#v", issues)
+	}
+	if root.Kind != document.Mapping || len(root.Mapping) != 5 {
+		t.Fatalf("root = %#v", root)
+	}
+	if root.Mapping[0].Value.Kind != document.Number || root.Mapping[0].Value.Text != "9223372036854775808" {
+		t.Fatalf("wide number = %#v", root.Mapping[0].Value)
+	}
+	if root.Mapping[0].KeyLocation.Line != 2 || root.Mapping[0].ValueLocation.Column != 7 {
+		t.Fatalf("locations = %#v", root.Mapping[0])
+	}
+	if root.Mapping[1].Value.Kind != document.String || root.Mapping[1].Value.Text != "2026-09-06" {
+		t.Fatalf("timestamp-like text = %#v", root.Mapping[1].Value)
+	}
+	if root.Mapping[3].Value.Sequence[1].Kind != document.Null || root.Mapping[3].Value.Sequence[2].Kind != document.Null {
+		t.Fatalf("null spellings = %#v", root.Mapping[3].Value.Sequence)
+	}
+	if root.Mapping[4].Value.Kind != document.Null {
+		t.Fatalf("empty value = %#v", root.Mapping[4].Value)
+	}
+}
+
+func TestParsePolicyTable(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		input string
+		code  string
+	}{
+		{name: "empty input", input: "", code: codeEmpty},
+		{name: "empty document", input: "---\n", code: codeEmpty},
+		{name: "sequence root", input: "- value\n", code: codeRootType},
+		{name: "scalar root", input: "value\n", code: codeRootType},
+		{name: "second document", input: "a: 1\n---\nb: 2\n", code: codeMultipleDocuments},
+		{name: "empty second document", input: "a: 1\n---\n", code: codeMultipleDocuments},
+		{name: "duplicate key", input: "a: 1\na: 2\n", code: codeDuplicateKey},
+		{name: "nested duplicate", input: "a: {b: 1, b: 2}\n", code: codeDuplicateKey},
+		{name: "explicit standard tag", input: "a: !!str value\n", code: codeTag},
+		{name: "explicit custom tag", input: "a: !custom value\n", code: codeTag},
+		{name: "anchor", input: "a: &name value\n", code: codeAnchor},
+		{name: "alias source anchor", input: "a: &name value\nb: *name\n", code: codeAnchor},
+		{name: "merge key", input: "<<: value\n", code: codeMergeKey},
+		{name: "quoted merge key", input: "\"<<\": value\n", code: codeMergeKey},
+		{name: "boolean key", input: "true: value\n", code: codeScalar},
+		{name: "number key", input: "1: value\n", code: codeScalar},
+		{name: "null key", input: "null: value\n", code: codeScalar},
+		{name: "legacy yes", input: "a: yes\n", code: codeScalar},
+		{name: "legacy mixed", input: "a: OfF\n", code: codeScalar},
+		{name: "reserved case", input: "a: TRUE\n", code: codeScalar},
+		{name: "hex", input: "a: 0x10\n", code: codeScalar},
+		{name: "octal", input: "a: 0o10\n", code: codeScalar},
+		{name: "separator", input: "a: 1_000\n", code: codeScalar},
+		{name: "leading plus", input: "a: +1\n", code: codeScalar},
+		{name: "leading zero", input: "a: 01\n", code: codeScalar},
+		{name: "negative leading zero", input: "a: -01\n", code: codeScalar},
+		{name: "leading dot", input: "a: .5\n", code: codeScalar},
+		{name: "trailing dot", input: "a: 1.\n", code: codeScalar},
+		{name: "incomplete exponent", input: "a: 1e\n", code: codeScalar},
+		{name: "incomplete signed exponent", input: "a: 1e+\n", code: codeScalar},
+		{name: "infinity", input: "a: .inf\n", code: codeScalar},
+		{name: "nan", input: "a: .NaN\n", code: codeScalar},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			_, issues := Parse(strings.NewReader(test.input), "source-1", generousLimits())
+			if len(issues) != 1 || issues[0].Code != test.code {
+				t.Fatalf("issues = %#v, want code %q", issues, test.code)
+			}
+		})
+	}
+}
+
+func TestTranslationRejectsAliasNodes(t *testing.T) {
+	t.Parallel()
+
+	node := &yaml.Node{Kind: yaml.AliasNode, Line: 2, Column: 3}
+	state := translationState{sourceID: "source-1", limits: generousLimits()}
+	if _, issue := state.translate(node, 0); issue == nil || issue.Code != codeAlias {
+		t.Fatalf("alias issue = %#v", issue)
+	}
+}
+
+func TestParseAcceptsSupportedSyntax(t *testing.T) {
+	t.Parallel()
+
+	inputs := []string{
+		"plain: text\nquoted_bool: 'ON'\nquoted_number: \"01\"\nblock: |\n  text\n",
+		"{zero: 0, negative: -1, fraction: 1.25, exponent: 1e+3, boolean: false}\n",
+		"\"true\": value\n\"1\": number key\n\"null\": null key\n",
+	}
+	for _, input := range inputs {
+		if _, issues := Parse(strings.NewReader(input), "source-1", generousLimits()); len(issues) != 0 {
+			t.Errorf("Parse(%q) issues = %#v", input, issues)
+		}
+	}
+}
+
+func TestDuplicateKeyReportsBothLocations(t *testing.T) {
+	t.Parallel()
+
+	_, issues := Parse(strings.NewReader("same: first\nsame: second\n"), "source-1", generousLimits())
+	if len(issues) != 1 || issues[0].Code != codeDuplicateKey || issues[0].Line != 2 || len(issues[0].Related) != 1 || issues[0].Related[0].Line != 1 {
+		t.Fatalf("duplicate issues = %#v", issues)
+	}
+}
+
+func TestParseEnforcesTreeAndParserLimits(t *testing.T) {
+	t.Parallel()
+
+	for _, input := range []string{"a:\n  b: value\n", "{a: {b: value}}\n"} {
+		if _, issues := Parse(strings.NewReader(input), "source-1", Limits{Depth: 2, Nodes: 20, ParserDepth: 20}); len(issues) != 0 {
+			t.Fatalf("depth boundary rejected for %q: %#v", input, issues)
+		}
+		if _, issues := Parse(strings.NewReader(input), "source-1", Limits{Depth: 1, Nodes: 20, ParserDepth: 20}); len(issues) != 1 || issues[0].Code != codeDepthLimit {
+			t.Fatalf("depth overflow for %q = %#v", input, issues)
+		}
+	}
+	if _, issues := Parse(strings.NewReader("a: 1\n"), "source-1", Limits{Depth: 2, Nodes: 3, ParserDepth: 20}); len(issues) != 0 {
+		t.Fatalf("node boundary rejected: %#v", issues)
+	}
+	if _, issues := Parse(strings.NewReader("a: 1\n"), "source-1", Limits{Depth: 2, Nodes: 2, ParserDepth: 20}); len(issues) != 1 || issues[0].Code != codeNodeLimit {
+		t.Fatalf("node overflow = %#v", issues)
+	}
+	if _, issues := Parse(strings.NewReader("a:\n  b:\n    c: value\n"), "source-1", Limits{Depth: 20, Nodes: 20, ParserDepth: 1}); len(issues) != 1 || issues[0].Code != codeDepthLimit {
+		t.Fatalf("parser depth overflow = %#v", issues)
+	}
+}
+
+func TestParserDepthCalibrationForBlockAndFlow(t *testing.T) {
+	t.Parallel()
+
+	inputs := []string{"a:\n  b: value\n", "{a: {b: value}}\n"}
+	for _, input := range inputs {
+		maximum := observedParserDepth(t, input)
+		if maximum > 4 {
+			t.Fatalf("parser depth %d exceeds root-zero tree depth plus safety offset", maximum)
+		}
+		if _, issues := Parse(strings.NewReader(input), "source-1", Limits{Depth: 2, Nodes: 20, ParserDepth: maximum}); len(issues) != 0 {
+			t.Fatalf("parser boundary %d rejected %q: %#v", maximum, input, issues)
+		}
+		if _, issues := Parse(strings.NewReader(input), "source-1", Limits{Depth: 20, Nodes: 20, ParserDepth: maximum - 1}); len(issues) != 1 || issues[0].Code != codeDepthLimit {
+			t.Fatalf("parser overflow at %d for %q: %#v", maximum-1, input, issues)
+		}
+	}
+}
+
+func observedParserDepth(t *testing.T, input string) int {
+	t.Helper()
+	maximum := 0
+	depthPlugin := limit.New(limit.DepthFunc(func(depth int, _ *limit.DepthContext) error {
+		maximum = max(maximum, depth)
+		return nil
+	}))
+	loader, err := yaml.NewLoader(strings.NewReader(input), yaml.WithPlugin(depthPlugin))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var node yaml.Node
+	if err := loader.Load(&node); err != nil && err != io.EOF {
+		t.Fatal(err)
+	}
+	return maximum
+}
+
+func TestMalformedParserDiagnosticIsSafe(t *testing.T) {
+	t.Parallel()
+
+	const canary = "CANARY_SECRET_VALUE"
+	_, issues := Parse(strings.NewReader("value: ["+canary+"\n"), "source-1", generousLimits())
+	if len(issues) != 1 || issues[0].Code != codeInvalid || issues[0].Line == 0 {
+		t.Fatalf("issues = %#v", issues)
+	}
+	if strings.Contains(issues[0].Message, canary) {
+		t.Fatal("parser diagnostic exposed input")
+	}
+}
+
+func generousLimits() Limits {
+	return Limits{Depth: 32, Nodes: 1_000, ParserDepth: 64}
+}

internal/yaml/write.go

@@ -0,0 +1,103 @@
+package yaml
+
+import (
+	"errors"
+	"io"
+	"strings"
+
+	"go.yaml.in/yaml/v4"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+var errWrite = errors.New("configkit: effective YAML rendering failed")
+
+// Write serializes one owned document tree with fully specified stable formatting.
+func Write(output io.Writer, root *document.Node) error {
+	if output == nil || root == nil {
+		return errWrite
+	}
+	dumper, err := yaml.NewDumper(strictWriter{output},
+		yaml.WithIndent(2),
+		yaml.WithCompactSeqIndent(false),
+		yaml.WithLineWidth(-1),
+		yaml.WithUnicode(true),
+		yaml.WithUniqueKeys(true),
+		yaml.WithCanonical(false),
+		yaml.WithLineBreak(yaml.LineBreakLN),
+		yaml.WithExplicitStart(false),
+		yaml.WithExplicitEnd(false),
+		yaml.WithFlowSimpleCollections(false),
+		yaml.WithQuotePreference(yaml.QuoteSingle),
+		yaml.WithAllDocuments(false),
+	)
+	if err != nil {
+		return errWrite
+	}
+	if err := dumper.Dump(adapterNode(root)); err != nil {
+		return safeWriteError(err)
+	}
+	if err := dumper.Close(); err != nil {
+		return safeWriteError(err)
+	}
+	return nil
+}
+
+func adapterNode(node *document.Node) *yaml.Node {
+	result := &yaml.Node{}
+	switch node.Kind {
+	case document.Mapping:
+		result.Kind, result.Tag = yaml.MappingNode, "!!map"
+		result.Content = make([]*yaml.Node, 0, len(node.Mapping)*2)
+		for _, entry := range node.Mapping {
+			result.Content = append(result.Content,
+				&yaml.Node{Kind: yaml.ScalarNode, Tag: "!!str", Value: entry.Key},
+				adapterNode(entry.Value),
+			)
+		}
+	case document.Sequence:
+		result.Kind, result.Tag = yaml.SequenceNode, "!!seq"
+		result.Content = make([]*yaml.Node, len(node.Sequence))
+		for index, child := range node.Sequence {
+			result.Content[index] = adapterNode(child)
+		}
+	case document.String:
+		result.Kind, result.Tag, result.Value = yaml.ScalarNode, "!!str", node.Text
+		if node.Text == "[REDACTED]" {
+			result.Style = yaml.SingleQuotedStyle
+		}
+	case document.Boolean:
+		result.Kind, result.Tag, result.Value = yaml.ScalarNode, "!!bool", node.Text
+	case document.Number:
+		result.Kind, result.Value = yaml.ScalarNode, node.Text
+		result.Tag = "!!int"
+		if strings.ContainsAny(node.Text, ".eE") {
+			result.Tag = "!!float"
+		}
+	case document.Null:
+		result.Kind, result.Tag, result.Value = yaml.ScalarNode, "!!null", "null"
+	}
+	return result
+}
+
+func safeWriteError(err error) error {
+	var dumpError *yaml.DumpError
+	if errors.As(err, &dumpError) && dumpError.Stage == yaml.WriterStage {
+		if cause := errors.Unwrap(dumpError); cause != nil {
+			return cause
+		}
+	}
+	return errWrite
+}
+
+type strictWriter struct {
+	output io.Writer
+}
+
+func (writer strictWriter) Write(data []byte) (int, error) {
+	written, err := writer.output.Write(data)
+	if err == nil && written != len(data) {
+		err = io.ErrShortWrite
+	}
+	return written, err
+}

internal/yaml/write_test.go

@@ -0,0 +1,59 @@
+package yaml
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestWriteUsesStableExplicitFormatting(t *testing.T) {
+	t.Parallel()
+	root := &document.Node{Kind: document.Mapping, Mapping: []document.Entry{
+		{Key: "empty_map", Value: &document.Node{Kind: document.Mapping, Mapping: []document.Entry{}}},
+		{Key: "empty_sequence", Value: &document.Node{Kind: document.Sequence, Sequence: []*document.Node{}}},
+		{Key: "number", Value: &document.Node{Kind: document.Number, Text: "1.25e+2"}},
+		{Key: "redacted", Value: &document.Node{Kind: document.String, Text: "[REDACTED]"}},
+		{Key: "sequence", Value: &document.Node{Kind: document.Sequence, Sequence: []*document.Node{
+			{Kind: document.String, Text: "first"},
+			{Kind: document.String, Text: "true"},
+		}}},
+		{Key: "nil_value", Value: &document.Node{Kind: document.Null}},
+	}}
+	var output bytes.Buffer
+	if err := Write(&output, root); err != nil {
+		t.Fatal(err)
+	}
+	want := "empty_map: {}\nempty_sequence: []\nnumber: 1.25e+2\nredacted: '[REDACTED]'\nsequence:\n  - first\n  - 'true'\nnil_value: null\n"
+	if output.String() != want {
+		t.Fatalf("output:\n%s\nwant:\n%s", output.String(), want)
+	}
+}
+
+func TestWritePropagatesWriterErrorsAndShortWrites(t *testing.T) {
+	t.Parallel()
+	root := &document.Node{Kind: document.Mapping, Mapping: []document.Entry{{Key: "value", Value: &document.Node{Kind: document.String, Text: "safe"}}}}
+	want := errors.New("write failed")
+	if err := Write(failingWriter{err: want}, root); !errors.Is(err, want) {
+		t.Fatalf("writer error = %v", err)
+	}
+	if err := Write(shortOutput{}, root); !errors.Is(err, io.ErrShortWrite) {
+		t.Fatalf("short writer error = %v", err)
+	}
+}
+
+type failingWriter struct {
+	err error
+}
+
+func (writer failingWriter) Write(data []byte) (int, error) {
+	return min(2, len(data)), writer.err
+}
+
+type shortOutput struct{}
+
+func (shortOutput) Write(data []byte) (int, error) {
+	return max(0, len(data)-1), nil
+}

load.go

@@ -0,0 +1,184 @@
+package configkit
+
+import (
+	"os"
+	"reflect"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+// Definition contains reusable application rules for a configuration type.
+// Secret is a pure predicate that can add sensitivity to semantic paths.
+// Definitions should be treated as read-only after being shared.
+type Definition[T any] struct {
+	Defaults func() T
+	Resolve  func(*T, *Resolution) error
+	Validate func(*T, *Validation)
+	Version  *VersionRule
+	Secret   func(Path) bool
+}
+
+// Options contains the application definition and per-load input selections.
+// Imports enables explicit path and glob imports; Profile selects one fixed profile
+// overlay; Overrides contains only values explicitly supplied by an application CLI.
+type Options[T any] struct {
+	Definition Definition[T]
+	Imports    bool
+	Profile    string
+	Filesystem *FilesystemContext
+	Overrides  []Override
+}
+
+// FilesystemContext supplies filesystem roots for imports and profiles.
+type FilesystemContext struct {
+	BaseDir string
+	Root    string
+}
+
+// Override describes one explicitly supplied application override. Value uses
+// parser-neutral built-in scalars, nil, []any, or map[string]any; Flag is a safe
+// display name and must not contain a raw argument or value.
+type Override struct {
+	Path  Path
+	Value any
+	Flag  string
+}
+
+// Resolution exposes surviving-input presence and records audited application
+// resolution changes. A resolver must not retain or concurrently use the context.
+type Resolution struct {
+	state *resolutionState
+}
+
+// VersionRule declares the input path and application checker for a schema version.
+// At that path, string and decimal-integer input is supplied as Version.Text.
+type VersionRule struct {
+	Path  Path
+	Check func(Version, *Validation) error
+}
+
+// Version preserves the absent, null, and textual states of a schema-version input.
+// Text is the exact spelling of accepted string or decimal-integer input.
+type Version struct {
+	Present bool
+	Null    bool
+	Text    string
+}
+
+// Result is a successful independently owned configuration load.
+type Result[T any] struct {
+	Config T
+
+	metadata         Metadata
+	sources          []SourceInfo
+	diagnostics      []Diagnostic
+	baseline         *document.Node
+	input            *document.Node
+	inputOrigins     map[string]Origin
+	final            *document.Node
+	resolution       []resolutionSnapshot
+	applications     []historyApplication
+	secrets          map[string]bool
+	secretReferences map[string]string
+}
+
+// Metadata returns detached effective presence, origin, and history records.
+func (r *Result[T]) Metadata() Metadata {
+	if r == nil {
+		return Metadata{}
+	}
+	return cloneMetadata(r.metadata)
+}
+
+// Sources returns a detached list of sources used by the load.
+func (r *Result[T]) Sources() []SourceInfo {
+	if r == nil {
+		return nil
+	}
+	return cloneSourceInfos(r.sources)
+}
+
+// Diagnostics returns detached nonfatal diagnostics from the load.
+func (r *Result[T]) Diagnostics() []Diagnostic {
+	if r == nil {
+		return nil
+	}
+	return cloneDiagnostics(r.diagnostics)
+}
+
+// Load parses and strictly decodes zero or one selected source, explicitly enabled
+// imports, and one selected profile. It accepts at most one Options value and
+// returns no result when any error is reported.
+func Load[T any](source Source, supplied ...Options[T]) (*Result[T], error) {
+	return loadWithSecretLookup(source, os.LookupEnv, supplied...)
+}
+
+func loadWithSecretLookup[T any](source Source, lookup secretLookup, supplied ...Options[T]) (*Result[T], error) {
+	accumulator := diagnosticAccumulator{}
+	options, diagnostics := selectOptions(source, supplied)
+	accumulator.add(diagnostics...)
+	if len(diagnostics) != 0 {
+		return nil, accumulator.loadError()
+	}
+
+	input, diagnostics := assembleBaseInput(source, options)
+	accumulator.sources = input.sources
+	accumulator.add(diagnostics...)
+	if len(diagnostics) != 0 {
+		return nil, accumulator.loadError()
+	}
+	schema, schemaIssue := buildSchema(reflect.TypeFor[T]())
+	if schemaIssue != nil {
+		accumulator.add(diagnosticFromSchemaIssue(schemaIssue))
+		return nil, accumulator.loadError()
+	}
+	versionDiagnostics := checkVersion(options.Definition.Version, input.document)
+	accumulator.add(versionDiagnostics...)
+	if hasErrors(versionDiagnostics) {
+		return nil, accumulator.loadError()
+	}
+	input, diagnostics = applyInputLayers(source, options, input)
+	accumulator.sources = input.sources
+	accumulator.add(diagnostics...)
+	if len(diagnostics) != 0 {
+		return nil, accumulator.loadError()
+	}
+
+	decoded, diagnostics := decodeLoadInput(schema, options.Definition, input)
+	accumulator.add(diagnostics...)
+	if len(diagnostics) != 0 {
+		return nil, accumulator.loadError()
+	}
+	resolved, diagnostics := resolveLoadInput(schema, options.Definition, input, decoded)
+	accumulator.add(diagnostics...)
+	if hasErrors(diagnostics) {
+		return nil, accumulator.loadError()
+	}
+	if !historyNodesWithinLimit(decoded.baseline, decoded.final, resolved.final, resolved.applications, resolved.snapshots) {
+		accumulator.add(Diagnostic{Code: CodeExplainLimitExceeded, Severity: SeverityError, Message: "retained configuration history exceeds the node limit"})
+		return nil, accumulator.loadError()
+	}
+
+	// Detach resolver-borrowed values before reference mutation or validation.
+	configuration, ownershipIssue := detachLoadConfiguration(schema, resolved.configuration, cloneUnresolvedSecrets)
+	if ownershipIssue != nil {
+		accumulator.add(*ownershipIssue)
+		return nil, accumulator.loadError()
+	}
+	secretResolution := resolveSecretReferences(reflect.ValueOf(&configuration).Elem(), schema, input.document, lookup)
+	accumulator.add(secretResolution.diagnostics...)
+	if hasErrors(secretResolution.diagnostics) {
+		return nil, accumulator.loadError()
+	}
+	if options.Definition.Validate != nil {
+		diagnostics = runValidation(&configuration, options.Definition.Validate, schema, resolved.final, input.document, secretResolution.lookups)
+		accumulator.add(diagnostics...)
+		if hasErrors(diagnostics) {
+			return nil, accumulator.loadError()
+		}
+	}
+	resolved.configuration = configuration
+	// Publication detaches equal-valued validator replacements, preserving the
+	// validated secret state, then finalizes metadata and sensitivity.
+	return publishLoadResult(schema, options.Definition, resolved, secretResolution.references, &accumulator)
+}

load_input.go

@@ -0,0 +1,88 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// assembledInput is the source/document handoff across schema and version checks.
+type assembledInput struct {
+	parsed      parsedSource
+	document    *document.Node
+	composition *composedDocument
+	sources     []SourceInfo
+}
+
+func assembleBaseInput[T any](source Source, options Options[T]) (assembledInput, []Diagnostic) {
+	parsed, diagnostics := parseSource(source, "source-1", defaultParseLimits())
+	input := assembledInput{parsed: parsed, document: parsed.document}
+	if len(diagnostics) != 0 {
+		input.sources = sourceInfos(parsed.source)
+		return input, diagnostics
+	}
+	if options.Imports {
+		input.composition, input.sources, diagnostics = loadImportedDocuments(source, options.Filesystem, parsed, options.Definition.Version, defaultImportLimits())
+		if len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+		input.document = input.composition.root
+	} else {
+		input.sources = sourceInfos(parsed.source)
+		if options.Filesystem != nil && !activeProfile(options.Profile) {
+			if diagnostic := validateFilesystemContext(source, options.Filesystem, parsed); diagnostic != nil {
+				return input, []Diagnostic{*diagnostic}
+			}
+		}
+	}
+	if !options.Imports && input.document != nil {
+		if diagnostics := disabledDirectiveDiagnostics(input.document, Path{}); len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+	}
+	return input, nil
+}
+
+func applyInputLayers[T any](source Source, options Options[T], input assembledInput) (assembledInput, []Diagnostic) {
+	if !activeProfile(options.Profile) && len(options.Overrides) == 0 {
+		return input, nil
+	}
+	composition, diagnostics := compositionForInput(input)
+	if len(diagnostics) != 0 {
+		return input, diagnostics
+	}
+	if activeProfile(options.Profile) {
+		composition, input.sources, diagnostics = loadProfileDocument(
+			source, options.Filesystem, input.parsed, composition, input.sources,
+			options.Profile, options.Definition.Version, defaultImportLimits(),
+		)
+		if len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+	}
+	if len(options.Overrides) != 0 {
+		composition, input.sources, diagnostics = applyOverrides(
+			composition, input.sources, options.Overrides, options.Definition.Version, defaultImportLimits(),
+		)
+		if len(diagnostics) != 0 {
+			return input, diagnostics
+		}
+	}
+	input.composition = composition
+	input.document = composition.root
+	return input, nil
+}
+
+// compositionForInput initializes one handoff for profiles and CLI, including
+// defaults-only CLI input. Raw source bytes and mapping keys retain their budgets.
+func compositionForInput(input assembledInput) (*composedDocument, []Diagnostic) {
+	if input.composition != nil {
+		return input.composition, nil
+	}
+	if input.document == nil {
+		return &composedDocument{root: &document.Node{Kind: document.Mapping}, origins: make(map[string]Origin)}, nil
+	}
+	composition, diagnostics := composeDocuments(input.document)
+	if len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	composition.inputBytes = input.parsed.bytes
+	composition.inputNodes = countInputNodes(input.document)
+	return composition, nil
+}

load_metadata.go

@@ -0,0 +1,289 @@
+package configkit
+
+import (
+	"slices"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func metadataFromDecoded(decoded decodedConfiguration) Metadata {
+	metadata := Metadata{
+		fields:  make(map[string]FieldInfo),
+		history: make(map[string][]Change),
+	}
+	baseline := make(map[string]*document.Node)
+	indexDocument(decoded.baseline, Path{}, baseline)
+	for pointer := range baseline {
+		path, err := ParsePath(pointer)
+		if err != nil {
+			continue
+		}
+		metadata.history[pointer] = append(metadata.history[pointer], Change{
+			Path: path, Origin: Origin{Layer: LayerDefault},
+		})
+	}
+	for pointer := range decoded.presence {
+		path, err := ParsePath(pointer)
+		if err != nil {
+			continue
+		}
+		origin := cloneOrigin(decoded.origins[pointer])
+		change := Change{Path: path, Origin: origin}
+		if _, ok := baseline[pointer]; ok {
+			change.Previous = []Origin{{Layer: LayerDefault}}
+		}
+		metadata.history[pointer] = append(metadata.history[pointer], change)
+	}
+	collectFieldInfo(decoded.final, Path{}, decoded, metadata.fields)
+	return metadata
+}
+
+func applyOverlayMetadata(metadata *Metadata, records []overlayRecord) {
+	baseline := make(map[string][]Change, len(metadata.history))
+	for pointer, history := range metadata.history {
+		for _, change := range history {
+			if change.Origin.Layer == LayerDefault {
+				baseline[pointer] = append(baseline[pointer], cloneChange(change))
+			}
+		}
+	}
+	events := make(map[string][]Change)
+	for _, record := range records {
+		exact := false
+		for _, displaced := range record.displaced {
+			pointer := displaced.path.String()
+			appendInputOriginEvent(events, baseline, pointer, displaced.path, displaced.origin)
+			appendInputChangeEvent(events, baseline, pointer, record.change)
+			if pointer == record.change.Path.String() {
+				exact = true
+			}
+		}
+		if !exact {
+			pointer := record.change.Path.String()
+			for _, previous := range record.change.Previous {
+				appendInputOriginEvent(events, baseline, pointer, record.change.Path, previous)
+			}
+			appendInputChangeEvent(events, baseline, pointer, record.change)
+		}
+	}
+	for pointer, info := range metadata.fields {
+		if !info.Explicit {
+			continue
+		}
+		path, err := ParsePath(pointer)
+		if err != nil {
+			continue
+		}
+		if history := events[pointer]; len(history) != 0 && history[len(history)-1].Origin.Layer == info.Origin.Layer && history[len(history)-1].Origin.Operation == info.Origin.Operation {
+			continue
+		}
+		appendInputOriginEvent(events, baseline, pointer, path, info.Origin)
+	}
+	metadata.history = make(map[string][]Change, len(baseline)+len(events))
+	for pointer, history := range baseline {
+		metadata.history[pointer] = cloneChanges(history)
+	}
+	for pointer, history := range events {
+		metadata.history[pointer] = append(metadata.history[pointer], cloneChanges(history)...)
+	}
+}
+
+func appendInputOriginEvent(events map[string][]Change, baseline map[string][]Change, pointer string, path Path, origin Origin) {
+	if origin.Layer == "" {
+		return
+	}
+	history := events[pointer]
+	if len(history) != 0 && originContainedBy(history[len(history)-1].Origin, origin) {
+		return
+	}
+	appendInputChangeEvent(events, baseline, pointer, Change{Path: path.clone(), Origin: cloneOrigin(origin)})
+}
+
+func appendInputChangeEvent(events map[string][]Change, baseline map[string][]Change, pointer string, value Change) {
+	change := cloneChange(value)
+	history := events[pointer]
+	if len(history) != 0 {
+		change.Previous = []Origin{cloneOrigin(history[len(history)-1].Origin)}
+	} else if defaults := baseline[pointer]; len(defaults) != 0 {
+		change.Previous = []Origin{cloneOrigin(defaults[len(defaults)-1].Origin)}
+	} else {
+		change.Previous = nil
+	}
+	events[pointer] = append(history, change)
+}
+
+func originContainedBy(candidate, container Origin) bool {
+	if candidate.Layer != container.Layer || candidate.Operation != container.Operation {
+		return false
+	}
+	for _, location := range candidate.Locations {
+		found := false
+		for _, available := range container.Locations {
+			if location == available {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return false
+		}
+	}
+	return true
+}
+
+func cloneChanges(changes []Change) []Change {
+	cloned := make([]Change, len(changes))
+	for index, change := range changes {
+		cloned[index] = cloneChange(change)
+	}
+	return cloned
+}
+
+func collectFieldInfo(node *document.Node, path Path, decoded decodedConfiguration, fields map[string]FieldInfo) {
+	pointer := path.String()
+	_, explicit := decoded.presence[pointer]
+	fields[pointer] = FieldInfo{
+		Explicit: explicit,
+		Null:     node.Kind == document.Null,
+		Origin:   cloneOrigin(decoded.origins[pointer]),
+	}
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			collectFieldInfo(entry.Value, path.Key(entry.Key), decoded, fields)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			collectFieldInfo(child, path.Index(index), decoded, fields)
+		}
+	}
+}
+
+func cloneMetadata(metadata Metadata) Metadata {
+	if metadata.fields == nil && metadata.history == nil {
+		return Metadata{}
+	}
+	cloned := Metadata{
+		fields:  make(map[string]FieldInfo, len(metadata.fields)),
+		history: make(map[string][]Change, len(metadata.history)),
+	}
+	cloned.effectivePaths = make(map[string]Path, len(metadata.effectivePaths))
+	for pointer, path := range metadata.effectivePaths {
+		cloned.effectivePaths[pointer] = path.clone()
+	}
+	cloned.historicalPaths = make(map[string][]Path, len(metadata.historicalPaths))
+	for pointer, paths := range metadata.historicalPaths {
+		cloned.historicalPaths[pointer] = clonePaths(paths)
+	}
+	for path, info := range metadata.fields {
+		cloned.fields[path] = cloneFieldInfo(info)
+	}
+	for path, changes := range metadata.history {
+		clonedChanges := make([]Change, len(changes))
+		for index, change := range changes {
+			clonedChanges[index] = cloneChange(change)
+		}
+		cloned.history[path] = clonedChanges
+	}
+	return cloned
+}
+
+func applyResolutionMetadata(metadata *Metadata, resolved resolutionResult) {
+	for _, snapshot := range resolved.snapshots {
+		pointer := snapshot.change.Path.String()
+		change := cloneChange(snapshot.change)
+		if info, ok := metadata.fields[pointer]; ok {
+			change.Previous = []Origin{cloneOrigin(info.Origin)}
+			info.Origin = cloneOrigin(change.Origin)
+			metadata.fields[pointer] = info
+		}
+		metadata.history[pointer] = append(metadata.history[pointer], change)
+	}
+	appendResolutionDescendantHistory(metadata, resolved)
+	for _, delta := range resolved.deltas {
+		record := coveringResolutionSnapshot(resolved.snapshots, delta.path)
+		if record == nil {
+			continue
+		}
+		for pointer, info := range metadata.fields {
+			path, err := ParsePath(pointer)
+			if err != nil || !pathContains(delta.path, path) {
+				continue
+			}
+			info.Origin = cloneOrigin(record.change.Origin)
+			metadata.fields[pointer] = info
+		}
+	}
+	refreshResolutionFields(metadata.fields, resolved.final, resolved.snapshots)
+}
+
+func appendResolutionDescendantHistory(metadata *Metadata, resolved resolutionResult) {
+	finalPaths := make(map[string]*document.Node)
+	indexDocument(resolved.final, Path{}, finalPaths)
+	for _, delta := range resolved.deltas {
+		record := coveringResolutionSnapshot(resolved.snapshots, delta.path)
+		if record == nil {
+			continue
+		}
+		affected := make(map[string]bool)
+		for pointer := range metadata.history {
+			if path, err := ParsePath(pointer); err == nil && pathContains(delta.path, path) {
+				affected[pointer] = true
+			}
+		}
+		for pointer := range finalPaths {
+			if path, err := ParsePath(pointer); err == nil && pathContains(delta.path, path) {
+				affected[pointer] = true
+			}
+		}
+		pointers := make([]string, 0, len(affected))
+		for pointer := range affected {
+			if pointer != record.change.Path.String() {
+				pointers = append(pointers, pointer)
+			}
+		}
+		slices.Sort(pointers)
+		for _, pointer := range pointers {
+			change := cloneChange(record.change)
+			if info, ok := metadata.fields[pointer]; ok {
+				change.Previous = []Origin{cloneOrigin(info.Origin)}
+			}
+			metadata.history[pointer] = append(metadata.history[pointer], change)
+		}
+	}
+}
+
+func coveringResolutionSnapshot(snapshots []resolutionSnapshot, path Path) *resolutionSnapshot {
+	var covering *resolutionSnapshot
+	for index := range snapshots {
+		snapshot := &snapshots[index]
+		if pathContains(snapshot.change.Path, path) && (covering == nil || len(snapshot.change.Path.segments) > len(covering.change.Path.segments)) {
+			covering = snapshot
+		}
+	}
+	return covering
+}
+
+func refreshResolutionFields(fields map[string]FieldInfo, final *document.Node, snapshots []resolutionSnapshot) {
+	finalPaths := make(map[string]*document.Node)
+	indexDocument(final, Path{}, finalPaths)
+	for pointer := range fields {
+		if _, ok := finalPaths[pointer]; !ok {
+			delete(fields, pointer)
+		}
+	}
+	for pointer, node := range finalPaths {
+		if info, ok := fields[pointer]; ok {
+			info.Null = node.Kind == document.Null
+			fields[pointer] = info
+			continue
+		}
+		origin := Origin{Layer: LayerResolution}
+		if path, err := ParsePath(pointer); err == nil {
+			if covering := coveringResolutionSnapshot(snapshots, path); covering != nil {
+				origin = cloneOrigin(covering.change.Origin)
+			}
+		}
+		fields[pointer] = FieldInfo{Null: node.Kind == document.Null, Origin: origin}
+	}
+}

load_result.go

@@ -0,0 +1,119 @@
+package configkit
+
+import (
+	"reflect"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+// resolvedLoad carries typed phase outputs needed for validation and publication;
+// it owns no source access, options, or long-lived schema state.
+type resolvedLoad[T any] struct {
+	configuration T
+	decoded       decodedConfiguration
+	metadata      Metadata
+	final         *document.Node
+	snapshots     []resolutionSnapshot
+	applications  []historyApplication
+}
+
+func decodeLoadInput[T any](schema *schemaDescriptor, definition Definition[T], input assembledInput) (decodedConfiguration, []Diagnostic) {
+	defaults := reflect.Zero(reflect.TypeFor[T]())
+	if definition.Defaults != nil {
+		defaults = reflect.ValueOf(definition.Defaults())
+	}
+	context := decodeContext{}
+	if definition.Version != nil {
+		context.versionPath = &definition.Version.Path
+	}
+	decoded, decodeIssues := decodeConfigurationWithContext(schema, defaults, input.document, context)
+	if len(decodeIssues) != 0 {
+		diagnostics := make([]Diagnostic, len(decodeIssues))
+		for i, issue := range decodeIssues {
+			diagnostics[i] = diagnosticFromDecodeIssue(issue)
+		}
+		return decodedConfiguration{}, diagnostics
+	}
+	if input.composition != nil {
+		applyCompositionOrigins(&decoded, input.composition.origins)
+	}
+
+	return decoded, nil
+}
+
+func resolveLoadInput[T any](schema *schemaDescriptor, definition Definition[T], input assembledInput, decoded decodedConfiguration) (resolvedLoad[T], []Diagnostic) {
+	var diagnostics []Diagnostic
+	configuration := decoded.value.Interface().(T)
+	metadata := metadataFromDecoded(decoded)
+	if input.composition != nil {
+		applyOverlayMetadata(&metadata, input.composition.overlays)
+	}
+	final := decoded.final
+	var resolutionSnapshots []resolutionSnapshot
+	if definition.Resolve != nil {
+		resolved := runResolution(&configuration, definition.Resolve, schema, decoded, input.document)
+		diagnostics = append(diagnostics, resolved.diagnostics...)
+		if hasErrors(resolved.diagnostics) {
+			return resolvedLoad[T]{}, diagnostics
+		}
+		final = resolved.final
+		resolutionSnapshots = resolved.snapshots
+		applyResolutionMetadata(&metadata, resolved)
+	}
+	applications := []historyApplication(nil)
+	if input.composition != nil {
+		applications = cloneHistoryApplications(input.composition.applications)
+	} else if input.document != nil {
+		applications = []historyApplication{{layer: LayerBase, value: input.document}}
+	}
+	return resolvedLoad[T]{configuration: configuration, decoded: decoded, metadata: metadata, final: final, snapshots: resolutionSnapshots, applications: applications}, diagnostics
+}
+
+func detachLoadConfiguration[T any](schema *schemaDescriptor, configuration T, mode secretCloneMode) (T, *Diagnostic) {
+	detached, issue := cloneConfigurationWithMode(schema, reflect.ValueOf(configuration), defaultProjectionLimits(), mode)
+	if issue != nil {
+		diagnostic := diagnosticFromDecodeIssue(decodeIssueFromProjection(issue))
+		var zero T
+		return zero, &diagnostic
+	}
+	return detached.Interface().(T), nil
+}
+
+func publishLoadResult[T any](schema *schemaDescriptor, definition Definition[T], resolved resolvedLoad[T], references map[string]string, accumulator *diagnosticAccumulator) (*Result[T], error) {
+	// A read-only validator can replace a value with an equal borrowed object.
+	// Detach without re-reading the environment or discarding validated state.
+	configuration, issue := detachLoadConfiguration(schema, resolved.configuration, cloneValidatedSecrets)
+	if issue != nil {
+		accumulator.add(*issue)
+		return nil, accumulator.loadError()
+	}
+	decoded, metadata := resolved.decoded, resolved.metadata
+	final, applications, resolutionSnapshots := resolved.final, resolved.applications, resolved.snapshots
+	finalizeMetadataPaths(&metadata, final, applications, decoded.baseline, decoded.final)
+	secrets := classifySecretPaths(schema, definition.Secret, metadata, final, applications, decoded.baseline, decoded.final)
+	return &Result[T]{
+		Config:           configuration,
+		metadata:         metadata,
+		sources:          cloneSourceInfos(accumulator.sources),
+		diagnostics:      canonicalDiagnostics(accumulator.diagnostics),
+		baseline:         decoded.baseline,
+		input:            decoded.final,
+		inputOrigins:     cloneOriginIndex(decoded.origins),
+		final:            final,
+		resolution:       resolutionSnapshots,
+		applications:     applications,
+		secrets:          secrets,
+		secretReferences: cloneSecretReferences(references),
+	}, nil
+}
+
+func diagnosticFromSchemaIssue(issue *schemaIssue) Diagnostic {
+	return Diagnostic{Code: issue.code, Severity: SeverityError, Path: issue.path.clone(), Message: issue.message}
+}
+
+func diagnosticFromDecodeIssue(issue decodeIssue) Diagnostic {
+	return Diagnostic{
+		Code: issue.code, Severity: SeverityError, Path: issue.path.clone(), Message: issue.message,
+		Primary: documentLocationPointer(issue.location),
+	}
+}

load_test.go

@@ -0,0 +1,604 @@
+package configkit
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestLoadSingleSourceKindsAndDefaultsOnly(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Name string `yaml:"name"`
+	}
+	input := []byte("name: configured\n")
+	temporary := t.TempDir()
+	filename := filepath.Join(temporary, "config.yml")
+	if err := os.WriteFile(filename, input, 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	tests := []struct {
+		name        string
+		source      func() Source
+		want        string
+		wantSources int
+	}{
+		{name: "file", source: func() Source { return File(filename) }, want: "configured", wantSources: 1},
+		{name: "bytes", source: func() Source { return Bytes("memory.yml", input) }, want: "configured", wantSources: 1},
+		{name: "reader", source: func() Source { return Reader("reader.yml", strings.NewReader(string(input))) }, want: "configured", wantSources: 1},
+		{name: "no source", source: NoSource, want: "default", wantSources: 0},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			var defaultsCalls atomic.Int32
+			result, err := Load(test.source(), Options[config]{Definition: Definition[config]{
+				Defaults: func() config {
+					defaultsCalls.Add(1)
+					return config{Name: "default"}
+				},
+			}})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if result.Config.Name != test.want || defaultsCalls.Load() != 1 || len(result.Sources()) != test.wantSources {
+				t.Fatalf("result = %#v, calls = %d, sources = %#v", result.Config, defaultsCalls.Load(), result.Sources())
+			}
+		})
+	}
+}
+
+func TestLoadVersionStatesWarningsAndSafeFailures(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		SchemaVersion *string `yaml:"schema_version"`
+	}
+	versionPath := Keys("schema_version")
+	tests := []struct {
+		name       string
+		source     Source
+		want       Version
+		wantLine   int
+		wantErr    Code
+		returnOnly bool
+	}{
+		{name: "absent", source: Bytes("absent.yml", []byte("{}\n")), want: Version{}, wantLine: 1},
+		{name: "no source absent", source: NoSource(), want: Version{}},
+		{name: "null", source: Bytes("null.yml", []byte("schema_version: null\n")), want: Version{Present: true, Null: true}, wantLine: 1},
+		{name: "string", source: Bytes("string.yml", []byte("schema_version: app.v1\n")), want: Version{Present: true, Text: "app.v1"}, wantLine: 1},
+		{name: "integer", source: Bytes("integer.yml", []byte("schema_version: 1\n")), want: Version{Present: true, Text: "1"}, wantLine: 1},
+		{name: "boolean", source: Bytes("boolean.yml", []byte("schema_version: true\n")), wantErr: CodeVersionType},
+		{name: "sequence", source: Bytes("sequence.yml", []byte("schema_version: []\n")), wantErr: CodeVersionType},
+		{name: "returned error", source: Bytes("return.yml", []byte("{}\n")), wantErr: CodeVersionHookFailed, returnOnly: true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			called := false
+			rule := &VersionRule{Path: versionPath, Check: func(version Version, validation *Validation) error {
+				called = true
+				if test.returnOnly {
+					return errors.New("CANARY_VERSION_ERROR")
+				}
+				if version != test.want {
+					t.Errorf("version = %#v, want %#v", version, test.want)
+				}
+				validation.Warning(versionPath, "review schema compatibility", Keys("missing_related"))
+				return nil
+			}}
+			result, err := Load(test.source, Options[config]{Definition: Definition[config]{Version: rule}})
+			if test.wantErr != "" {
+				if result != nil || err == nil {
+					t.Fatalf("result, err = %#v, %v", result, err)
+				}
+				var loadError *LoadError
+				if !errors.As(err, &loadError) {
+					t.Fatalf("error type = %T", err)
+				}
+				diagnostics := loadError.Diagnostics()
+				if len(diagnostics) != 1 || diagnostics[0].Code != test.wantErr || strings.Contains(diagnostics[0].Message, "CANARY") {
+					t.Fatalf("diagnostics = %#v", diagnostics)
+				}
+				if test.wantErr == CodeVersionType && called {
+					t.Fatal("checker ran for an unsupported version kind")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			diagnostics := result.Diagnostics()
+			if len(diagnostics) != 1 || diagnostics[0].Code != CodeVersionWarning || diagnostics[0].Severity != SeverityWarning {
+				t.Fatalf("diagnostics = %#v", diagnostics)
+			}
+			if test.wantLine == 0 && (diagnostics[0].Primary != nil || len(diagnostics[0].Related) != 0) {
+				t.Fatalf("synthetic diagnostic locations = %#v", diagnostics[0])
+			}
+			if test.wantLine != 0 && (diagnostics[0].Primary == nil || diagnostics[0].Primary.Line != test.wantLine || len(diagnostics[0].Related) != 1) {
+				t.Fatalf("diagnostic locations = %#v", diagnostics[0])
+			}
+		})
+	}
+}
+
+func TestLoadDecodesIntegerVersionOnlyAtConfiguredStringPath(t *testing.T) {
+	t.Parallel()
+
+	type stringConfig struct {
+		Version string `yaml:"version"`
+	}
+	type pointerConfig struct {
+		Version *string `yaml:"version"`
+	}
+	versionPath := Keys("version")
+
+	for _, test := range []struct {
+		name   string
+		source string
+		load   func(Source, *VersionRule) (string, error)
+	}{
+		{name: "integer into string", source: "version: 4\n", load: func(source Source, rule *VersionRule) (string, error) {
+			result, err := Load(source, Options[stringConfig]{Definition: Definition[stringConfig]{Version: rule}})
+			if err != nil {
+				return "", err
+			}
+			return result.Config.Version, nil
+		}},
+		{name: "string into string", source: "version: \"4\"\n", load: func(source Source, rule *VersionRule) (string, error) {
+			result, err := Load(source, Options[stringConfig]{Definition: Definition[stringConfig]{Version: rule}})
+			if err != nil {
+				return "", err
+			}
+			return result.Config.Version, nil
+		}},
+		{name: "integer into pointer", source: "version: 4\n", load: func(source Source, rule *VersionRule) (string, error) {
+			result, err := Load(source, Options[pointerConfig]{Definition: Definition[pointerConfig]{Version: rule}})
+			if err != nil {
+				return "", err
+			}
+			if result.Config.Version == nil {
+				t.Fatal("version pointer is nil")
+			}
+			return *result.Config.Version, nil
+		}},
+		{name: "string into pointer", source: "version: \"4\"\n", load: func(source Source, rule *VersionRule) (string, error) {
+			result, err := Load(source, Options[pointerConfig]{Definition: Definition[pointerConfig]{Version: rule}})
+			if err != nil {
+				return "", err
+			}
+			if result.Config.Version == nil {
+				t.Fatal("version pointer is nil")
+			}
+			return *result.Config.Version, nil
+		}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			var checked Version
+			rule := &VersionRule{Path: versionPath, Check: func(version Version, _ *Validation) error {
+				checked = version
+				return nil
+			}}
+			got, err := test.load(Bytes("version.yml", []byte(test.source)), rule)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got != "4" || checked != (Version{Present: true, Text: "4"}) {
+				t.Fatalf("value = %q, checked = %#v", got, checked)
+			}
+		})
+	}
+
+	if result, err := Load(Bytes("version.yml", []byte("version: 4\n")), Options[stringConfig]{}); result != nil {
+		t.Fatalf("load without rule = %#v, %v", result, err)
+	} else {
+		var loadError *LoadError
+		if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != CodeDecodeType {
+			t.Fatalf("load without rule error = %v", err)
+		}
+	}
+	type unrelatedConfig struct {
+		Version string `yaml:"version"`
+		Label   string `yaml:"label"`
+	}
+	rule := &VersionRule{Path: versionPath, Check: func(Version, *Validation) error { return nil }}
+	if result, err := Load(Bytes("version.yml", []byte("version: 4\nlabel: 5\n")), Options[unrelatedConfig]{Definition: Definition[unrelatedConfig]{Version: rule}}); result != nil {
+		t.Fatalf("unrelated coercion = %#v, %v", result, err)
+	} else {
+		var loadError *LoadError
+		if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != CodeDecodeType || loadError.Diagnostics()[0].Path.String() != "/label" {
+			t.Fatalf("unrelated coercion error = %v", err)
+		}
+	}
+}
+
+func TestLoadVersionScalarAndPathBoundaries(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Version string `yaml:"version"`
+	}
+	for _, test := range []struct {
+		name  string
+		value string
+		want  string
+	}{
+		{name: "zero", value: "0", want: "0"},
+		{name: "negative", value: "-7", want: "-7"},
+		{name: "negative zero", value: "-0", want: "-0"},
+		{name: "beyond int64", value: "123456789012345678901234567890", want: "123456789012345678901234567890"},
+		{name: "empty string", value: `""`, want: ""},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			var checked Version
+			rule := &VersionRule{Path: Keys("version"), Check: func(version Version, _ *Validation) error {
+				checked = version
+				return nil
+			}}
+			result, err := Load(Bytes("version.yml", []byte("version: "+test.value+"\n")), Options[config]{Definition: Definition[config]{Version: rule}})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if result.Config.Version != test.want || checked.Text != test.want || !checked.Present || checked.Null {
+				t.Fatalf("configuration = %#v, checked = %#v", result.Config, checked)
+			}
+		})
+	}
+
+	for _, test := range []struct {
+		name  string
+		value string
+	}{
+		{name: "fraction", value: "4.0"},
+		{name: "exponent", value: "4e0"},
+		{name: "boolean", value: "true"},
+		{name: "mapping", value: "{}"},
+		{name: "sequence", value: "[]"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			called := false
+			rule := &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error {
+				called = true
+				return nil
+			}}
+			result, err := Load(Bytes("version.yml", []byte("version: "+test.value+"\n")), Options[config]{Definition: Definition[config]{Version: rule}})
+			if result != nil || called {
+				t.Fatalf("result = %#v, checker called = %v", result, called)
+			}
+			requireDiagnostic(t, err, CodeVersionType, "/version")
+		})
+	}
+
+	type namedVersion string
+	type nested struct {
+		Versions map[string]**namedVersion `yaml:"versions"`
+		Sibling  namedVersion              `yaml:"sibling"`
+	}
+	parsed, err := ParsePath("/versions/0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	rule := &VersionRule{Path: parsed, Check: func(version Version, _ *Validation) error {
+		if version.Text != "4" {
+			t.Errorf("version = %#v", version)
+		}
+		return nil
+	}}
+	result, err := Load(Bytes("nested.yml", []byte("versions:\n  \"0\": 4\nsibling: quoted\n")), Options[nested]{Definition: Definition[nested]{Version: rule}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Config.Versions["0"] == nil || *result.Config.Versions["0"] == nil || **result.Config.Versions["0"] != "4" || result.Config.Sibling != "quoted" {
+		t.Fatalf("nested configuration = %#v", result.Config)
+	}
+	type sequenceConfig struct {
+		Versions []namedVersion `yaml:"versions"`
+	}
+	sequenceResult, err := Load(Bytes("sequence.yml", []byte("versions: [6]\n")), Options[sequenceConfig]{Definition: Definition[sequenceConfig]{Version: &VersionRule{Path: parsed, Check: func(Version, *Validation) error { return nil }}}})
+	if err != nil || len(sequenceResult.Config.Versions) != 1 || sequenceResult.Config.Versions[0] != "6" {
+		t.Fatalf("sequence configuration = %#v, %v", sequenceResult, err)
+	}
+
+	type escaped struct {
+		Versions map[string]namedVersion `yaml:"versions"`
+	}
+	escapedPath, err := ParsePath("/versions/a~1b")
+	if err != nil {
+		t.Fatal(err)
+	}
+	escapedResult, err := Load(Bytes("escaped.yml", []byte("versions:\n  a/b: 9\n")), Options[escaped]{Definition: Definition[escaped]{Version: &VersionRule{Path: escapedPath, Check: func(Version, *Validation) error { return nil }}}})
+	if err != nil || escapedResult.Config.Versions["a/b"] != "9" {
+		t.Fatalf("escaped configuration = %#v, %v", escapedResult, err)
+	}
+
+	siblingRule := &VersionRule{Path: Keys("versions", "target"), Check: func(Version, *Validation) error { return nil }}
+	if result, err := Load(Bytes("sibling.yml", []byte("versions:\n  target: 4\n  other: 5\n")), Options[escaped]{Definition: Definition[escaped]{Version: siblingRule}}); result != nil {
+		t.Fatalf("same-type sibling result = %#v, %v", result, err)
+	} else {
+		requireDiagnostic(t, err, CodeDecodeType, "/versions/other")
+	}
+}
+
+func TestLoadVersionDestinationTypesKeepOrdinaryDecoding(t *testing.T) {
+	t.Parallel()
+
+	check := func(Version, *Validation) error { return nil }
+	t.Run("integer destination", func(t *testing.T) {
+		type config struct {
+			Version int8 `yaml:"version"`
+		}
+		result, err := Load(Bytes("version.yml", []byte("version: 4\n")), Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: check}}})
+		if err != nil || result.Config.Version != 4 {
+			t.Fatalf("result = %#v, %v", result, err)
+		}
+		result, err = Load(Bytes("version.yml", []byte("version: 999\n")), Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: check}}})
+		if result != nil {
+			t.Fatalf("range result = %#v", result)
+		}
+		requireDiagnostic(t, err, CodeDecodeRange, "/version")
+		result, err = Load(Bytes("version.yml", []byte("version: \"4\"\n")), Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: check}}})
+		if result != nil {
+			t.Fatalf("quoted integer result = %#v", result)
+		}
+		requireDiagnostic(t, err, CodeDecodeType, "/version")
+	})
+
+	t.Run("dynamic destination", func(t *testing.T) {
+		type config struct {
+			Version any `yaml:"version"`
+		}
+		result, err := Load(Bytes("version.yml", []byte("version: 4\n")), Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: check}}})
+		if err != nil || result.Config.Version != int64(4) {
+			t.Fatalf("result = %#v, %v", result, err)
+		}
+		result, err = Load(Bytes("version.yml", []byte("version: 123456789012345678901234567890\n")), Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: check}}})
+		if result != nil {
+			t.Fatalf("range result = %#v", result)
+		}
+		requireDiagnostic(t, err, CodeDecodeRange, "/version")
+	})
+
+	for _, test := range []struct {
+		name string
+		load func() error
+	}{
+		{name: "text codec", load: func() error {
+			type config struct {
+				Version validatedText `yaml:"version"`
+			}
+			_, err := Load(Bytes("version.yml", []byte("version: 4\n")), Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: check}}})
+			return err
+		}},
+		{name: "duration", load: func() error {
+			type config struct {
+				Version time.Duration `yaml:"version"`
+			}
+			_, err := Load(Bytes("version.yml", []byte("version: 4\n")), Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: check}}})
+			return err
+		}},
+		{name: "secret", load: func() error {
+			type config struct {
+				Version Secret `yaml:"version"`
+			}
+			_, err := Load(Bytes("version.yml", []byte("version: 4\n")), Options[config]{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version"), Check: check}}})
+			return err
+		}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			requireDiagnostic(t, test.load(), CodeDecodeType, "/version")
+		})
+	}
+}
+
+func TestLoadVersionPolicyStopsLaterHooks(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Version string `yaml:"version"`
+	}
+	var defaultsCalled, resolutionCalled, validationCalled bool
+	rule := &VersionRule{Path: Keys("version"), Check: func(version Version, validation *Validation) error {
+		if version.Text != "4" {
+			t.Errorf("version = %#v", version)
+		}
+		validation.Error(Keys("version"), "unsupported configuration version")
+		return nil
+	}}
+	result, err := Load(Bytes("version.yml", []byte("version: 4\n")), Options[config]{Definition: Definition[config]{
+		Defaults: func() config { defaultsCalled = true; return config{} },
+		Version:  rule,
+		Resolve:  func(*config, *Resolution) error { resolutionCalled = true; return nil },
+		Validate: func(*config, *Validation) { validationCalled = true },
+	}})
+	if result != nil || defaultsCalled || resolutionCalled || validationCalled {
+		t.Fatalf("result = %#v, hooks = %v/%v/%v", result, defaultsCalled, resolutionCalled, validationCalled)
+	}
+	requireDiagnostic(t, err, CodeVersionError, "/version")
+}
+
+func requireDiagnostic(t *testing.T, err error, code Code, path string) {
+	t.Helper()
+	var loadError *LoadError
+	if !errors.As(err, &loadError) {
+		t.Fatalf("error = %T %v", err, err)
+	}
+	diagnostics := loadError.Diagnostics()
+	if len(diagnostics) != 1 || diagnostics[0].Code != code || diagnostics[0].Path.String() != path || diagnostics[0].Primary == nil {
+		t.Fatalf("diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestLoadVersionReportSuppressesReturnedErrorTextAndDefaults(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Name string `yaml:"name"`
+	}
+	var defaultsCalls atomic.Int32
+	result, err := Load(Bytes("version.yml", []byte("name: value\n")), Options[config]{Definition: Definition[config]{
+		Defaults: func() config {
+			defaultsCalls.Add(1)
+			return config{Name: "default"}
+		},
+		Version: &VersionRule{Path: Keys("schema_version"), Check: func(_ Version, validation *Validation) error {
+			validation.Error(Keys("schema_version"), "schema_version is required")
+			return errors.New("CANARY_RETURNED_ERROR")
+		}},
+	}})
+	if result != nil || err == nil || defaultsCalls.Load() != 0 {
+		t.Fatalf("result = %#v, err = %v, calls = %d", result, err, defaultsCalls.Load())
+	}
+	var loadError *LoadError
+	if !errors.As(err, &loadError) {
+		t.Fatalf("error type = %T", err)
+	}
+	diagnostics := loadError.Diagnostics()
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeVersionError || strings.Contains(diagnostics[0].Message, "CANARY") {
+		t.Fatalf("diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestLoadMetadataReflectsDefaultsInputAndDetachedRecords(t *testing.T) {
+	t.Parallel()
+
+	type nested struct {
+		Kept string `yaml:"kept"`
+		Set  string `yaml:"set"`
+	}
+	type config struct {
+		Nested   nested            `yaml:"nested"`
+		Nullable map[string]string `yaml:"nullable"`
+	}
+	result, err := Load(Bytes("metadata.yml", []byte("nested: {set: configured}\nnullable: null\n")), Options[config]{Definition: Definition[config]{
+		Defaults: func() config {
+			return config{Nested: nested{Kept: "default", Set: "default"}, Nullable: map[string]string{"key": "value"}}
+		},
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	metadata := result.Metadata()
+	kept, ok := metadata.Lookup(Keys("nested", "kept"))
+	if !ok || kept.Explicit || kept.Origin.Layer != LayerDefault {
+		t.Fatalf("kept = %#v, %v", kept, ok)
+	}
+	set, ok := metadata.Lookup(Keys("nested", "set"))
+	if !ok || !set.Explicit || set.Origin.Layer != LayerBase || len(set.Origin.Locations) != 1 {
+		t.Fatalf("set = %#v, %v", set, ok)
+	}
+	nullable, ok := metadata.Lookup(Keys("nullable"))
+	if !ok || !nullable.Explicit || !nullable.Null || nullable.Origin.Layer != LayerBase {
+		t.Fatalf("nullable = %#v, %v", nullable, ok)
+	}
+	if metadata.IsExplicit(Keys("nested", "kept")) || !metadata.IsExplicit(Keys("nested", "set")) {
+		t.Fatal("explicit presence disagrees with lookup")
+	}
+	if history := metadata.History(Keys("nested", "set")); len(history) != 2 || history[0].Origin.Layer != LayerDefault || history[1].Origin.Layer != LayerBase || len(history[1].Previous) != 1 {
+		t.Fatalf("history = %#v", history)
+	}
+
+	set.Origin.Locations[0].Line = 99
+	history := metadata.History(Keys("nested", "set"))
+	history[1].Origin.Locations[0].Line = 99
+	sources := result.Sources()
+	sources[0].Label = "changed"
+	diagnostics := result.Diagnostics()
+	if diagnostics != nil {
+		diagnostics[0].Message = "changed"
+	}
+	fresh, _ := result.Metadata().Lookup(Keys("nested", "set"))
+	if fresh.Origin.Locations[0].Line == 99 || result.Sources()[0].Label != "metadata.yml" || result.Diagnostics() != nil {
+		t.Fatal("result inspection aliases internal records")
+	}
+}
+
+func TestLoadRejectsUnavailableOptionsAndDisabledDirectives(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Name string `yaml:"name"`
+	}
+	validSource := func() Source { return Bytes("options.yml", []byte("name: value\n")) }
+	tests := []struct {
+		name    string
+		source  Source
+		options []Options[config]
+		code    Code
+	}{
+		{name: "multiple options", source: validSource(), options: []Options[config]{{}, {}}, code: CodeOptionsInvalid},
+		{name: "imports without filesystem context", source: validSource(), options: []Options[config]{{Imports: true}}, code: CodeOptionsInvalid},
+		{name: "incomplete filesystem", source: validSource(), options: []Options[config]{{Filesystem: &FilesystemContext{}}}, code: CodeOptionsInvalid},
+		{name: "missing version checker", source: validSource(), options: []Options[config]{{Definition: Definition[config]{Version: &VersionRule{Path: Keys("version")}}}}, code: CodeOptionsInvalid},
+		{name: "no-source filesystem", source: NoSource(), options: []Options[config]{{Filesystem: &FilesystemContext{}}}, code: CodeOptionsInvalid},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := Load(test.source, test.options...)
+			if result != nil || err == nil {
+				t.Fatalf("result, err = %#v, %v", result, err)
+			}
+			var loadError *LoadError
+			if !errors.As(err, &loadError) || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != test.code {
+				t.Fatalf("error = %#v", err)
+			}
+		})
+	}
+
+	result, err := Load[config](Bytes("directive.yml", []byte("nested:\n  configkit: {}\n")))
+	if result != nil || err == nil {
+		t.Fatalf("result, err = %#v, %v", result, err)
+	}
+	var loadError *LoadError
+	if !errors.As(err, &loadError) {
+		t.Fatal(err)
+	}
+	diagnostic := loadError.Diagnostics()[0]
+	if diagnostic.Code != CodeImportDisabled || diagnostic.Path.String() != "/nested/configkit" || diagnostic.Primary == nil || diagnostic.Primary.Line != 2 {
+		t.Fatalf("diagnostic = %#v", diagnostic)
+	}
+}
+
+func TestLoadReturnsNilForSourceSchemaAndDecodeFailures(t *testing.T) {
+	t.Parallel()
+
+	type invalidSchema struct {
+		Channel chan int `yaml:"channel"`
+	}
+	type validConfig struct {
+		Known int `yaml:"known"`
+	}
+	tests := []struct {
+		name string
+		run  func() (any, error)
+		code Code
+	}{
+		{name: "source", run: func() (any, error) { return Load[validConfig](File(filepath.Join(t.TempDir(), "missing.yml"))) }, code: CodeSourceMissing},
+		{name: "schema", run: func() (any, error) { return Load[invalidSchema](Bytes("schema.yml", []byte("{}\n"))) }, code: CodeSchemaUnsupportedType},
+		{name: "decode", run: func() (any, error) { return Load[validConfig](Bytes("decode.yml", []byte("unknown: 1\n"))) }, code: CodeDecodeUnknownField},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := test.run()
+			if !isNilInterface(result) || err == nil {
+				t.Fatalf("result, err = %#v, %v", result, err)
+			}
+			var loadError *LoadError
+			if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != test.code {
+				t.Fatalf("error = %#v", err)
+			}
+		})
+	}
+}
+
+func isNilInterface(value any) bool {
+	if value == nil {
+		return true
+	}
+	reflected := reflect.ValueOf(value)
+	return reflected.Kind() == reflect.Pointer && reflected.IsNil()
+}

metadata.go

@@ -0,0 +1,140 @@
+package configkit
+
+import (
+	"sort"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+// Metadata provides detached effective presence/origin and chronological history
+// queries for a load. Its zero value contains no paths.
+type Metadata struct {
+	effectivePaths  map[string]Path
+	historicalPaths map[string][]Path
+	fields          map[string]FieldInfo
+	history         map[string][]Change
+}
+
+// Lookup returns information for an effective path. Unknown and invalid paths
+// return false.
+func (m Metadata) Lookup(path Path) (FieldInfo, bool) {
+	if !path.valid() || !matchesPathShape(path, m.effectivePaths[path.String()]) {
+		return FieldInfo{}, false
+	}
+	info, ok := m.fields[path.String()]
+	if !ok {
+		return FieldInfo{}, false
+	}
+	return cloneFieldInfo(info), true
+}
+
+// IsExplicit reports whether a surviving input contribution explicitly defined
+// path.
+func (m Metadata) IsExplicit(path Path) bool {
+	info, ok := m.Lookup(path)
+	return ok && info.Explicit
+}
+
+// History returns detached definition and resolution records for path. A record's
+// Path can be an ancestor when that container operation replaced or removed path.
+// Removed historical paths can have history even though Lookup returns false.
+// Unknown and invalid paths return nil.
+func (m Metadata) History(path Path) []Change {
+	if !m.hasHistoricalPath(path) {
+		return nil
+	}
+	history, ok := m.history[path.String()]
+	if !ok {
+		return nil
+	}
+	cloned := make([]Change, len(history))
+	for i, change := range history {
+		cloned[i] = cloneChange(change)
+	}
+	return cloned
+}
+
+// HistoricalPaths returns the paths observed during loading, including effective
+// paths and paths removed by overlays or recorded resolution. It includes default
+// paths and the root, but not paths that never existed. The zero Metadata returns
+// an empty slice.
+//
+// Paths are detached, unique by canonical JSON Pointer, and sorted by that
+// spelling. Like ParsePath results, numeric tokens are resolved contextually;
+// one pointer can identify a mapping key in an earlier layer and a sequence index
+// in a later one. Use History to inspect definitions and Lookup to check whether
+// a path remains effective. Enumeration exposes no configuration values and does
+// not perform another load or resolve secrets.
+func (m Metadata) HistoricalPaths() []Path {
+	pointers := make([]string, 0, len(m.historicalPaths))
+	for pointer := range m.historicalPaths {
+		pointers = append(pointers, pointer)
+	}
+	sort.Strings(pointers)
+	paths := make([]Path, 0, len(pointers))
+	for _, pointer := range pointers {
+		// The index contains only canonical pointers produced by Path.String.
+		path, _ := ParsePath(pointer)
+		paths = append(paths, path)
+	}
+	return paths
+}
+
+// Textual pointers are lookup keys, but explicit key/index kinds remain part of
+// the query. Candidate paths are constructed from actual document containers.
+func matchesPathShape(query, candidate Path) bool {
+	if !query.valid() || query.String() != candidate.String() || len(query.segments) != len(candidate.segments) {
+		return false
+	}
+	for i, segment := range query.segments {
+		actual := candidate.segments[i]
+		if actual.kind == pathSegmentIndex {
+			index, ok := schemaIndex(segment)
+			if !ok || index != actual.index {
+				return false
+			}
+		} else if segment.kind == pathSegmentIndex {
+			return false
+		}
+	}
+	return true
+}
+
+func (m Metadata) hasHistoricalPath(path Path) bool {
+	if !path.valid() {
+		return false
+	}
+	for _, candidate := range m.historicalPaths[path.String()] {
+		if matchesPathShape(path, candidate) {
+			return true
+		}
+	}
+	return false
+}
+
+func finalizeMetadataPaths(m *Metadata, final *document.Node, applications []historyApplication, earlier ...*document.Node) {
+	m.effectivePaths = make(map[string]Path)
+	m.historicalPaths = make(map[string][]Path)
+	walkDocument(final, Path{}, func(path Path, _ *document.Node) { m.effectivePaths[path.String()] = path.clone() })
+	add := func(path Path, _ *document.Node) {
+		pointer := path.String()
+		for _, prior := range m.historicalPaths[pointer] {
+			if matchesPathShape(path, prior) {
+				return
+			}
+		}
+		m.historicalPaths[pointer] = append(m.historicalPaths[pointer], path.clone())
+	}
+	walkDocument(final, Path{}, add)
+	for _, tree := range earlier {
+		walkDocument(tree, Path{}, add)
+	}
+	for _, application := range applications {
+		// Overrides have mapping-only prefixes; parsed tokens are keys in this layer.
+		prefix := Path{}
+		for _, segment := range application.path.segments {
+			prefix = prefix.Key(segment.key)
+		}
+		walkDocument(application.value, prefix, add)
+	}
+}

metadata_test.go

@@ -0,0 +1,191 @@
+package configkit
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+)
+
+func TestMetadataHistoricalPaths(t *testing.T) {
+	t.Parallel()
+	result, err := Load[map[string]any](Bytes("base", []byte("retired: {child: 1}\nshape: {'0': base}\n'a/b~': {'': 2}\n")), Options[map[string]any]{
+		Overrides: []Override{
+			{Path: Keys("retired"), Value: nil, Flag: "--retired"},
+			{Path: Keys("shape"), Value: []any{"replacement"}, Flag: "--shape"},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	metadata := result.Metadata()
+	paths := metadata.HistoricalPaths()
+	var got []string
+	for _, path := range paths {
+		got = append(got, path.String())
+		if len(metadata.History(path)) == 0 {
+			t.Fatalf("enumerated path %s has no history", path)
+		}
+	}
+	want := []string{"", "/a~1b~0", "/a~1b~0/", "/retired", "/retired/child", "/shape", "/shape/0"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("historical paths = %v, want %v", got, want)
+	}
+	if _, ok := metadata.Lookup(paths[4]); ok {
+		t.Fatal("removed descendant remains effective")
+	}
+	if _, ok := metadata.Lookup(paths[6]); !ok {
+		t.Fatal("enumerated numeric path cannot query the new sequence shape")
+	}
+	// Returned slices and nested paths must not alias retained metadata.
+	paths[1].segments[0].key = "mutated"
+	paths[2] = Keys("replacement")
+	if again := metadata.HistoricalPaths(); again[1].String() != want[1] || again[2].String() != want[2] {
+		t.Fatal("enumeration returned aliased paths")
+	}
+	if len((Metadata{}).HistoricalPaths()) != 0 {
+		t.Fatal("zero metadata contains paths")
+	}
+}
+
+func TestHistoricalPathsExposeHiddenBaseOwnershipWithoutResolvingBaseSecrets(t *testing.T) {
+	t.Setenv("CONFIGKIT_HISTORY_UNUSED_PROD_KEY", "")
+	t.Setenv("CONFIGKIT_HISTORY_TEST_KEY", "synthetic-test-key")
+	type config struct {
+		Credential  *Secret                      `yaml:"credential"`
+		Definitions map[string]map[string]string `yaml:"definitions"`
+	}
+	for _, credential := range []string{"CONFIGKIT_HISTORY_TEST_KEY", "null"} {
+		t.Run(credential, func(t *testing.T) {
+			dir := t.TempDir()
+			root := writeImportFile(t, dir, "main.yml", "configkit:\n  imports: [{path: first.yml}, {path: second.yml}]\ncredential: CONFIGKIT_HISTORY_UNUSED_PROD_KEY\n")
+			writeImportFile(t, dir, "first.yml", "definitions: {recap: {prompt: recap}}\n")
+			writeImportFile(t, dir, "second.yml", "definitions: {recap: {output: recap.md}}\n")
+			writeImportFile(t, dir, "profiles/testing.yml", "definitions: null\ncredential: "+credential+"\n")
+			result, err := Load[config](File(root), Options[config]{Imports: true, Profile: "testing"})
+			if err != nil {
+				t.Fatal(err)
+			}
+			metadata := result.Metadata()
+			found := false
+			for _, path := range metadata.HistoricalPaths() {
+				if path.String() != "/definitions/recap" {
+					continue
+				}
+				found = true
+				owners := map[string]bool{}
+				for _, change := range metadata.History(path) {
+					if change.Origin.Layer == LayerBase {
+						for _, location := range change.Origin.Locations {
+							owners[location.SourceID] = true
+						}
+					}
+				}
+				if len(owners) != 2 {
+					t.Fatalf("hidden declaration owners = %v", owners)
+				}
+			}
+			if !found {
+				t.Fatal("hidden base declaration was not discoverable")
+			}
+		})
+	}
+}
+
+func TestMetadataQueriesReturnDetachedRecords(t *testing.T) {
+	t.Parallel()
+
+	path := Keys("service", "workers")
+	metadata := Metadata{
+		effectivePaths:  map[string]Path{path.String(): path},
+		historicalPaths: map[string][]Path{path.String(): {path}},
+		fields: map[string]FieldInfo{
+			path.String(): {
+				Explicit: true,
+				Origin: Origin{
+					Layer:     LayerBase,
+					Locations: []Location{{SourceID: "source-1", Line: 2, Column: 3}},
+				},
+			},
+		},
+		history: map[string][]Change{
+			path.String(): {{
+				Path:         path,
+				Origin:       Origin{Layer: LayerResolution, Locations: []Location{{SourceID: "source-1"}}},
+				Previous:     []Origin{{Layer: LayerDefault, Locations: []Location{{SourceID: "default"}}}},
+				Reasons:      []string{"bounded worker count"},
+				Dependencies: []Path{Keys("limits")},
+			}},
+		},
+	}
+
+	info, ok := metadata.Lookup(path)
+	if !ok || !info.Explicit || !metadata.IsExplicit(path) {
+		t.Fatalf("Lookup() = %#v, %v", info, ok)
+	}
+	info.Origin.Locations[0].SourceID = "mutated"
+	again, _ := metadata.Lookup(path)
+	if again.Origin.Locations[0].SourceID != "source-1" {
+		t.Fatal("Lookup returned aliased locations")
+	}
+
+	history := metadata.History(path)
+	history[0].Reasons[0] = "mutated"
+	history[0].Previous[0].Locations[0].SourceID = "mutated"
+	history[0].Dependencies[0].segments[0].key = "mutated"
+	againHistory := metadata.History(path)
+	if againHistory[0].Reasons[0] != "bounded worker count" ||
+		againHistory[0].Previous[0].Locations[0].SourceID != "default" ||
+		againHistory[0].Dependencies[0].String() != "/limits" {
+		t.Fatal("History returned aliased nested records")
+	}
+
+	missing := Keys("missing")
+	if _, ok := metadata.Lookup(missing); ok || metadata.IsExplicit(missing) || metadata.History(missing) != nil {
+		t.Fatal("unknown path produced metadata")
+	}
+	if _, ok := metadata.Lookup(Path{}.Index(-1)); ok {
+		t.Fatal("invalid path produced metadata")
+	}
+}
+
+func TestQueriesRespectActualHistoricalAndEffectiveContainers(t *testing.T) {
+	result, err := Load[map[string]any](Bytes("base", []byte("items: [1]\nm: {'0': 2}\nx: 3\nretired: {child: 4}\n")), Options[map[string]any]{Overrides: []Override{{Path: Keys("retired"), Value: "removed", Flag: "--retired"}}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, path := range []Path{Keys("items", "0"), Keys("m").Index(0), Keys("x", "missing"), Keys("items").Index(8)} {
+		if _, ok := result.Metadata().Lookup(path); ok {
+			t.Fatalf("invalid lookup %s", path)
+		}
+		if result.Metadata().History(path) != nil {
+			t.Fatalf("invalid history %s", path)
+		}
+		if _, ok := result.Explain(path); ok {
+			t.Fatalf("invalid explanation %s", path)
+		}
+		var out bytes.Buffer
+		if err := result.WriteExplanation(&out, path); err == nil || out.Len() != 0 {
+			t.Fatalf("invalid path wrote output: %s", path)
+		}
+	}
+	for _, pointer := range []string{"/items/0", "/m/0"} {
+		path, _ := ParsePath(pointer)
+		if _, ok := result.Metadata().Lookup(path); !ok {
+			t.Fatal("parsed path lost lookup")
+		}
+		if _, ok := result.Explain(path); !ok {
+			t.Fatal("parsed path lost explanation")
+		}
+	}
+	retired := Keys("retired", "child")
+	if _, ok := result.Metadata().Lookup(retired); ok {
+		t.Fatal("removed path remains effective")
+	}
+	if len(result.Metadata().History(retired)) == 0 {
+		t.Fatal("removed history missing")
+	}
+	x, ok := result.Explain(retired)
+	if !ok || x.Effective.Present || len(x.History) == 0 {
+		t.Fatal("removed explanation missing")
+	}
+}

options.go

@@ -0,0 +1,41 @@
+package configkit
+
+func selectOptions[T any](source Source, supplied []Options[T]) (Options[T], []Diagnostic) {
+	if len(supplied) > 1 {
+		return Options[T]{}, []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "Load accepts at most one options value")}
+	}
+	var options Options[T]
+	if len(supplied) == 1 {
+		options = supplied[0]
+	}
+	var diagnostics []Diagnostic
+	definition := options.Definition
+	if !validProfileName(options.Profile) {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "profile name must contain only ASCII letters, digits, underscore, or hyphen"))
+	}
+	if options.Filesystem != nil {
+		diagnostics = append(diagnostics, filesystemOptionDiagnostics(source, options.Filesystem)...)
+	}
+	if source.kind == sourceNone && options.Imports {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "NoSource cannot enable imports"))
+	}
+	if source.kind == sourceNone && options.Profile != "" && options.Profile != "default" {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "NoSource cannot select an active profile"))
+	}
+	if options.Filesystem == nil && (source.kind == sourceBytes || source.kind == sourceReader) && (options.Imports || activeProfile(options.Profile)) {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "byte and reader imports or profiles require filesystem context"))
+	}
+	if definition.Version != nil {
+		if !definition.Version.Path.valid() {
+			diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "schema-version path is invalid"))
+		}
+		if definition.Version.Check == nil {
+			diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "schema-version rule requires a checker"))
+		}
+	}
+	return options, canonicalDiagnostics(diagnostics)
+}
+
+func optionDiagnostic(code Code, message string) Diagnostic {
+	return Diagnostic{Code: code, Severity: SeverityError, Message: message}
+}

overlay.go

@@ -0,0 +1,161 @@
+package configkit
+
+import (
+	"slices"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type overlayRecord struct {
+	change    Change
+	displaced []displacedDefinition
+}
+
+type displacedDefinition struct {
+	path   Path
+	origin Origin
+}
+
+func overlayDocuments(base *composedDocument, overlay *document.Node) (*composedDocument, []Diagnostic) {
+	if base == nil || base.root == nil || base.root.Kind != document.Mapping || overlay == nil || overlay.Kind != document.Mapping {
+		return nil, []Diagnostic{{
+			Code: CodeProfileInvalid, Severity: SeverityError,
+			Message: "profile overlay requires mapping document roots",
+		}}
+	}
+	result := cloneComposedDocument(base)
+	mergeOverlayOrigin(result.origins, Path{}, overlay.Location, LayerProfile)
+	mergeOverlayMappings(result, result.root, overlay, Path{}, LayerProfile, true)
+	result.applications = append(result.applications, historyApplication{layer: LayerProfile, value: overlay})
+	return result, nil
+}
+
+func mergeOverlayMappings(result *composedDocument, target, overlay *document.Node, current Path, layer Layer, recordReplacements bool) {
+	for _, incoming := range overlay.Mapping {
+		path := current.Key(incoming.Key)
+		index := documentEntryIndex(target.Mapping, incoming.Key)
+		if index < 0 {
+			cloned := incoming
+			cloned.Value = document.Clone(incoming.Value)
+			target.Mapping = append(target.Mapping, cloned)
+			collectOverlayOrigins(result.origins, incoming.Value, path, layer)
+			continue
+		}
+
+		existing := target.Mapping[index].Value
+		if existing.Kind == document.Mapping && incoming.Value.Kind == document.Mapping {
+			mergeOverlayOrigin(result.origins, path, incoming.Value.Location, layer)
+			mergeOverlayMappings(result, existing, incoming.Value, path, layer, recordReplacements)
+			continue
+		}
+
+		displaced := displacedOrigins(existing, result.origins, path)
+		previous := cloneOrigin(result.origins[path.String()])
+		deleteOriginSubtree(result.origins, path)
+		target.Mapping[index] = incoming
+		target.Mapping[index].Value = document.Clone(incoming.Value)
+		collectOverlayOrigins(result.origins, incoming.Value, path, layer)
+		if recordReplacements {
+			result.overlays = append(result.overlays, overlayRecord{
+				change: Change{
+					Path: path.clone(), Origin: overlayOrigin(layer, incoming.Value.Location),
+					Previous: previousOrigins(previous),
+				},
+				displaced: displaced,
+			})
+		}
+	}
+}
+
+func collectOverlayOrigins(origins map[string]Origin, node *document.Node, path Path, layer Layer) {
+	origins[path.String()] = overlayOrigin(layer, node.Location)
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			collectOverlayOrigins(origins, entry.Value, path.Key(entry.Key), layer)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			collectOverlayOrigins(origins, child, path.Index(index), layer)
+		}
+	}
+}
+
+func mergeOverlayOrigin(origins map[string]Origin, path Path, location document.Location, layer Layer) {
+	pointer := path.String()
+	origin := cloneOrigin(origins[pointer])
+	origin.Layer = layer
+	origin.Locations = append(origin.Locations, publicLocation(location))
+	origins[pointer] = origin
+}
+
+func overlayOrigin(layer Layer, location document.Location) Origin {
+	return Origin{Layer: layer, Locations: []Location{publicLocation(location)}}
+}
+
+func previousOrigins(origin Origin) []Origin {
+	if origin.Layer == "" {
+		return nil
+	}
+	return []Origin{origin}
+}
+
+func displacedOrigins(node *document.Node, origins map[string]Origin, path Path) []displacedDefinition {
+	var displaced []displacedDefinition
+	collectDisplacedOrigins(&displaced, node, origins, path, Origin{})
+	slices.SortFunc(displaced, func(left, right displacedDefinition) int {
+		return strings.Compare(left.path.String(), right.path.String())
+	})
+	return displaced
+}
+
+func collectDisplacedOrigins(displaced *[]displacedDefinition, node *document.Node, origins map[string]Origin, path Path, inherited Origin) {
+	origin, ok := origins[path.String()]
+	if !ok {
+		origin = cloneOrigin(inherited)
+		origin.Locations = []Location{publicLocation(node.Location)}
+	}
+	*displaced = append(*displaced, displacedDefinition{path: path.clone(), origin: cloneOrigin(origin)})
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			collectDisplacedOrigins(displaced, entry.Value, origins, path.Key(entry.Key), origin)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			collectDisplacedOrigins(displaced, child, origins, path.Index(index), origin)
+		}
+	}
+}
+
+func deleteOriginSubtree(origins map[string]Origin, path Path) {
+	prefix := path.String()
+	for pointer := range origins {
+		if pointer == prefix || strings.HasPrefix(pointer, prefix+"/") {
+			delete(origins, pointer)
+		}
+	}
+}
+
+func cloneOriginIndex(origins map[string]Origin) map[string]Origin {
+	cloned := make(map[string]Origin, len(origins))
+	for pointer, origin := range origins {
+		cloned[pointer] = cloneOrigin(origin)
+	}
+	return cloned
+}
+
+func cloneOverlayRecords(records []overlayRecord) []overlayRecord {
+	cloned := make([]overlayRecord, len(records))
+	for index, record := range records {
+		cloned[index].change = cloneChange(record.change)
+		cloned[index].displaced = make([]displacedDefinition, len(record.displaced))
+		for displacedIndex, displaced := range record.displaced {
+			cloned[index].displaced[displacedIndex] = displacedDefinition{
+				path: displaced.path.clone(), origin: cloneOrigin(displaced.origin),
+			}
+		}
+	}
+	return cloned
+}

overlay_test.go

@@ -0,0 +1,78 @@
+package configkit
+
+import (
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestOverlayDocumentsRecursesMappingsAndReplacesAtomicSubtrees(t *testing.T) {
+	baseRoot := parseCompositionDocument(t, "source-1", `shared:
+  kept: base
+  changed: old
+sequence: [one, two]
+nullable: value
+to_map: scalar
+to_scalar: {child: old}
+`)
+	base, diagnostics := composeDocuments(baseRoot)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	overlay := parseCompositionDocument(t, "source-2", `shared:
+  changed: new
+  added: profile
+sequence: []
+nullable: null
+to_map: {child: new}
+to_scalar: scalar
+`)
+	baseBefore := document.Clone(base.root)
+	overlayBefore := document.Clone(overlay)
+
+	result, diagnostics := overlayDocuments(base, overlay)
+	if len(diagnostics) != 0 {
+		t.Fatalf("overlayDocuments() diagnostics = %#v", diagnostics)
+	}
+	checks := []struct {
+		path Path
+		kind document.Kind
+		text string
+	}{
+		{path: Keys("shared", "kept"), kind: document.String, text: "base"},
+		{path: Keys("shared", "changed"), kind: document.String, text: "new"},
+		{path: Keys("shared", "added"), kind: document.String, text: "profile"},
+		{path: Keys("sequence"), kind: document.Sequence},
+		{path: Keys("nullable"), kind: document.Null},
+		{path: Keys("to_map"), kind: document.Mapping},
+		{path: Keys("to_map", "child"), kind: document.String, text: "new"},
+		{path: Keys("to_scalar"), kind: document.String, text: "scalar"},
+	}
+	for _, check := range checks {
+		node, ok := documentAtPath(result.root, check.path)
+		if !ok || node.Kind != check.kind || node.Text != check.text {
+			t.Errorf("node %s = %#v, %v", check.path.String(), node, ok)
+		}
+	}
+	if _, exists := result.origins["/sequence/0"]; exists {
+		t.Fatal("replaced empty sequence retained a stale index origin")
+	}
+	if _, exists := result.origins["/to_scalar/child"]; exists {
+		t.Fatal("parent replacement retained a stale child origin")
+	}
+	shared := result.origins["/shared"]
+	if shared.Layer != LayerProfile || len(shared.Locations) != 2 {
+		t.Fatalf("shared mapping origin = %#v", shared)
+	}
+	if len(result.overlays) != 5 {
+		t.Fatalf("overlay records = %#v, want five replacements", result.overlays)
+	}
+	if !reflect.DeepEqual(base.root, baseBefore) || !reflect.DeepEqual(overlay, overlayBefore) {
+		t.Fatal("overlay mutated an input document")
+	}
+	result.root.Mapping[0].Value.Mapping[0].Value.Text = "changed"
+	if !reflect.DeepEqual(base.root, baseBefore) || !reflect.DeepEqual(overlay, overlayBefore) {
+		t.Fatal("overlay result aliases an input document")
+	}
+}

override.go

@@ -0,0 +1,223 @@
+package configkit
+
+import (
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+const maxOverrides = 1024
+
+type preparedOverride struct {
+	path   Path
+	value  *document.Node
+	source SourceInfo
+	bytes  int64
+	nodes  int
+}
+
+func prepareOverrides(base *composedDocument, sources []SourceInfo, overrides []Override, rule *VersionRule, limits importLimits) ([]preparedOverride, []SourceInfo, []Diagnostic) {
+	if len(overrides) > maxOverrides || len(sources)+len(overrides) > limits.sources {
+		return nil, cloneSourceInfos(sources), []Diagnostic{{
+			Code: CodeOverrideLimitExceeded, Severity: SeverityError,
+			Message: "configuration overrides exceed the record or source-count limit",
+		}}
+	}
+
+	var diagnostics []Diagnostic
+	for index, override := range overrides {
+		if !validOverrideLabel(override.Flag) {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOverrideInvalidLabel, Severity: SeverityError,
+				Message: "override record " + strconv.Itoa(index+1) + " requires a safe nonempty flag name",
+			})
+		}
+	}
+	if len(diagnostics) != 0 {
+		return nil, cloneSourceInfos(sources), canonicalDiagnostics(diagnostics)
+	}
+
+	prepared := make([]preparedOverride, len(overrides))
+	allSources := cloneSourceInfos(sources)
+	for index, override := range overrides {
+		sourceID := "source-" + strconv.Itoa(len(sources)+index+1)
+		info := SourceInfo{ID: sourceID, Label: override.Flag, Layer: LayerCLI}
+		allSources = append(allSources, info)
+		prepared[index].source = info
+	}
+
+	seen := make(map[string]int, len(overrides))
+	pathAccepted := make([]bool, len(overrides))
+	var usedBytes int64
+	usedNodes := 0
+	for index, override := range overrides {
+		location := &Location{SourceID: prepared[index].source.ID}
+		if issue := validateOverridePath(override.Path, base.root); issue != nil {
+			issue.Primary = location
+			diagnostics = append(diagnostics, *issue)
+			continue
+		}
+		pointer := override.Path.String()
+		if previous, duplicate := seen[pointer]; duplicate {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOverrideDuplicate, Severity: SeverityError, Path: override.Path.clone(),
+				Message: "configuration override path is repeated", Primary: location,
+				Related: []Location{{SourceID: prepared[previous].source.ID}},
+			})
+			continue
+		}
+		seen[pointer] = index
+		if conflicting := overlappingOverride(overrides[:index], pathAccepted[:index], override.Path); conflicting >= 0 {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOverrideOverlap, Severity: SeverityError, Path: override.Path.clone(),
+				Message: "configuration override paths overlap", Primary: location,
+				Related: []Location{{SourceID: prepared[conflicting].source.ID}},
+			})
+			continue
+		}
+		if rule != nil && (pathContains(override.Path, rule.Path) || pathContains(rule.Path, override.Path)) {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOverrideVersion, Severity: SeverityError, Path: override.Path.clone(),
+				Message: "configuration overrides cannot replace the schema-version path", Primary: location,
+			})
+			continue
+		}
+		if pathContainsDirective(override.Path) {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOverrideDirective, Severity: SeverityError, Path: override.Path.clone(),
+				Message: "configuration overrides cannot create configuration directives", Primary: location,
+			})
+			continue
+		}
+		pathAccepted[index] = true
+
+		builder := overrideValueBuilder{sourceID: prepared[index].source.ID, active: make(map[valueVisit]bool)}
+		value, issue := builder.build(override.Value, override.Path, len(override.Path.segments))
+		if issue != nil {
+			issue.Primary = location
+			diagnostics = append(diagnostics, *issue)
+			continue
+		}
+		if relative, ok := overrideDirectivePath(value, Path{}); ok {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOverrideDirective, Severity: SeverityError, Path: appendOverridePath(override.Path, relative),
+				Message: "configuration overrides cannot create configuration directives", Primary: location,
+			})
+			continue
+		}
+		nodes := builder.nodes + len(override.Path.segments)*2
+		if nodes > limits.nodes || base.inputNodes+usedNodes+nodes > limits.nodes {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOverrideLimitExceeded, Severity: SeverityError, Path: override.Path.clone(),
+				Message: "configuration overrides exceed the aggregate node limit", Primary: location,
+			})
+			continue
+		}
+		if builder.bytes > maxSourceBytes || base.inputBytes+usedBytes+builder.bytes > limits.bytes {
+			diagnostics = append(diagnostics, Diagnostic{
+				Code: CodeOverrideLimitExceeded, Severity: SeverityError, Path: override.Path.clone(),
+				Message: "configuration overrides exceed a record or aggregate string-byte limit", Primary: location,
+			})
+			continue
+		}
+		prepared[index].path = override.Path.clone()
+		prepared[index].value = value
+		prepared[index].bytes = builder.bytes
+		prepared[index].nodes = nodes
+		usedBytes += builder.bytes
+		usedNodes += nodes
+	}
+	if len(diagnostics) != 0 {
+		return nil, allSources, canonicalDiagnostics(diagnostics)
+	}
+
+	return prepared, allSources, nil
+}
+
+func validOverrideLabel(label string) bool {
+	if label == "" || strings.TrimSpace(label) != label || !utf8.ValidString(label) {
+		return false
+	}
+	for _, character := range label {
+		if unicode.IsControl(character) {
+			return false
+		}
+	}
+	return true
+}
+
+func validateOverridePath(path Path, root *document.Node) *Diagnostic {
+	if !path.valid() || len(path.segments) == 0 {
+		return &Diagnostic{Code: CodeOverrideInvalidPath, Severity: SeverityError, Path: path.clone(), Message: "configuration override path must be a valid non-root mapping path"}
+	}
+	current := root
+	for index, segment := range path.segments {
+		if segment.kind == pathSegmentIndex {
+			return &Diagnostic{Code: CodeOverrideInvalidPath, Severity: SeverityError, Path: path.clone(), Message: "configuration override paths cannot contain sequence indices"}
+		}
+		if current == nil {
+			continue
+		}
+		if current.Kind != document.Mapping {
+			return &Diagnostic{Code: CodeOverrideInvalidPath, Severity: SeverityError, Path: path.clone(), Message: "configuration override path traverses a non-mapping value"}
+		}
+		position := documentEntryIndex(current.Mapping, segment.key)
+		if index == len(path.segments)-1 {
+			continue
+		}
+		if position < 0 {
+			current = nil
+			continue
+		}
+		current = current.Mapping[position].Value
+	}
+	return nil
+}
+
+func overlappingOverride(previous []Override, valid []bool, path Path) int {
+	for index, override := range previous {
+		if valid[index] && (pathContains(override.Path, path) || pathContains(path, override.Path)) {
+			return index
+		}
+	}
+	return -1
+}
+
+func pathContainsDirective(path Path) bool {
+	for _, segment := range path.segments {
+		if segment.key == directiveKey {
+			return true
+		}
+	}
+	return false
+}
+
+func appendOverridePath(base, relative Path) Path {
+	result := base.clone()
+	result.segments = append(result.segments, clonePathSegments(relative.segments, 0)...)
+	return result
+}
+
+func overrideDirectivePath(node *document.Node, path Path) (Path, bool) {
+	if node.Kind == document.Mapping {
+		for _, entry := range node.Mapping {
+			child := path.Key(entry.Key)
+			if entry.Key == directiveKey {
+				return child, true
+			}
+			if found, ok := overrideDirectivePath(entry.Value, child); ok {
+				return found, true
+			}
+		}
+	} else if node.Kind == document.Sequence {
+		for index, child := range node.Sequence {
+			if found, ok := overrideDirectivePath(child, path.Index(index)); ok {
+				return found, true
+			}
+		}
+	}
+	return Path{}, false
+}

override_apply.go

@@ -0,0 +1,100 @@
+package configkit
+
+import (
+	"slices"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func applyOverrides(base *composedDocument, sources []SourceInfo, overrides []Override, rule *VersionRule, limits importLimits) (*composedDocument, []SourceInfo, []Diagnostic) {
+	if len(overrides) == 0 {
+		return base, cloneSourceInfos(sources), nil
+	}
+	prepared, allSources, diagnostics := prepareOverrides(base, sources, overrides, rule, limits)
+	if len(diagnostics) != 0 {
+		return nil, allSources, diagnostics
+	}
+	result := cloneComposedDocument(base)
+	for _, override := range prepared {
+		applyPreparedOverride(result, override)
+		result.inputBytes += override.bytes
+		result.inputNodes += override.nodes
+	}
+	return result, allSources, nil
+}
+
+func applyPreparedOverride(result *composedDocument, override preparedOverride) {
+	location := document.Location{SourceID: override.source.ID}
+	mergeOverlayOrigin(result.origins, Path{}, location, LayerCLI)
+	current := result.root
+	currentPath := Path{}
+	for index, segment := range override.path.segments {
+		path := currentPath.Key(segment.key)
+		position := documentEntryIndex(current.Mapping, segment.key)
+		if index < len(override.path.segments)-1 {
+			if position < 0 {
+				child := &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{}}
+				current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: child})
+				collectOverlayOrigins(result.origins, child, path, LayerCLI)
+				current = child
+			} else {
+				current = current.Mapping[position].Value
+				mergeOverlayOrigin(result.origins, path, location, LayerCLI)
+			}
+			currentPath = path
+			continue
+		}
+
+		previous := cloneOrigin(result.origins[path.String()])
+		var displaced []displacedDefinition
+		if position < 0 {
+			incoming := document.Clone(override.value)
+			current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: incoming})
+			collectOverlayOrigins(result.origins, incoming, path, LayerCLI)
+		} else {
+			existing := current.Mapping[position].Value
+			displaced = affectedOverrideOrigins(existing, override.value, result.origins, path)
+			if existing.Kind == document.Mapping && override.value.Kind == document.Mapping {
+				mergeOverlayOrigin(result.origins, path, location, LayerCLI)
+				mergeOverlayMappings(result, existing, override.value, path, LayerCLI, false)
+			} else {
+				deleteOriginSubtree(result.origins, path)
+				current.Mapping[position] = document.Entry{Key: segment.key, KeyLocation: location, ValueLocation: location, Value: document.Clone(override.value)}
+				collectOverlayOrigins(result.origins, current.Mapping[position].Value, path, LayerCLI)
+			}
+		}
+		result.overlays = append(result.overlays, overlayRecord{change: Change{
+			Path: path.clone(), Origin: overlayOrigin(LayerCLI, location), Previous: previousOrigins(previous),
+		}, displaced: displaced})
+		result.applications = append(result.applications, historyApplication{
+			layer: LayerCLI, path: path.clone(), value: override.value,
+			origin: overlayOrigin(LayerCLI, location),
+		})
+	}
+}
+
+func affectedOverrideOrigins(existing, incoming *document.Node, origins map[string]Origin, path Path) []displacedDefinition {
+	displaced := []displacedDefinition{{path: path.clone(), origin: cloneOrigin(origins[path.String()])}}
+	if existing.Kind == document.Mapping && incoming.Kind == document.Mapping {
+		for _, entry := range incoming.Mapping {
+			position := documentEntryIndex(existing.Mapping, entry.Key)
+			if position < 0 {
+				continue
+			}
+			childPath := path.Key(entry.Key)
+			childExisting := existing.Mapping[position].Value
+			if childExisting.Kind == document.Mapping && entry.Value.Kind == document.Mapping {
+				displaced = append(displaced, affectedOverrideOrigins(childExisting, entry.Value, origins, childPath)...)
+			} else {
+				displaced = append(displaced, displacedOrigins(childExisting, origins, childPath)...)
+			}
+		}
+	} else {
+		displaced = displacedOrigins(existing, origins, path)
+	}
+	slices.SortFunc(displaced, func(left, right displacedDefinition) int {
+		return strings.Compare(left.path.String(), right.path.String())
+	})
+	return displaced
+}

override_test.go

@@ -0,0 +1,307 @@
+package configkit
+
+import (
+	"errors"
+	"math"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type overrideLifecycleSettings struct {
+	Target  string            `yaml:"target"`
+	Count   int               `yaml:"count"`
+	Enabled bool              `yaml:"enabled"`
+	Text    string            `yaml:"text"`
+	Items   []string          `yaml:"items"`
+	Maybe   *string           `yaml:"maybe"`
+	Labels  map[string]string `yaml:"labels"`
+	APIKey  Secret            `yaml:"api_key"`
+}
+
+type overrideLifecycleConfig struct {
+	Version  string                    `yaml:"version"`
+	Settings overrideLifecycleSettings `yaml:"settings"`
+}
+
+func TestOverridesRunAtHighestInputPrecedenceBeforeResolutionAndValidation(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", "configkit:\n  imports: [{path: base.yml}]\nversion: v1\n")
+	writeImportFile(t, directory, "base.yml", `settings:
+  target: base
+  count: 4
+  enabled: true
+  text: base
+  items: [base]
+  maybe: base
+  labels: {base: retained}
+  api_key: BASE_SECRET
+`)
+	writeImportFile(t, directory, filepath.Join("profiles", "testing.yml"), `settings:
+  target: profile
+  labels: {profile: retained}
+`)
+	validated := false
+	overrides := []Override{
+		{Path: Keys("settings", "target"), Value: " cli ", Flag: "--target"},
+		{Path: Keys("settings", "count"), Value: 0, Flag: "--count"},
+		{Path: Keys("settings", "enabled"), Value: false, Flag: "--enabled"},
+		{Path: Keys("settings", "text"), Value: "", Flag: "--text"},
+		{Path: Keys("settings", "items"), Value: []any{}, Flag: "--items"},
+		{Path: Keys("settings", "maybe"), Value: nil, Flag: "--maybe"},
+		{Path: Keys("settings", "labels"), Value: map[string]any{"cli": "selected"}, Flag: "--label"},
+		{Path: Keys("settings", "api_key"), Value: "CLI_SECRET", Flag: "--api-key-env"},
+	}
+	result, err := loadWithSecretLookup(File(mainPath), func(name string) (string, bool) {
+		if name != "CLI_SECRET" {
+			t.Fatalf("looked up superseded secret name %q", name)
+		}
+		return "credential-value", true
+	}, Options[overrideLifecycleConfig]{
+		Imports: true, Profile: "testing", Overrides: overrides,
+		Definition: Definition[overrideLifecycleConfig]{
+			Defaults: func() overrideLifecycleConfig {
+				return overrideLifecycleConfig{Settings: overrideLifecycleSettings{Target: "default", Count: 9, Text: "default", Items: []string{"default"}}}
+			},
+			Version: &VersionRule{Path: Keys("version"), Check: func(version Version, validation *Validation) error {
+				if version.Text != "v1" {
+					validation.Error(Keys("version"), "unsupported version")
+				}
+				return nil
+			}},
+			Resolve: func(configuration *overrideLifecycleConfig, resolution *Resolution) error {
+				Normalize(resolution, Keys("settings", "target"), &configuration.Settings.Target, strings.TrimSpace(configuration.Settings.Target), "trimmed target")
+				return nil
+			},
+			Validate: func(configuration *overrideLifecycleConfig, validation *Validation) {
+				validated = configuration.Settings.Target == "cli" && configuration.Settings.APIKey.Value() == "credential-value"
+			},
+		},
+	})
+	if err != nil {
+		t.Fatalf("loadWithSecretLookup() error = %v", err)
+	}
+	want := overrideLifecycleSettings{
+		Target: "cli", Labels: map[string]string{"base": "retained", "profile": "retained", "cli": "selected"},
+		APIKey: result.Config.Settings.APIKey,
+	}
+	if !validated || result.Config.Settings.Count != 0 || result.Config.Settings.Enabled || result.Config.Settings.Text != "" ||
+		len(result.Config.Settings.Items) != 0 || result.Config.Settings.Items == nil || result.Config.Settings.Maybe != nil ||
+		result.Config.Settings.Target != want.Target || !reflect.DeepEqual(result.Config.Settings.Labels, want.Labels) {
+		t.Fatalf("effective settings = %#v", result.Config.Settings)
+	}
+	sources := result.Sources()
+	if len(sources) != 3+len(overrides) || sources[0].Layer != LayerBase || sources[1].Layer != LayerBase || sources[2].Layer != LayerProfile {
+		t.Fatalf("sources = %#v", sources)
+	}
+	for index, override := range overrides {
+		if sources[index+3].Layer != LayerCLI || sources[index+3].Label != override.Flag || sources[index+3].Path != "" {
+			t.Fatalf("CLI source %d = %#v", index, sources[index+3])
+		}
+	}
+	history := result.Metadata().History(Keys("settings", "target"))
+	wantLayers := []Layer{LayerDefault, LayerBase, LayerProfile, LayerCLI, LayerResolution}
+	if got := historyLayers(history); !reflect.DeepEqual(got, wantLayers) {
+		t.Fatalf("target history layers = %#v, want %#v; history %#v", got, wantLayers, history)
+	}
+}
+
+func TestOverridesSupportDefaultsOnlyMapMergeListReplacementAndNumericKeys(t *testing.T) {
+	type config struct {
+		Labels  map[string]string `yaml:"labels"`
+		Items   []int             `yaml:"items"`
+		Numbers map[string]int    `yaml:"numbers"`
+		Dynamic any               `yaml:"dynamic"`
+	}
+	labels := map[string]any{"0": "zero", "new": "value"}
+	items := []any{0, 2}
+	dynamic := map[string]any{
+		"int": int8(1), "uint": uint16(2), "float": float32(3.5), "bool": false,
+		"null": nil, "list": []any{"value"}, "map": map[string]any{"key": "value"},
+	}
+	result, err := Load(NoSource(), Options[config]{
+		Definition: Definition[config]{Defaults: func() config {
+			return config{Labels: map[string]string{"kept": "default"}, Items: []int{9}}
+		}},
+		Overrides: []Override{
+			{Path: parseOverrideTestPath(t, "/labels"), Value: labels, Flag: "--labels"},
+			{Path: Keys("items"), Value: items, Flag: "--items"},
+			{Path: parseOverrideTestPath(t, "/numbers/0"), Value: 7, Flag: "--number"},
+			{Path: Keys("dynamic"), Value: dynamic, Flag: "--dynamic"},
+		},
+	})
+	labels["new"] = "mutated"
+	items[0] = 99
+	dynamic["int"] = 99
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(result.Config.Labels, map[string]string{"kept": "default", "0": "zero", "new": "value"}) || !reflect.DeepEqual(result.Config.Items, []int{0, 2}) || result.Config.Numbers["0"] != 7 || result.Config.Dynamic.(map[string]any)["int"] != int64(1) {
+		t.Fatalf("defaults-only overrides = %#v", result.Config)
+	}
+	var effective strings.Builder
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String(), "mutated") || !strings.Contains(effective.String(), "new: value") {
+		t.Fatalf("detached effective output = %q", effective.String())
+	}
+	if info, ok := result.Metadata().Lookup(Keys("labels", "0")); !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
+		t.Fatalf("numeric mapping key metadata = %#v, %v", info, ok)
+	}
+}
+
+func TestOverrideValidationRejectsPathsOverlapValuesAndProtectedTargets(t *testing.T) {
+	type config struct {
+		Version string         `yaml:"version"`
+		Scalar  string         `yaml:"scalar"`
+		Items   []string       `yaml:"items"`
+		Nested  map[string]any `yaml:"nested"`
+		Unknown string         `yaml:"unknown"`
+	}
+	base := Bytes("override-errors.yml", []byte("version: 4\nscalar: value\nitems: [one]\nnested: {}\n"))
+	version := &VersionRule{Path: Keys("version"), Check: func(Version, *Validation) error { return nil }}
+	cycle := map[string]any{}
+	cycle["self"] = cycle
+	deep := any("leaf")
+	for range maxDocumentDepth + 1 {
+		deep = []any{deep}
+	}
+	for _, test := range []struct {
+		name      string
+		overrides []Override
+		code      Code
+	}{
+		{name: "root", overrides: []Override{{Path: Keys(), Value: "x", Flag: "--root"}}, code: CodeOverrideInvalidPath},
+		{name: "index", overrides: []Override{{Path: Keys("items").Index(0), Value: "x", Flag: "--item"}}, code: CodeOverrideInvalidPath},
+		{name: "parsed sequence index", overrides: []Override{{Path: parseOverrideTestPath(t, "/items/0"), Value: "x", Flag: "--item"}}, code: CodeOverrideInvalidPath},
+		{name: "scalar ancestor", overrides: []Override{{Path: Keys("scalar", "child"), Value: "x", Flag: "--child"}}, code: CodeOverrideInvalidPath},
+		{name: "duplicate", overrides: []Override{{Path: Keys("scalar"), Value: "a", Flag: "--first"}, {Path: Keys("scalar"), Value: "b", Flag: "--second"}}, code: CodeOverrideDuplicate},
+		{name: "overlap", overrides: []Override{{Path: Keys("nested"), Value: map[string]any{}, Flag: "--nested"}, {Path: Keys("nested", "child"), Value: "x", Flag: "--child"}}, code: CodeOverrideOverlap},
+		{name: "unsupported", overrides: []Override{{Path: Keys("scalar"), Value: time.Second, Flag: "--scalar"}}, code: CodeOverrideInvalidValue},
+		{name: "cycle", overrides: []Override{{Path: Keys("nested"), Value: cycle, Flag: "--nested"}}, code: CodeOverrideInvalidValue},
+		{name: "nonfinite", overrides: []Override{{Path: Keys("scalar"), Value: math.Inf(1), Flag: "--scalar"}}, code: CodeOverrideInvalidValue},
+		{name: "depth", overrides: []Override{{Path: Keys("nested"), Value: deep, Flag: "--nested"}}, code: CodeOverrideLimitExceeded},
+		{name: "record bytes", overrides: []Override{{Path: Keys("scalar"), Value: strings.Repeat("x", maxSourceBytes+1), Flag: "--scalar"}}, code: CodeOverrideLimitExceeded},
+		{name: "directive path", overrides: []Override{{Path: Keys("configkit", "x"), Value: true, Flag: "--directive"}}, code: CodeOverrideDirective},
+		{name: "directive value", overrides: []Override{{Path: Keys("nested"), Value: map[string]any{"configkit": map[string]any{}}, Flag: "--nested"}}, code: CodeOverrideDirective},
+		{name: "version", overrides: []Override{{Path: Keys("version"), Value: "v2", Flag: "--version"}}, code: CodeOverrideVersion},
+		{name: "version ancestor", overrides: []Override{{Path: Keys("version"), Value: map[string]any{"child": "v2"}, Flag: "--version"}}, code: CodeOverrideVersion},
+		{name: "label", overrides: []Override{{Path: Keys("scalar"), Value: "x", Flag: "\nCANARY"}}, code: CodeOverrideInvalidLabel},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := Load(base, Options[config]{Definition: Definition[config]{Version: version}, Overrides: test.overrides})
+			if result != nil {
+				t.Fatalf("result = %#v, want nil", result)
+			}
+			loadError := requireSecretLoadCode(t, err, test.code)
+			if strings.Contains(fmtLoadError(loadError), strings.Repeat("x", 64)) {
+				t.Fatal("override diagnostic leaked a raw value")
+			}
+		})
+	}
+}
+
+func TestOverrideCannotReplaceSchemaVersionAncestor(t *testing.T) {
+	type schemaConfig struct {
+		Version string `yaml:"version"`
+	}
+	type config struct {
+		Schema schemaConfig `yaml:"schema"`
+	}
+	_, err := Load(Bytes("version.yml", []byte("schema: {version: 4}\n")), Options[config]{
+		Definition: Definition[config]{Version: &VersionRule{Path: Keys("schema", "version"), Check: func(Version, *Validation) error { return nil }}},
+		Overrides:  []Override{{Path: Keys("schema"), Value: map[string]any{"other": "value"}, Flag: "--schema"}},
+	})
+	requireSecretLoadCode(t, err, CodeOverrideVersion)
+}
+
+func TestOverrideDecodeFailureUsesFlagSourceWithoutRawValue(t *testing.T) {
+	type config struct {
+		Count int `yaml:"count"`
+	}
+	result, err := Load(Bytes("base.yml", []byte("count: 1\n")), Options[config]{Overrides: []Override{{
+		Path: Keys("missing"), Value: "RAW-ARG-CANARY", Flag: "--missing",
+	}}})
+	if result != nil {
+		t.Fatalf("result = %#v", result)
+	}
+	var loadError *LoadError
+	if !errors.As(err, &loadError) {
+		t.Fatalf("error = %T", err)
+	}
+	diagnostics := loadError.Diagnostics()
+	sources := loadError.Sources()
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeDecodeUnknownField || diagnostics[0].Primary == nil ||
+		len(sources) != 2 || sources[1].Label != "--missing" || diagnostics[0].Primary.SourceID != sources[1].ID {
+		t.Fatalf("diagnostics/sources = %#v / %#v", diagnostics, sources)
+	}
+	if strings.Contains(fmtLoadError(loadError), "RAW-ARG-CANARY") {
+		t.Fatal("decode failure leaked override value")
+	}
+}
+
+func TestOverrideTypeMismatchUsesNormalStrictDecode(t *testing.T) {
+	type config struct {
+		Count int `yaml:"count"`
+	}
+	_, err := Load(Bytes("base.yml", []byte("count: 1\n")), Options[config]{Overrides: []Override{{
+		Path: Keys("count"), Value: "not-an-integer", Flag: "--count",
+	}}})
+	loadError := requireSecretLoadCode(t, err, CodeDecodeType)
+	if loadError.Diagnostics()[0].Primary == nil || loadError.Sources()[1].Label != "--count" || strings.Contains(fmtLoadError(loadError), "not-an-integer") {
+		t.Fatalf("type mismatch diagnostics/sources = %#v / %#v", loadError.Diagnostics(), loadError.Sources())
+	}
+}
+
+func TestOverrideEqualReplacementStillAppearsInHistory(t *testing.T) {
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	result, err := Load(Bytes("equal.yml", []byte("value: same\n")), Options[config]{Overrides: []Override{{Path: Keys("value"), Value: "same", Flag: "--value"}}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := historyLayers(result.Metadata().History(Keys("value"))); !reflect.DeepEqual(got, []Layer{LayerDefault, LayerBase, LayerCLI}) {
+		t.Fatalf("equal override history = %#v", result.Metadata().History(Keys("value")))
+	}
+}
+
+func TestOverrideAggregateLimitsFailBeforeApplication(t *testing.T) {
+	base, diagnostics := composeDocuments(&document.Node{Kind: document.Mapping})
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	base.inputNodes = 1
+	result, sources, diagnostics := applyOverrides(base, nil, []Override{{Path: Keys("value"), Value: "x", Flag: "--value"}}, nil, importLimits{sources: 1, bytes: 1, nodes: 2})
+	if result != nil || len(sources) != 1 || len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideLimitExceeded || len(base.root.Mapping) != 0 {
+		t.Fatalf("bounded apply = %#v, %#v, %#v; base %#v", result, sources, diagnostics, base.root)
+	}
+}
+
+func historyLayers(history []Change) []Layer {
+	layers := make([]Layer, len(history))
+	for index, change := range history {
+		layers[index] = change.Origin.Layer
+	}
+	return layers
+}
+
+func parseOverrideTestPath(t *testing.T, value string) Path {
+	t.Helper()
+	path, err := ParsePath(value)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
+
+func fmtLoadError(loadError *LoadError) string {
+	var output strings.Builder
+	_ = WriteDiagnostics(&output, loadError.Diagnostics())
+	return loadError.Error() + output.String()
+}

override_value.go

@@ -0,0 +1,150 @@
+package configkit
+
+import (
+	"math"
+	"reflect"
+	"slices"
+	"strconv"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type overrideValueBuilder struct {
+	sourceID string
+	active   map[valueVisit]bool
+	bytes    int64
+	nodes    int
+}
+
+func (b *overrideValueBuilder) build(value any, path Path, depth int) (*document.Node, *Diagnostic) {
+	if depth > maxDocumentDepth {
+		return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the depth limit")
+	}
+	b.nodes++
+	if b.nodes > maxDocumentNodes {
+		return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the node limit")
+	}
+	location := document.Location{SourceID: b.sourceID}
+	switch value := value.(type) {
+	case nil:
+		return &document.Node{Kind: document.Null, Location: location}, nil
+	case string:
+		if !b.addBytes(len(value)) {
+			return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the record string-byte limit")
+		}
+		return &document.Node{Kind: document.String, Text: value, Location: location}, nil
+	case bool:
+		return &document.Node{Kind: document.Boolean, Text: strconv.FormatBool(value), Location: location}, nil
+	case int:
+		return overrideNumber(strconv.FormatInt(int64(value), 10), location), nil
+	case int8:
+		return overrideNumber(strconv.FormatInt(int64(value), 10), location), nil
+	case int16:
+		return overrideNumber(strconv.FormatInt(int64(value), 10), location), nil
+	case int32:
+		return overrideNumber(strconv.FormatInt(int64(value), 10), location), nil
+	case int64:
+		return overrideNumber(strconv.FormatInt(value, 10), location), nil
+	case uint:
+		return overrideNumber(strconv.FormatUint(uint64(value), 10), location), nil
+	case uint8:
+		return overrideNumber(strconv.FormatUint(uint64(value), 10), location), nil
+	case uint16:
+		return overrideNumber(strconv.FormatUint(uint64(value), 10), location), nil
+	case uint32:
+		return overrideNumber(strconv.FormatUint(uint64(value), 10), location), nil
+	case uint64:
+		return overrideNumber(strconv.FormatUint(value, 10), location), nil
+	case float32:
+		if math.IsNaN(float64(value)) || math.IsInf(float64(value), 0) {
+			return nil, overrideValueDiagnostic(CodeOverrideInvalidValue, path, "configuration override float must be finite")
+		}
+		return overrideNumber(strconv.FormatFloat(float64(value), 'g', -1, 32), location), nil
+	case float64:
+		if math.IsNaN(value) || math.IsInf(value, 0) {
+			return nil, overrideValueDiagnostic(CodeOverrideInvalidValue, path, "configuration override float must be finite")
+		}
+		return overrideNumber(strconv.FormatFloat(value, 'g', -1, 64), location), nil
+	case []any:
+		if value == nil {
+			return &document.Node{Kind: document.Null, Location: location}, nil
+		}
+		leave, diagnostic := b.enter(reflect.ValueOf(value), path)
+		if diagnostic != nil {
+			return nil, diagnostic
+		}
+		defer leave()
+		if len(value) > maxDocumentNodes-b.nodes {
+			return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the node limit")
+		}
+		node := &document.Node{Kind: document.Sequence, Location: location, Sequence: make([]*document.Node, len(value))}
+		for index, child := range value {
+			built, diagnostic := b.build(child, path.Index(index), depth+1)
+			if diagnostic != nil {
+				return nil, diagnostic
+			}
+			node.Sequence[index] = built
+		}
+		return node, nil
+	case map[string]any:
+		if value == nil {
+			return &document.Node{Kind: document.Null, Location: location}, nil
+		}
+		leave, diagnostic := b.enter(reflect.ValueOf(value), path)
+		if diagnostic != nil {
+			return nil, diagnostic
+		}
+		defer leave()
+		if len(value) > (maxDocumentNodes-b.nodes)/2 {
+			return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path, "configuration override exceeds the node limit")
+		}
+		keys := make([]string, 0, len(value))
+		for key := range value {
+			keys = append(keys, key)
+		}
+		slices.Sort(keys)
+		node := &document.Node{Kind: document.Mapping, Location: location, Mapping: make([]document.Entry, 0, len(keys))}
+		for _, key := range keys {
+			b.nodes++ // mapping key
+			if !b.addBytes(len(key)) {
+				return nil, overrideValueDiagnostic(CodeOverrideLimitExceeded, path.Key(key), "configuration override exceeds the record string-byte limit")
+			}
+			built, diagnostic := b.build(value[key], path.Key(key), depth+1)
+			if diagnostic != nil {
+				return nil, diagnostic
+			}
+			node.Mapping = append(node.Mapping, document.Entry{Key: key, KeyLocation: location, ValueLocation: location, Value: built})
+		}
+		return node, nil
+	default:
+		return nil, overrideValueDiagnostic(CodeOverrideInvalidValue, path, "configuration override value has an unsupported type")
+	}
+}
+
+func (b *overrideValueBuilder) addBytes(count int) bool {
+	if count < 0 || int64(count) > maxSourceBytes-b.bytes {
+		return false
+	}
+	b.bytes += int64(count)
+	return true
+}
+
+func (b *overrideValueBuilder) enter(value reflect.Value, path Path) (func(), *Diagnostic) {
+	visit, ok := visitFor(value)
+	if !ok {
+		return func() {}, nil
+	}
+	if b.active[visit] {
+		return nil, overrideValueDiagnostic(CodeOverrideInvalidValue, path, "configuration override values must not contain cycles")
+	}
+	b.active[visit] = true
+	return func() { delete(b.active, visit) }, nil
+}
+
+func overrideValueDiagnostic(code Code, path Path, message string) *Diagnostic {
+	return &Diagnostic{Code: code, Severity: SeverityError, Path: path.clone(), Message: message}
+}
+
+func overrideNumber(text string, location document.Location) *document.Node {
+	return &document.Node{Kind: document.Number, Text: text, Location: location}
+}

parse.go

@@ -0,0 +1,62 @@
+package configkit
+
+import (
+	"bytes"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+const (
+	maxDocumentDepth = 256
+	maxDocumentNodes = 1_000_000
+)
+
+type parseLimits struct {
+	source   sourceLimits
+	document yamladapter.Limits
+}
+
+func defaultParseLimits() parseLimits {
+	return parseLimits{
+		source: defaultSourceLimits(),
+		document: yamladapter.Limits{
+			Depth: maxDocumentDepth,
+			Nodes: maxDocumentNodes,
+		},
+	}
+}
+
+type parsedSource struct {
+	document *document.Node
+	source   *SourceInfo
+	bytes    int64
+}
+
+// parserDiagnostics translates adapter issues into detached, ordered public records.
+func parserDiagnostics(sourceID string, issues []yamladapter.Issue) []Diagnostic {
+	diagnostics := make([]Diagnostic, len(issues))
+	for index, issue := range issues {
+		diagnostics[index] = Diagnostic{
+			Code: Code(issue.Code), Severity: SeverityError, Message: issue.Message,
+			Primary: &Location{SourceID: sourceID, Line: issue.Line, Column: issue.Column},
+		}
+		for _, related := range issue.Related {
+			diagnostics[index].Related = append(diagnostics[index].Related, publicLocation(related))
+		}
+	}
+	return canonicalDiagnostics(diagnostics)
+}
+
+func parseSource(source Source, sourceID string, limits parseLimits) (parsedSource, []Diagnostic) {
+	read, diagnostics := readSource(source, sourceID, limits.source)
+	if len(diagnostics) != 0 || read.info == nil {
+		return parsedSource{source: read.info}, diagnostics
+	}
+
+	root, issues := yamladapter.Parse(bytes.NewReader(read.data), sourceID, limits.document)
+	if len(issues) != 0 {
+		return parsedSource{source: read.info, bytes: int64(len(read.data))}, parserDiagnostics(sourceID, issues)
+	}
+	return parsedSource{document: root, source: read.info, bytes: int64(len(read.data))}, nil
+}

parse_test.go

@@ -0,0 +1,38 @@
+package configkit
+
+import (
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+func TestParseSourceOwnsDocumentAndSafeDiagnostics(t *testing.T) {
+	t.Parallel()
+
+	limits := defaultParseLimits()
+	parsed, diagnostics := parseSource(Bytes("memory.yml", []byte("name: config\n")), "source-1", limits)
+	if len(diagnostics) != 0 || parsed.source == nil || parsed.source.Label != "memory.yml" || parsed.document.Kind != document.Mapping {
+		t.Fatalf("parse result = %#v, diagnostics = %#v", parsed, diagnostics)
+	}
+
+	const canary = "CANARY_PRIVATE_INPUT"
+	failed, diagnostics := parseSource(Bytes("unsafe\nlabel", []byte("name: ["+canary+"\n")), "source-1", limits)
+	if failed.document != nil || failed.source == nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeYAMLInvalid {
+		t.Fatalf("failed parse = %#v, diagnostics = %#v", failed, diagnostics)
+	}
+	var output strings.Builder
+	if err := WriteDiagnostics(&output, diagnostics); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(output.String(), canary) || strings.Count(output.String(), "\n") != 1 {
+		t.Fatalf("unsafe rendered diagnostic = %q", output.String())
+	}
+
+	limits.document = yaml.Limits{Depth: 1, Nodes: 20, ParserDepth: 20}
+	_, diagnostics = parseSource(Bytes("deep", []byte("a:\n  b: value\n")), "source-1", limits)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeYAMLDepthLimit || diagnostics[0].Primary.Line == 0 {
+		t.Fatalf("depth diagnostics = %#v", diagnostics)
+	}
+}

path.go

@@ -0,0 +1,148 @@
+package configkit
+
+import (
+	"errors"
+	"strconv"
+	"strings"
+)
+
+// Path identifies a configuration value using YAML field names. Its textual form
+// is a JSON Pointer: the root is empty, ~ escapes as ~0, and / escapes as ~1.
+// Paths are semantic rather than filesystem or URL paths: empty, dot, dot-dot,
+// percent-containing, and numeric keys retain their literal meaning. Parsed
+// numeric tokens are resolved only when traversal knows their containing type.
+//
+// Paths are immutable. The zero value is the root path.
+type Path struct {
+	segments []pathSegment
+	invalid  bool
+}
+
+type pathSegmentKind uint8
+
+const (
+	pathSegmentKey pathSegmentKind = iota
+	pathSegmentIndex
+	pathSegmentParsed
+)
+
+type pathSegment struct {
+	kind  pathSegmentKind
+	key   string
+	index int
+}
+
+// Keys constructs a path of mapping keys. Keys with no arguments returns the
+// root path.
+func Keys(keys ...string) Path {
+	p := Path{}
+	for _, key := range keys {
+		p = p.Key(key)
+	}
+	return p
+}
+
+// ParsePath parses a JSON Pointer. Numeric tokens remain unresolved until schema
+// or document traversal determines whether the containing value is a mapping or
+// sequence.
+func ParsePath(pointer string) (Path, error) {
+	if pointer == "" {
+		return Path{}, nil
+	}
+	if pointer[0] != '/' {
+		return Path{}, errors.New("configkit: path must be empty or begin with slash")
+	}
+
+	parts := strings.Split(pointer[1:], "/")
+	segments := make([]pathSegment, 0, len(parts))
+	for _, part := range parts {
+		key, ok := unescapePathToken(part)
+		if !ok {
+			return Path{}, errors.New("configkit: path contains an invalid tilde escape")
+		}
+		segments = append(segments, pathSegment{kind: pathSegmentParsed, key: key})
+	}
+	return Path{segments: segments}, nil
+}
+
+// Key returns a new path with key appended as a mapping-key segment.
+func (p Path) Key(key string) Path {
+	segments := clonePathSegments(p.segments, 1)
+	segments = append(segments, pathSegment{kind: pathSegmentKey, key: key})
+	return Path{segments: segments, invalid: p.invalid}
+}
+
+// Index returns a new path with index appended as a sequence-index segment.
+// A negative index produces an invalid path that configuration operations reject.
+func (p Path) Index(index int) Path {
+	segments := clonePathSegments(p.segments, 1)
+	segments = append(segments, pathSegment{kind: pathSegmentIndex, index: index})
+	return Path{segments: segments, invalid: p.invalid || index < 0}
+}
+
+// String returns the path's canonical JSON Pointer spelling.
+func (p Path) String() string {
+	if len(p.segments) == 0 {
+		return ""
+	}
+
+	var b strings.Builder
+	for _, segment := range p.segments {
+		b.WriteByte('/')
+		if segment.kind == pathSegmentIndex {
+			b.WriteString(strconv.Itoa(segment.index))
+			continue
+		}
+		b.WriteString(escapePathToken(segment.key))
+	}
+	return b.String()
+}
+
+func (p Path) clone() Path {
+	return Path{segments: clonePathSegments(p.segments, 0), invalid: p.invalid}
+}
+
+func (p Path) valid() bool {
+	return !p.invalid
+}
+
+func clonePathSegments(segments []pathSegment, extra int) []pathSegment {
+	if len(segments) == 0 && extra == 0 {
+		return nil
+	}
+	cloned := make([]pathSegment, len(segments), len(segments)+extra)
+	copy(cloned, segments)
+	return cloned
+}
+
+func escapePathToken(token string) string {
+	token = strings.ReplaceAll(token, "~", "~0")
+	return strings.ReplaceAll(token, "/", "~1")
+}
+
+func unescapePathToken(token string) (string, bool) {
+	if !strings.Contains(token, "~") {
+		return token, true
+	}
+
+	var b strings.Builder
+	for i := 0; i < len(token); i++ {
+		if token[i] != '~' {
+			b.WriteByte(token[i])
+			continue
+		}
+		if i+1 >= len(token) {
+			return "", false
+		}
+		i++
+		switch token[i] {
+		case '0':
+			b.WriteByte('~')
+		case '1':
+			b.WriteByte('/')
+		default:
+			return "", false
+		}
+	}
+	return b.String(), true
+}

path_fuzz_test.go

@@ -0,0 +1,27 @@
+package configkit
+
+import "testing"
+
+func FuzzParsePath(f *testing.F) {
+	for _, seed := range []string{"", "/", "/service/port", "/a~1b/~0key", "/0", "relative", "/bad~2escape"} {
+		f.Add(seed)
+	}
+
+	f.Fuzz(func(t *testing.T, input string) {
+		if len(input) > 4096 {
+			return
+		}
+		path, err := ParsePath(input)
+		if err != nil {
+			return
+		}
+		canonical := path.String()
+		reparsed, err := ParsePath(canonical)
+		if err != nil {
+			t.Fatalf("canonical path %q did not parse: %v", canonical, err)
+		}
+		if reparsed.String() != canonical {
+			t.Fatalf("path round trip = %q, want %q", reparsed.String(), canonical)
+		}
+	})
+}

path_test.go

@@ -0,0 +1,88 @@
+package configkit
+
+import "testing"
+
+func TestPathRoundTripsSemanticKeys(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		pointer string
+	}{
+		{name: "root", pointer: ""},
+		{name: "slash and tilde", pointer: "/a~1b/c~0d"},
+		{name: "empty key", pointer: "/"},
+		{name: "dotted key", pointer: "/outputs/a.b"},
+		{name: "numeric key", pointer: "/outputs/0"},
+		{name: "dot keys", pointer: "/./.."},
+		{name: "percent text", pointer: "/%2F/%7E"},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			parsed, err := ParsePath(test.pointer)
+			if err != nil {
+				t.Fatalf("ParsePath() error = %v", err)
+			}
+			if got := parsed.String(); got != test.pointer {
+				t.Fatalf("String() = %q, want %q", got, test.pointer)
+			}
+		})
+	}
+
+	if got := Keys("a/b", "c~d", "", "..").String(); got != "/a~1b/c~0d//.." {
+		t.Fatalf("Keys().String() = %q", got)
+	}
+	if got := Keys("items").Index(12).Key("name").String(); got != "/items/12/name" {
+		t.Fatalf("indexed path = %q", got)
+	}
+}
+
+func TestPathPreservesSegmentIntentAndImmutability(t *testing.T) {
+	t.Parallel()
+
+	key := Keys("items", "0")
+	index := Keys("items").Index(0)
+	parsed, err := ParsePath("/items/0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if key.String() != index.String() || index.String() != parsed.String() {
+		t.Fatal("equivalent textual paths should share a JSON Pointer spelling")
+	}
+	if key.segments[1].kind != pathSegmentKey {
+		t.Fatal("Keys numeric segment lost mapping-key intent")
+	}
+	if index.segments[1].kind != pathSegmentIndex {
+		t.Fatal("Index segment lost sequence intent")
+	}
+	if parsed.segments[1].kind != pathSegmentParsed {
+		t.Fatal("parsed numeric segment was resolved before traversal")
+	}
+
+	base := Keys("parent")
+	child := base.Key("child")
+	child.segments[0].key = "changed"
+	if got := base.String(); got != "/parent" {
+		t.Fatalf("builder mutated parent: %q", got)
+	}
+
+	negative := Path{}.Index(-1)
+	if negative.valid() {
+		t.Fatal("negative index must produce an invalid path")
+	}
+}
+
+func TestParsePathRejectsMalformedPointers(t *testing.T) {
+	t.Parallel()
+
+	for _, pointer := range []string{"relative", "~0", "/bad~", "/bad~2", "/bad~01~"} {
+		if _, err := ParsePath(pointer); err == nil {
+			t.Errorf("ParsePath(%q) succeeded", pointer)
+		}
+	}
+
+	if parsed, err := ParsePath("/missing"); err != nil || !parsed.valid() {
+		t.Fatalf("valid missing path rejected: path=%v error=%v", parsed, err)
+	}
+}

profile.go

@@ -0,0 +1,131 @@
+package configkit
+
+import (
+	"bytes"
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+func activeProfile(name string) bool {
+	return name != "" && name != "default"
+}
+
+func validProfileName(name string) bool {
+	if !activeProfile(name) {
+		return true
+	}
+	for _, character := range name {
+		if (character >= 'a' && character <= 'z') ||
+			(character >= 'A' && character <= 'Z') ||
+			(character >= '0' && character <= '9') || character == '_' || character == '-' {
+			continue
+		}
+		return false
+	}
+	return true
+}
+
+func loadProfileDocument(source Source, context *FilesystemContext, main parsedSource, base *composedDocument, sources []SourceInfo, name string, rule *VersionRule, limits importLimits) (*composedDocument, []SourceInfo, []Diagnostic) {
+	rootPath, _, _, diagnostic := importFilesystem(source, context, main)
+	if diagnostic != nil {
+		return nil, cloneSourceInfos(sources), []Diagnostic{*diagnostic}
+	}
+	profilePath := filepath.Join(rootPath, "profiles", name+".yml")
+	canonical, err := canonicalExistingPath(profilePath)
+	if err != nil {
+		code := CodeProfileInvalid
+		message := "selected profile could not be resolved"
+		if errors.Is(err, fs.ErrNotExist) {
+			code, message = CodeProfileMissing, "selected profile does not exist"
+		} else if errors.Is(err, fs.ErrPermission) {
+			code, message = CodeImportPermission, "selected profile cannot be accessed"
+		}
+		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(code, message, nil)}
+	}
+	if !containedPath(rootPath, canonical) {
+		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeProfileOutsideRoot, "selected profile resolves outside the configuration root", nil)}
+	}
+	for _, existing := range sources {
+		if existing.Path != "" && existing.Path == canonical {
+			return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeProfileRepeated, "selected profile is already present in the base source graph", nil)}
+		}
+	}
+	if len(sources)+1 > limits.sources {
+		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeProfileLimitExceeded, "selected profile exceeds the source-count limit", nil)}
+	}
+
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		value := globFilesystemDiagnostic(err, "configuration root could not be opened for the selected profile", Location{})
+		value.Primary = nil
+		return nil, cloneSourceInfos(sources), []Diagnostic{value}
+	}
+	rootClosed := false
+	defer func() {
+		if !rootClosed {
+			_ = root.Close()
+		}
+	}()
+	relative, err := filepath.Rel(rootPath, canonical)
+	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+		return nil, cloneSourceInfos(sources), []Diagnostic{importDiagnostic(CodeProfileOutsideRoot, "selected profile could not be addressed inside the configuration root", nil)}
+	}
+
+	sourceID := "source-" + strconv.Itoa(len(sources)+1)
+	profileLocation := &Location{SourceID: sourceID}
+	profileSource := SourceInfo{ID: sourceID, Label: filepath.ToSlash(filepath.Join("profiles", name+".yml")), Path: canonical, Layer: LayerProfile}
+	sources = append(cloneSourceInfos(sources), profileSource)
+	data, diagnostic := readProfileFile(root, relative, sourceID, limits.bytes-base.inputBytes)
+	if diagnostic != nil {
+		return nil, sources, []Diagnostic{*diagnostic}
+	}
+	profileRoot, issues := yamladapter.Parse(bytes.NewReader(data), sourceID, yamladapter.Limits{Depth: maxDocumentDepth, Nodes: maxDocumentNodes})
+	if len(issues) != 0 {
+		return nil, sources, parserDiagnostics(sourceID, issues)
+	}
+	nodeCount := countInputNodes(profileRoot)
+	if base.inputNodes+nodeCount > limits.nodes {
+		return nil, sources, []Diagnostic{importDiagnostic(CodeProfileLimitExceeded, "selected profile exceeds the aggregate node limit", profileLocation)}
+	}
+	if diagnostics := profileDirectiveDiagnostics(profileRoot, Path{}); len(diagnostics) != 0 {
+		return nil, sources, diagnostics
+	}
+	if versionNode := fragmentVersionClaim(profileRoot, rule); versionNode != nil {
+		location := publicLocation(versionNode.Location)
+		return nil, sources, []Diagnostic{{
+			Code: CodeProfileVersion, Severity: SeverityError, Path: rule.Path.clone(),
+			Message: "profiles cannot contribute to or replace the schema-version path", Primary: &location,
+		}}
+	}
+	closeErr := root.Close()
+	rootClosed = true
+	if closeErr != nil {
+		return nil, sources, []Diagnostic{importDiagnostic(CodeImportRead, "configuration root could not be closed after reading the selected profile", nil)}
+	}
+
+	profiled, diagnostics := overlayDocuments(base, profileRoot)
+	if profiled != nil {
+		profiled.inputBytes = base.inputBytes + int64(len(data))
+		profiled.inputNodes = base.inputNodes + nodeCount
+	}
+	return profiled, sources, diagnostics
+}
+
+func readProfileFile(root *os.Root, relative, sourceID string, remaining int64) ([]byte, *Diagnostic) {
+	limit := min(int64(maxSourceBytes), remaining)
+	if limit < 0 {
+		limit = 0
+	}
+	data, failure := readRegularFile(relative, root.Stat, root.Open, limit)
+	if failure != nil {
+		value := sourceReadDiagnostic(failure, sourceID, CodeProfileLimitExceeded, "selected profile exceeds a source or aggregate byte limit")
+		return nil, &value
+	}
+	return data, nil
+}

profile_test.go

@@ -0,0 +1,267 @@
+package configkit
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"testing"
+)
+
+type profileTestConfig struct {
+	Settings struct {
+		Kept    string `yaml:"kept"`
+		Changed string `yaml:"changed"`
+		Added   string `yaml:"added"`
+	} `yaml:"settings"`
+	Values   []string          `yaml:"values"`
+	Nullable *string           `yaml:"nullable"`
+	Value    any               `yaml:"value"`
+	Schema   map[string]string `yaml:"schema"`
+}
+
+func TestLoadAppliesSelectedProfileAfterBaseAndBeforeDefaults(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports: [{path: base.yml}]
+settings:
+  kept: main
+  changed: base
+values: [base, stale]
+nullable: base
+value: {child: base}
+`)
+	basePath := writeImportFile(t, directory, "base.yml", "settings: {added: imported}\n")
+	profilePath := writeImportFile(t, directory, "profiles/testing.yml", `settings:
+  changed: profile
+values: [profile]
+nullable: null
+value: scalar
+`)
+
+	result, err := Load[profileTestConfig](File(mainPath), Options[profileTestConfig]{
+		Imports: true,
+		Profile: "testing",
+		Definition: Definition[profileTestConfig]{Defaults: func() profileTestConfig {
+			defaultValue := "default"
+			return profileTestConfig{
+				Values: []string{"default"}, Nullable: &defaultValue,
+				Settings: struct {
+					Kept    string `yaml:"kept"`
+					Changed string `yaml:"changed"`
+					Added   string `yaml:"added"`
+				}{Kept: "default", Changed: "default", Added: "default"},
+			}
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Config.Settings.Kept != "main" || result.Config.Settings.Changed != "profile" || result.Config.Settings.Added != "imported" {
+		t.Fatalf("settings = %#v", result.Config.Settings)
+	}
+	if !reflect.DeepEqual(result.Config.Values, []string{"profile"}) || result.Config.Nullable != nil || result.Config.Value != "scalar" {
+		t.Fatalf("profile values = values %#v nullable %#v value %#v", result.Config.Values, result.Config.Nullable, result.Config.Value)
+	}
+	sources := result.Sources()
+	wantPaths := []string{canonicalTestPath(t, mainPath), canonicalTestPath(t, basePath), canonicalTestPath(t, profilePath)}
+	gotPaths := make([]string, len(sources))
+	for index, source := range sources {
+		gotPaths[index] = source.Path
+	}
+	if !reflect.DeepEqual(gotPaths, wantPaths) || sources[2].Layer != LayerProfile {
+		t.Fatalf("sources = %#v", sources)
+	}
+	changed, _ := result.Metadata().Lookup(Keys("settings", "changed"))
+	kept, _ := result.Metadata().Lookup(Keys("settings", "kept"))
+	if changed.Origin.Layer != LayerProfile || changed.Origin.Locations[0].SourceID != "source-3" || kept.Origin.Layer != LayerBase {
+		t.Fatalf("origins = changed %#v kept %#v", changed, kept)
+	}
+	if _, ok := result.Metadata().Lookup(Keys("value", "child")); ok {
+		t.Fatal("removed child remains effective")
+	}
+	childHistory := result.Metadata().History(Keys("value", "child"))
+	if len(childHistory) < 2 || childHistory[len(childHistory)-1].Path.String() != "/value" || childHistory[len(childHistory)-1].Origin.Layer != LayerProfile {
+		t.Fatalf("removed child history = %#v", childHistory)
+	}
+	if _, ok := result.Metadata().Lookup(Keys("values").Index(1)); ok {
+		t.Fatal("removed sequence index remains effective")
+	}
+	indexHistory := result.Metadata().History(Keys("values").Index(1))
+	if len(indexHistory) < 2 || indexHistory[len(indexHistory)-1].Path.String() != "/values" {
+		t.Fatalf("removed sequence history = %#v", indexHistory)
+	}
+}
+
+func TestLoadProfileSelectionUsesOneFixedYAMLFile(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", "settings: {kept: base}\n")
+	writeImportFile(t, directory, "profiles/testing.yml", "settings: {changed: selected}\n")
+	writeImportFile(t, directory, "profiles/default.yml", "settings: {changed: forbidden}\n")
+
+	for _, name := range []string{"", "default"} {
+		result, err := Load[profileTestConfig](File(mainPath), Options[profileTestConfig]{Profile: name})
+		if err != nil || result.Config.Settings.Changed != "" || len(result.Sources()) != 1 {
+			t.Fatalf("inactive profile %q = %#v, %v", name, result, err)
+		}
+	}
+	selected, err := Load[profileTestConfig](File(mainPath), Options[profileTestConfig]{Profile: "testing"})
+	if err != nil || selected.Config.Settings.Changed != "selected" || len(selected.Sources()) != 2 {
+		t.Fatalf("selected profile = %#v, %v", selected, err)
+	}
+
+	for _, name := range []string{"../testing", "testing.yml", "one,two", "with space", "é", "bad\x00name"} {
+		t.Run(name, func(t *testing.T) {
+			requireProfileLoadCode(t, File(mainPath), Options[profileTestConfig]{Profile: name}, CodeOptionsInvalid)
+		})
+	}
+	requireProfileLoadCode(t, File(mainPath), Options[profileTestConfig]{Profile: "missing"}, CodeProfileMissing)
+	writeImportFile(t, directory, "profiles/fallback.yaml", "settings: {changed: fallback}\n")
+	requireProfileLoadCode(t, File(mainPath), Options[profileTestConfig]{Profile: "fallback"}, CodeProfileMissing)
+}
+
+func TestLoadProfileRejectsDirectivesVersionsAndBaseAliases(t *testing.T) {
+	versionRule := &VersionRule{Path: Keys("schema", "version"), Check: func(version Version, _ *Validation) error {
+		if !version.Present || version.Text != "4" {
+			return errors.New("unexpected version")
+		}
+		return nil
+	}}
+	for _, test := range []struct {
+		name    string
+		profile string
+		code    Code
+	}{
+		{name: "root directive", profile: "configkit: {}\n", code: CodeProfileDirective},
+		{name: "nested directive", profile: "settings: {configkit: {}}\n", code: CodeProfileDirective},
+		{name: "equal integer version", profile: "schema: {version: 4}\n", code: CodeProfileVersion},
+		{name: "ancestor scalar", profile: "schema: null\n", code: CodeProfileVersion},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", "schema: {version: 4}\n")
+			writeImportFile(t, directory, "profiles/testing.yml", test.profile)
+			requireProfileLoadCode(t, File(mainPath), Options[profileTestConfig]{Profile: "testing", Definition: Definition[profileTestConfig]{Version: versionRule}}, test.code)
+		})
+	}
+
+	t.Run("mapping sibling is allowed", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "schema: {version: 4}\n")
+		writeImportFile(t, directory, "profiles/testing.yml", "schema: {other: allowed}\n")
+		result, err := Load[profileTestConfig](File(mainPath), Options[profileTestConfig]{Profile: "testing", Definition: Definition[profileTestConfig]{Version: versionRule}})
+		if err != nil || result.Config.Schema["other"] != "allowed" {
+			t.Fatalf("mapping sibling = %#v, %v", result, err)
+		}
+	})
+
+	t.Run("profile aliases imported base", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: profiles/testing.yml}]}\n")
+		writeImportFile(t, directory, "profiles/testing.yml", "settings: {changed: value}\n")
+		requireProfileLoadCode(t, File(mainPath), Options[profileTestConfig]{Imports: true, Profile: "testing"}, CodeProfileRepeated)
+	})
+}
+
+func TestLoadProfileKeepsBaseFailuresAndFilesystemContainment(t *testing.T) {
+	t.Run("base parse precedes profile", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "settings: [\n")
+		writeImportFile(t, directory, "profiles/testing.yml", "settings: {changed: profile}\n")
+		requireProfileLoadCode(t, File(mainPath), Options[profileTestConfig]{Profile: "testing"}, CodeYAMLInvalid)
+	})
+
+	t.Run("profile parse", func(t *testing.T) {
+		directory := t.TempDir()
+		mainPath := writeImportFile(t, directory, "main.yml", "{}\n")
+		writeImportFile(t, directory, "profiles/testing.yml", "settings: [\n")
+		loadError := requireProfileLoadCode(t, File(mainPath), Options[profileTestConfig]{Profile: "testing"}, CodeYAMLInvalid)
+		if loadError.Diagnostics()[0].Primary.SourceID != "source-2" || len(loadError.Sources()) != 2 {
+			t.Fatalf("profile parse attribution = %#v / %#v", loadError.Diagnostics(), loadError.Sources())
+		}
+	})
+
+	if runtime.GOOS == "linux" || runtime.GOOS == "darwin" {
+		t.Run("symlink escape", func(t *testing.T) {
+			root := t.TempDir()
+			outside := t.TempDir()
+			outsideProfile := writeImportFile(t, outside, "testing.yml", "settings: {changed: outside}\n")
+			if err := os.Mkdir(filepath.Join(root, "profiles"), 0o755); err != nil {
+				t.Fatal(err)
+			}
+			if err := os.Symlink(outsideProfile, filepath.Join(root, "profiles/testing.yml")); err != nil {
+				t.Skipf("symlinks unavailable: %v", err)
+			}
+			mainPath := writeImportFile(t, root, "main.yml", "{}\n")
+			requireProfileLoadCode(t, File(mainPath), Options[profileTestConfig]{Profile: "testing"}, CodeProfileOutsideRoot)
+		})
+	}
+
+	t.Run("byte source context", func(t *testing.T) {
+		directory := t.TempDir()
+		writeImportFile(t, directory, "profiles/testing.yml", "settings: {changed: selected}\n")
+		result, err := Load[profileTestConfig](Bytes("s3://bucket/main.yml", []byte("{}\n")), Options[profileTestConfig]{
+			Profile: "testing", Filesystem: &FilesystemContext{BaseDir: directory, Root: directory},
+		})
+		if err != nil || result.Config.Settings.Changed != "selected" {
+			t.Fatalf("byte profile = %#v, %v", result, err)
+		}
+		requireProfileLoadCode(t, Bytes("memory.yml", []byte("{}\n")), Options[profileTestConfig]{Profile: "testing"}, CodeOptionsInvalid)
+	})
+}
+
+func TestLoadProfileCountsAgainstAggregateBudgets(t *testing.T) {
+	for _, test := range []struct {
+		name   string
+		limits func(parsedSource, int) importLimits
+	}{
+		{name: "source count", limits: func(_ parsedSource, _ int) importLimits {
+			return importLimits{sources: 1, bytes: maxTotalBytes, nodes: maxDocumentNodes}
+		}},
+		{name: "aggregate bytes", limits: func(main parsedSource, profileBytes int) importLimits {
+			return importLimits{sources: 2, bytes: main.bytes + int64(profileBytes) - 1, nodes: maxDocumentNodes}
+		}},
+		{name: "aggregate nodes", limits: func(main parsedSource, _ int) importLimits {
+			return importLimits{sources: 2, bytes: maxTotalBytes, nodes: countInputNodes(main.document) + 1}
+		}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			directory := t.TempDir()
+			mainPath := writeImportFile(t, directory, "main.yml", "{}\n")
+			profileData := "settings: {changed: selected}\n"
+			writeImportFile(t, directory, "profiles/testing.yml", profileData)
+			main, diagnostics := parseSource(File(mainPath), "source-1", defaultParseLimits())
+			if len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			base, diagnostics := composeDocuments(main.document)
+			if len(diagnostics) != 0 {
+				t.Fatal(diagnostics)
+			}
+			base.inputBytes = main.bytes
+			base.inputNodes = countInputNodes(main.document)
+			_, _, diagnostics = loadProfileDocument(File(mainPath), nil, main, base, sourceInfos(main.source), "testing", nil, test.limits(main, len(profileData)))
+			if len(diagnostics) == 0 || diagnostics[0].Code != CodeProfileLimitExceeded {
+				t.Fatalf("diagnostics = %#v", diagnostics)
+			}
+		})
+	}
+}
+
+func requireProfileLoadCode(t *testing.T, source Source, options Options[profileTestConfig], code Code) *LoadError {
+	t.Helper()
+	result, err := Load[profileTestConfig](source, options)
+	if result != nil || err == nil {
+		t.Fatalf("result, error = %#v, %v", result, err)
+	}
+	var loadError *LoadError
+	if !errors.As(err, &loadError) {
+		t.Fatalf("error type = %T", err)
+	}
+	diagnostics := loadError.Diagnostics()
+	if len(diagnostics) == 0 || diagnostics[0].Code != code {
+		t.Fatalf("diagnostics = %#v, want %s", diagnostics, code)
+	}
+	return loadError
+}

projection.go

@@ -0,0 +1,248 @@
+package configkit
+
+import (
+	"cmp"
+	"encoding"
+	"math"
+	"reflect"
+	"slices"
+	"strconv"
+	"time"
+	"unicode/utf8"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func projectConfiguration(schema *schemaDescriptor, value reflect.Value, limits projectionLimits) (*document.Node, *projectionIssue) {
+	if schema == nil || schema.root == nil || !value.IsValid() || value.Type() != schema.root.typeOf {
+		return nil, &projectionIssue{code: CodeSchemaUnsupportedType, message: "configuration value does not match its schema"}
+	}
+	operation := valueOperation{schema: schema, limits: limits, active: make(map[valueVisit]bool)}
+	return operation.project(value, schema.root, Path{}, 0)
+}
+
+func (o *valueOperation) project(value reflect.Value, descriptor *typeDescriptor, path Path, depth int) (*document.Node, *projectionIssue) {
+	if issue := o.checkDepth(path, depth); issue != nil {
+		return nil, issue
+	}
+	if descriptor.kind == descriptorPointer {
+		if value.IsNil() {
+			return o.node(document.Null, path, depth)
+		}
+		leave, issue := o.enter(value, path)
+		if issue != nil {
+			return nil, issue
+		}
+		defer leave()
+		return o.project(value.Elem(), descriptor.element, path, depth)
+	}
+
+	switch descriptor.kind {
+	case descriptorStruct:
+		result, issue := o.node(document.Mapping, path, depth)
+		if issue != nil {
+			return nil, issue
+		}
+		result.Mapping = make([]document.Entry, 0, len(descriptor.fields))
+		seen := make(map[string]bool, len(descriptor.fields))
+		for _, member := range descriptor.members {
+			memberValue := value.Field(member.index)
+			if member.inline {
+				if isNilValue(memberValue) {
+					continue
+				}
+				inline, issue := o.project(memberValue, member.descriptor, path, depth)
+				if issue != nil {
+					return nil, issue
+				}
+				if inline.Kind != document.Mapping {
+					return nil, &projectionIssue{code: CodeSchemaUnsupportedType, path: path.clone(), message: "inline value did not project to a mapping"}
+				}
+				o.nodes-- // The flattened container is not retained in the output tree.
+				for _, entry := range inline.Mapping {
+					if seen[entry.Key] {
+						return nil, &projectionIssue{code: CodeSchemaAmbiguousField, path: path.Key(entry.Key), message: "inline value conflicts with a declared field"}
+					}
+					seen[entry.Key] = true
+					result.Mapping = append(result.Mapping, entry)
+				}
+				continue
+			}
+			childPath := path.Key(member.name)
+			if seen[member.name] {
+				return nil, &projectionIssue{code: CodeSchemaAmbiguousField, path: childPath, message: "inline value conflicts with a declared field"}
+			}
+			if issue := o.charge(childPath); issue != nil { // mapping key
+				return nil, issue
+			}
+			child, issue := o.project(memberValue, member.descriptor, childPath, depth+1)
+			if issue != nil {
+				return nil, issue
+			}
+			seen[member.name] = true
+			result.Mapping = append(result.Mapping, document.Entry{Key: member.name, Value: child})
+		}
+		for _, entry := range result.Mapping {
+			field := descriptor.inlineMap
+			if index, ok := descriptor.fieldByName[entry.Key]; ok {
+				field = &descriptor.fields[index]
+			}
+			if field != nil && field.secret {
+				entry.Value.Sensitive = true
+			}
+		}
+		return result, nil
+	case descriptorMap:
+		if value.IsNil() {
+			return o.node(document.Null, path, depth)
+		}
+		leave, issue := o.enter(value, path)
+		if issue != nil {
+			return nil, issue
+		}
+		defer leave()
+		result, issue := o.node(document.Mapping, path, depth)
+		if issue != nil {
+			return nil, issue
+		}
+		result.Mapping = make([]document.Entry, 0, value.Len())
+		keys := value.MapKeys()
+		slices.SortFunc(keys, func(left, right reflect.Value) int {
+			return cmp.Compare(left.String(), right.String())
+		})
+		for _, key := range keys {
+			childPath := path.Key(key.String())
+			if issue := o.charge(childPath); issue != nil {
+				return nil, issue
+			}
+			child, issue := o.project(value.MapIndex(key), descriptor.element, childPath, depth+1)
+			if issue != nil {
+				return nil, issue
+			}
+			result.Mapping = append(result.Mapping, document.Entry{Key: key.String(), Value: child})
+		}
+		return result, nil
+	case descriptorSlice:
+		if value.IsNil() {
+			return o.node(document.Null, path, depth)
+		}
+		leave, issue := o.enter(value, path)
+		if issue != nil {
+			return nil, issue
+		}
+		defer leave()
+		fallthrough
+	case descriptorArray:
+		result, issue := o.node(document.Sequence, path, depth)
+		if issue != nil {
+			return nil, issue
+		}
+		result.Sequence = make([]*document.Node, 0, value.Len())
+		for i := 0; i < value.Len(); i++ {
+			childPath := path.Index(i)
+			child, issue := o.project(value.Index(i), descriptor.element, childPath, depth+1)
+			if issue != nil {
+				return nil, issue
+			}
+			result.Sequence = append(result.Sequence, child)
+		}
+		return result, nil
+	case descriptorDynamic:
+		if value.IsNil() {
+			return o.node(document.Null, path, depth)
+		}
+		concrete := value.Elem()
+		dynamic, issue := o.dynamicDescriptor(concrete.Type(), path)
+		if issue != nil {
+			return nil, issue
+		}
+		return o.project(concrete, dynamic, path, depth)
+	case descriptorString:
+		node, issue := o.node(document.String, path, depth)
+		if issue == nil {
+			node.Text = value.String()
+		}
+		return node, issue
+	case descriptorBoolean:
+		node, issue := o.node(document.Boolean, path, depth)
+		if issue == nil {
+			node.Text = strconv.FormatBool(value.Bool())
+		}
+		return node, issue
+	case descriptorSigned:
+		node, issue := o.node(document.Number, path, depth)
+		if issue == nil {
+			node.Text = strconv.FormatInt(value.Int(), 10)
+		}
+		return node, issue
+	case descriptorUnsigned:
+		node, issue := o.node(document.Number, path, depth)
+		if issue == nil {
+			node.Text = strconv.FormatUint(value.Uint(), 10)
+		}
+		return node, issue
+	case descriptorFloat:
+		if math.IsNaN(value.Float()) || math.IsInf(value.Float(), 0) {
+			return nil, &projectionIssue{code: CodeSchemaNonFinite, path: path.clone(), message: "configuration float must be finite"}
+		}
+		node, issue := o.node(document.Number, path, depth)
+		if issue == nil {
+			node.Text = strconv.FormatFloat(value.Float(), 'g', -1, value.Type().Bits())
+		}
+		return node, issue
+	case descriptorDuration:
+		node, issue := o.node(document.String, path, depth)
+		if issue == nil {
+			node.Text = time.Duration(value.Int()).String()
+		}
+		return node, issue
+	case descriptorSecret:
+		node, issue := o.node(document.String, path, depth)
+		if issue == nil {
+			node.Text = value.Interface().(Secret).EnvName()
+		}
+		return node, issue
+	case descriptorText:
+		text, issue := marshalCanonicalText(value, path)
+		if issue != nil {
+			return nil, issue
+		}
+		node, nodeIssue := o.node(document.String, path, depth)
+		if nodeIssue == nil {
+			node.Text = text
+		}
+		return node, nodeIssue
+	default:
+		return nil, &projectionIssue{code: CodeSchemaUnsupportedType, path: path.clone(), message: "configuration type is not supported"}
+	}
+}
+
+func (o *valueOperation) node(kind document.Kind, path Path, depth int) (*document.Node, *projectionIssue) {
+	if issue := o.checkDepth(path, depth); issue != nil {
+		return nil, issue
+	}
+	if issue := o.charge(path); issue != nil {
+		return nil, issue
+	}
+	return &document.Node{Kind: kind}, nil
+}
+
+func marshalCanonicalText(value reflect.Value, path Path) (string, *projectionIssue) {
+	var marshaler encoding.TextMarshaler
+	if value.CanInterface() {
+		marshaler, _ = value.Interface().(encoding.TextMarshaler)
+	}
+	if marshaler == nil {
+		addressable := reflect.New(value.Type())
+		addressable.Elem().Set(value)
+		marshaler, _ = addressable.Interface().(encoding.TextMarshaler)
+	}
+	if marshaler == nil {
+		return "", &projectionIssue{code: CodeSchemaCodec, path: path.clone(), message: "custom text scalar has no usable text marshaler"}
+	}
+	text, err := marshaler.MarshalText()
+	if err != nil || !utf8.Valid(text) {
+		return "", &projectionIssue{code: CodeSchemaCodec, path: path.clone(), message: "custom text scalar could not be projected"}
+	}
+	return string(text), nil
+}

projection_test.go

@@ -0,0 +1,394 @@
+package configkit
+
+import (
+	"errors"
+	"math"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type mutableText struct {
+	data []byte
+}
+
+func (value mutableText) MarshalText() ([]byte, error) {
+	return value.data, nil
+}
+
+func (value *mutableText) UnmarshalText(text []byte) error {
+	value.data = append(value.data[:0], text...)
+	return nil
+}
+
+type failingText struct {
+	failUnmarshal bool
+}
+
+func (value failingText) MarshalText() ([]byte, error) {
+	return []byte("CANARY_CODEC_TEXT"), errors.New("CANARY_MARSHAL_ERROR")
+}
+
+func (value *failingText) UnmarshalText([]byte) error {
+	if value.failUnmarshal {
+		return errors.New("CANARY_UNMARSHAL_ERROR")
+	}
+	return nil
+}
+
+type reconstructFailText string
+
+func (value reconstructFailText) MarshalText() ([]byte, error) {
+	return []byte(value), nil
+}
+
+func (value *reconstructFailText) UnmarshalText([]byte) error {
+	return errors.New("CANARY_RECONSTRUCT_ERROR")
+}
+
+func TestProjectionPreservesCanonicalTypesAndNilEmptyDistinctions(t *testing.T) {
+	t.Parallel()
+
+	type nested struct {
+		Value string `yaml:"value"`
+	}
+	type config struct {
+		Zero       int                `yaml:"zero"`
+		Wide       uint64             `yaml:"wide"`
+		Delay      time.Duration      `yaml:"delay"`
+		Pointer    *nested            `yaml:"pointer"`
+		NilPointer *nested            `yaml:"nil_pointer"`
+		EmptyMap   map[string]string  `yaml:"empty_map"`
+		NilMap     map[string]string  `yaml:"nil_map"`
+		EmptySlice []string           `yaml:"empty_slice"`
+		NilSlice   []string           `yaml:"nil_slice"`
+		Objects    map[string]*nested `yaml:"objects"`
+		Fixed      [2]int             `yaml:"fixed"`
+		Custom     mutableText        `yaml:"custom"`
+		Dynamic    any                `yaml:"dynamic"`
+		Ignored    *nested            `yaml:"-"`
+		private    *nested
+	}
+	value := config{
+		Wide:       math.MaxUint64,
+		Delay:      1500 * time.Millisecond,
+		Pointer:    &nested{Value: "pointer"},
+		EmptyMap:   map[string]string{},
+		EmptySlice: []string{},
+		Objects: map[string]*nested{
+			"z": {Value: "last"},
+			"a": {Value: "first"},
+		},
+		Fixed:   [2]int{1, 2},
+		Custom:  mutableText{data: []byte("canonical")},
+		Dynamic: map[string]any{"count": int64(9), "items": []any{"x", false}},
+		Ignored: &nested{Value: "ignored"},
+		private: &nested{Value: "private"},
+	}
+	schema := mustSchemaFor[config](t)
+	projected, issue := projectConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits())
+	if issue != nil {
+		t.Fatalf("projectConfiguration() issue = %#v", issue)
+	}
+
+	assertDocument(t, mappingValue(t, projected, "zero"), document.Number, "0")
+	assertDocument(t, mappingValue(t, projected, "wide"), document.Number, "18446744073709551615")
+	assertDocument(t, mappingValue(t, projected, "delay"), document.String, "1.5s")
+	assertDocument(t, mappingValue(t, projected, "custom"), document.String, "canonical")
+	if mappingValue(t, projected, "nil_pointer").Kind != document.Null || mappingValue(t, projected, "nil_map").Kind != document.Null || mappingValue(t, projected, "nil_slice").Kind != document.Null {
+		t.Fatal("nil values did not project as null")
+	}
+	if empty := mappingValue(t, projected, "empty_map"); empty.Kind != document.Mapping || empty.Mapping == nil || len(empty.Mapping) != 0 {
+		t.Fatalf("empty map = %#v", empty)
+	}
+	if empty := mappingValue(t, projected, "empty_slice"); empty.Kind != document.Sequence || empty.Sequence == nil || len(empty.Sequence) != 0 {
+		t.Fatalf("empty slice = %#v", empty)
+	}
+	objects := mappingValue(t, projected, "objects")
+	if len(objects.Mapping) != 2 || objects.Mapping[0].Key != "a" || objects.Mapping[1].Key != "z" {
+		t.Fatalf("map order = %#v", objects.Mapping)
+	}
+	if hasMappingKey(projected, "ignored") || hasMappingKey(projected, "private") {
+		t.Fatal("ignored field entered projection")
+	}
+	clonedValue, cloneIssue := cloneConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits())
+	if cloneIssue != nil {
+		t.Fatal(cloneIssue.message)
+	}
+	cloned := clonedValue.Interface().(config)
+	if cloned.NilMap != nil || cloned.NilSlice != nil || cloned.NilPointer != nil || cloned.EmptyMap == nil || cloned.EmptySlice == nil {
+		t.Fatalf("clone lost nil/empty distinctions: %#v", cloned)
+	}
+}
+
+func TestCloneConfigurationDetachesAllSupportedMutableValues(t *testing.T) {
+	t.Parallel()
+
+	type nested struct {
+		Values []string `yaml:"values"`
+	}
+	type config struct {
+		First   *nested            `yaml:"first"`
+		Second  *nested            `yaml:"second"`
+		Map     map[string]*nested `yaml:"map"`
+		Slice   []map[string]int   `yaml:"slice"`
+		Custom  mutableText        `yaml:"custom"`
+		Dynamic any                `yaml:"dynamic"`
+		Ignored *nested            `yaml:"-"`
+	}
+	shared := &nested{Values: []string{"original"}}
+	value := config{
+		First:  shared,
+		Second: shared,
+		Map:    map[string]*nested{"entry": {Values: []string{"map"}}},
+		Slice:  []map[string]int{{"count": 1}},
+		Custom: mutableText{data: []byte("text")},
+		Dynamic: map[string]any{
+			"items": []any{map[string]any{"name": "dynamic"}},
+		},
+		Ignored: &nested{Values: []string{"ignored"}},
+	}
+	schema := mustSchemaFor[config](t)
+	clonedValue, issue := cloneConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits())
+	if issue != nil {
+		t.Fatalf("cloneConfiguration() issue = %#v", issue)
+	}
+	cloned := clonedValue.Interface().(config)
+	if cloned.First == cloned.Second || cloned.First == shared || cloned.Custom.data == nil || &cloned.Custom.data[0] == &value.Custom.data[0] {
+		t.Fatal("clone retained mutable identity")
+	}
+	if cloned.Ignored != nil {
+		t.Fatal("ignored field was retained")
+	}
+
+	value.First.Values[0] = "changed"
+	value.Map["entry"].Values[0] = "changed"
+	value.Slice[0]["count"] = 2
+	value.Custom.data[0] = 'X'
+	value.Dynamic.(map[string]any)["items"].([]any)[0].(map[string]any)["name"] = "changed"
+	if cloned.First.Values[0] != "original" || cloned.Map["entry"].Values[0] != "map" || cloned.Slice[0]["count"] != 1 || string(cloned.Custom.data) != "text" || cloned.Dynamic.(map[string]any)["items"].([]any)[0].(map[string]any)["name"] != "dynamic" {
+		t.Fatalf("clone changed after source mutation: %#v", cloned)
+	}
+}
+
+func TestProjectionAndCloneRejectCyclesAndInvalidDynamicValues(t *testing.T) {
+	t.Parallel()
+
+	type recursive struct {
+		Next *recursive `yaml:"next"`
+	}
+	type config struct {
+		Tree    *recursive `yaml:"tree"`
+		Dynamic any        `yaml:"dynamic"`
+	}
+	cyclic := &recursive{}
+	cyclic.Next = cyclic
+	schema := mustSchemaFor[config](t)
+	acyclic := config{Tree: &recursive{Next: &recursive{}}}
+	if _, issue := projectConfiguration(schema, reflect.ValueOf(acyclic), defaultProjectionLimits()); issue != nil {
+		t.Fatalf("acyclic recursive projection issue = %#v", issue)
+	}
+	if _, issue := cloneConfiguration(schema, reflect.ValueOf(acyclic), defaultProjectionLimits()); issue != nil {
+		t.Fatalf("acyclic recursive clone issue = %#v", issue)
+	}
+	value := config{Tree: cyclic}
+	if _, issue := projectConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaCycle || issue.path.String() != "/tree/next" {
+		t.Fatalf("projection cycle issue = %#v", issue)
+	}
+	if _, issue := cloneConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaCycle {
+		t.Fatalf("clone cycle issue = %#v", issue)
+	}
+
+	dynamicCycle := map[string]any{}
+	dynamicCycle["self"] = dynamicCycle
+	value = config{Dynamic: dynamicCycle}
+	if _, issue := projectConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaCycle {
+		t.Fatalf("dynamic cycle issue = %#v", issue)
+	}
+	value = config{Dynamic: make(chan int)}
+	if _, issue := projectConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaDynamicType {
+		t.Fatalf("dynamic type issue = %#v", issue)
+	}
+	if _, issue := cloneConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaDynamicType {
+		t.Fatalf("dynamic clone issue = %#v", issue)
+	}
+}
+
+func TestProjectionRejectsNonFiniteLimitsAndCodecFailuresSafely(t *testing.T) {
+	t.Parallel()
+
+	type numbers struct {
+		Value float64 `yaml:"value"`
+	}
+	numberSchema := mustSchemaFor[numbers](t)
+	for _, value := range []float64{math.NaN(), math.Inf(1), math.Inf(-1)} {
+		if _, issue := projectConfiguration(numberSchema, reflect.ValueOf(numbers{Value: value}), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaNonFinite {
+			t.Fatalf("non-finite issue = %#v", issue)
+		}
+	}
+
+	type codecConfig struct {
+		Value failingText `yaml:"value"`
+	}
+	codecSchema := mustSchemaFor[codecConfig](t)
+	if _, issue := projectConfiguration(codecSchema, reflect.ValueOf(codecConfig{}), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaCodec || strings.Contains(issue.message, "CANARY") {
+		t.Fatalf("codec projection issue = %#v", issue)
+	}
+
+	type reconstructConfig struct {
+		Value reconstructFailText `yaml:"value"`
+	}
+	reconstructSchema := mustSchemaFor[reconstructConfig](t)
+	if _, issue := cloneConfiguration(reconstructSchema, reflect.ValueOf(reconstructConfig{Value: "safe-name"}), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaCodec || strings.Contains(issue.message, "CANARY") {
+		t.Fatalf("codec clone issue = %#v", issue)
+	}
+
+	type simple struct {
+		Value string `yaml:"value"`
+	}
+	simpleSchema := mustSchemaFor[simple](t)
+	if _, issue := projectConfiguration(simpleSchema, reflect.ValueOf(simple{}), projectionLimits{depth: 1, nodes: 3}); issue != nil {
+		t.Fatalf("exact node limit rejected: %#v", issue)
+	}
+	if _, issue := projectConfiguration(simpleSchema, reflect.ValueOf(simple{}), projectionLimits{depth: 1, nodes: 2}); issue == nil || issue.code != CodeSchemaProjectionLimit {
+		t.Fatalf("node overflow issue = %#v", issue)
+	}
+	if _, issue := projectConfiguration(simpleSchema, reflect.ValueOf(simple{}), projectionLimits{depth: 0, nodes: 3}); issue == nil || issue.code != CodeSchemaProjectionLimit {
+		t.Fatalf("depth overflow issue = %#v", issue)
+	}
+}
+
+func TestInlineProjectionAndCloneUsePhysicalMembers(t *testing.T) {
+	t.Parallel()
+
+	type Embedded struct {
+		Name  string            `yaml:"name"`
+		Items map[string]string `yaml:"items"`
+	}
+	type config struct {
+		*Embedded `yaml:",inline"`
+		Extras    map[string]any `yaml:",inline"`
+	}
+	value := config{
+		Embedded: &Embedded{Name: "value", Items: map[string]string{"a": "b"}},
+		Extras:   map[string]any{"extra": []string{"one"}},
+	}
+	schema := mustSchemaFor[config](t)
+	projected, issue := projectConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits())
+	if issue != nil || !hasMappingKey(projected, "name") || !hasMappingKey(projected, "items") || !hasMappingKey(projected, "extra") {
+		t.Fatalf("inline projection = %#v, issue=%#v", projected, issue)
+	}
+	clonedValue, issue := cloneConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits())
+	if issue != nil {
+		t.Fatal(issue.message)
+	}
+	cloned := clonedValue.Interface().(config)
+	value.Items["a"] = "changed"
+	value.Extras["extra"].([]string)[0] = "changed"
+	if cloned.Items["a"] != "b" || cloned.Extras["extra"].([]string)[0] != "one" {
+		t.Fatal("inline clone retained aliases")
+	}
+
+	value.Extras["name"] = "collision"
+	if _, issue := projectConfiguration(schema, reflect.ValueOf(value), defaultProjectionLimits()); issue == nil || issue.code != CodeSchemaAmbiguousField || issue.path.String() != "/name" {
+		t.Fatalf("inline collision issue = %#v", issue)
+	}
+}
+
+func TestCompareDocumentTreesTracksSemanticChanges(t *testing.T) {
+	t.Parallel()
+
+	type nested struct {
+		Value string `yaml:"value"`
+	}
+	type config struct {
+		Nested   nested         `yaml:"nested"`
+		Mapping  map[string]int `yaml:"mapping"`
+		Sequence []string       `yaml:"sequence"`
+		Maybe    map[string]int `yaml:"maybe"`
+	}
+	schema := mustSchemaFor[config](t)
+	before, issue := projectConfiguration(schema, reflect.ValueOf(config{
+		Nested:   nested{Value: "before"},
+		Mapping:  map[string]int{"remove": 1, "same": 2},
+		Sequence: []string{"a", "b"},
+	}), defaultProjectionLimits())
+	if issue != nil {
+		t.Fatal(issue.message)
+	}
+	after, issue := projectConfiguration(schema, reflect.ValueOf(config{
+		Nested:   nested{Value: "after"},
+		Mapping:  map[string]int{"add": 3, "same": 2},
+		Sequence: []string{"a", "changed"},
+		Maybe:    map[string]int{},
+	}), defaultProjectionLimits())
+	if issue != nil {
+		t.Fatal(issue.message)
+	}
+	deltas := compareConfigurationValues(before, after)
+	want := []struct {
+		path string
+		kind documentDeltaKind
+	}{
+		{path: "/mapping/add", kind: documentAdded},
+		{path: "/mapping/remove", kind: documentRemoved},
+		{path: "/maybe", kind: documentReplaced},
+		{path: "/nested/value", kind: documentReplaced},
+		{path: "/sequence", kind: documentReplaced},
+	}
+	if len(deltas) != len(want) {
+		t.Fatalf("deltas = %#v", deltas)
+	}
+	for i := range want {
+		if deltas[i].path.String() != want[i].path || deltas[i].kind != want[i].kind {
+			t.Fatalf("delta[%d] = %#v, want %#v", i, deltas[i], want[i])
+		}
+	}
+	for _, delta := range deltas {
+		if strings.HasPrefix(delta.path.String(), "/sequence/") {
+			t.Fatal("sequence comparison created index correspondence")
+		}
+	}
+	if equal := compareConfigurationValues(after, document.Clone(after)); len(equal) != 0 {
+		t.Fatalf("equal trees changed = %#v", equal)
+	}
+}
+
+func mustSchemaFor[T any](t *testing.T) *schemaDescriptor {
+	t.Helper()
+	schema, issue := buildSchema(reflect.TypeFor[T]())
+	if issue != nil {
+		t.Fatalf("buildSchema() issue = %#v", issue)
+	}
+	return schema
+}
+
+func mappingValue(t *testing.T, node *document.Node, key string) *document.Node {
+	t.Helper()
+	for _, entry := range node.Mapping {
+		if entry.Key == key {
+			return entry.Value
+		}
+	}
+	t.Fatalf("mapping key %q missing", key)
+	return nil
+}
+
+func hasMappingKey(node *document.Node, key string) bool {
+	for _, entry := range node.Mapping {
+		if entry.Key == key {
+			return true
+		}
+	}
+	return false
+}
+
+func assertDocument(t *testing.T, node *document.Node, kind document.Kind, text string) {
+	t.Helper()
+	if node.Kind != kind || node.Text != text {
+		t.Fatalf("node = %#v, want kind=%v text=%q", node, kind, text)
+	}
+}

provenance_test.go

@@ -0,0 +1,218 @@
+package configkit
+
+import (
+	"errors"
+	"reflect"
+	"sync"
+	"testing"
+)
+
+type layeredItem struct {
+	Value string `yaml:"value"`
+}
+
+type layeredConfig struct {
+	Shared struct {
+		Equal    string `yaml:"equal"`
+		Imported string `yaml:"imported"`
+		Profile  string `yaml:"profile"`
+	} `yaml:"shared"`
+	Values   []int                  `yaml:"values"`
+	Items    map[string]layeredItem `yaml:"items"`
+	Value    any                    `yaml:"value"`
+	Nullable *string                `yaml:"nullable"`
+	Derived  int                    `yaml:"derived"`
+	Empty    map[string]string      `yaml:"empty"`
+}
+
+func TestLayeredProvenanceAgreesWithResolvedSnapshot(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports: [{path: imported.yml}]
+shared: {equal: same}
+values: [1, 2]
+items:
+  keep: {value: 'yes'}
+  remove: {value: 'no'}
+value: {child: base}
+nullable: base
+empty: {}
+`)
+	importPath := writeImportFile(t, directory, "imported.yml", "shared: {imported: base}\n")
+	profilePath := writeImportFile(t, directory, "profiles/testing.yml", `shared: {profile: selected}
+values: [3, 4]
+value: selected
+nullable: null
+empty: {}
+`)
+	defaultNullable := "default"
+	definition := Definition[layeredConfig]{
+		Defaults: func() layeredConfig {
+			defaults := layeredConfig{
+				Values: []int{0, 0}, Value: map[string]any{"child": "default"},
+				Nullable: &defaultNullable, Empty: map[string]string{"retained": "default"},
+			}
+			defaults.Shared.Equal = "same"
+			return defaults
+		},
+		Resolve: func(configuration *layeredConfig, resolution *Resolution) error {
+			Normalize(resolution, Keys("values"), &configuration.Values, []int{5},
+				"bounded values", Keys("shared", "imported"), Keys("shared", "imported"))
+			Normalize(resolution, Keys("values"), &configuration.Values, []int{5},
+				"bounded values", Keys("shared", "imported"))
+			delete(configuration.Items, "remove")
+			resolution.Record(Keys("items"), ChangeNormalization, "removed disabled items")
+			Default(resolution, Keys("derived"), &configuration.Derived, 7,
+				"derived from equal", Keys("shared", "equal"), Keys("shared", "equal"))
+			resolution.Warning(Keys("values"), "values were bounded", Keys("shared", "imported"))
+			return nil
+		},
+	}
+
+	result, err := Load[layeredConfig](File(mainPath), Options[layeredConfig]{
+		Imports: true, Profile: "testing", Definition: definition,
+	})
+	if err != nil {
+		var loadError *LoadError
+		_ = errors.As(err, &loadError)
+		t.Fatalf("%v: %#v", err, loadError.Diagnostics())
+	}
+	wantSources := []string{canonicalTestPath(t, mainPath), canonicalTestPath(t, importPath), canonicalTestPath(t, profilePath)}
+	sources := result.Sources()
+	gotSources := make([]string, len(sources))
+	for index, source := range sources {
+		gotSources[index] = source.Path
+	}
+	if !reflect.DeepEqual(gotSources, wantSources) || sources[2].Layer != LayerProfile {
+		t.Fatalf("sources = %#v", sources)
+	}
+	if result.Config.Shared.Equal != "same" || result.Config.Shared.Imported != "base" || result.Config.Shared.Profile != "selected" ||
+		!reflect.DeepEqual(result.Config.Values, []int{5}) || result.Config.Nullable != nil || result.Config.Derived != 7 ||
+		result.Config.Empty["retained"] != "default" {
+		t.Fatalf("configuration = %#v", result.Config)
+	}
+
+	metadata := result.Metadata()
+	equal, _ := metadata.Lookup(Keys("shared", "equal"))
+	derived, _ := metadata.Lookup(Keys("derived"))
+	nullable, _ := metadata.Lookup(Keys("nullable"))
+	empty, _ := metadata.Lookup(Keys("empty"))
+	firstValue, _ := metadata.Lookup(Keys("values").Index(0))
+	if !equal.Explicit || equal.Origin.Layer != LayerBase || derived.Explicit || derived.Origin.Operation != ChangeDerivedDefault ||
+		!nullable.Explicit || !nullable.Null || nullable.Origin.Layer != LayerProfile ||
+		!empty.Explicit || empty.Origin.Layer != LayerProfile || len(empty.Origin.Locations) != 2 ||
+		!firstValue.Explicit || firstValue.Origin.Operation != ChangeNormalization {
+		t.Fatalf("metadata = equal %#v derived %#v nullable %#v empty %#v value %#v", equal, derived, nullable, empty, firstValue)
+	}
+	if _, ok := metadata.Lookup(Keys("value", "child")); ok {
+		t.Fatal("profile-removed child remains effective")
+	}
+	if _, ok := metadata.Lookup(Keys("values").Index(1)); ok {
+		t.Fatal("resolution-removed sequence index remains effective")
+	}
+	if _, ok := metadata.Lookup(Keys("items", "remove", "value")); ok {
+		t.Fatal("resolution-removed mapping child remains effective")
+	}
+
+	assertHistoryLayersAndPaths(t, metadata.History(Keys("values").Index(1)),
+		[]Layer{LayerDefault, LayerBase, LayerProfile, LayerResolution},
+		[]string{"/values/1", "/values/1", "/values", "/values"})
+	assertHistoryLayersAndPaths(t, metadata.History(Keys("value", "child")),
+		[]Layer{LayerDefault, LayerBase, LayerProfile},
+		[]string{"/value/child", "/value/child", "/value"})
+	removedHistory := metadata.History(Keys("items", "remove", "value"))
+	assertHistoryLayersAndPaths(t, removedHistory,
+		[]Layer{LayerBase, LayerResolution},
+		[]string{"/items/remove/value", "/items"})
+	if !reflect.DeepEqual(removedHistory[1].Reasons, []string{"removed disabled items"}) {
+		t.Fatalf("removed history = %#v", removedHistory)
+	}
+	valuesHistory := metadata.History(Keys("values").Index(0))
+	lastValue := valuesHistory[len(valuesHistory)-1]
+	if !reflect.DeepEqual(lastValue.Reasons, []string{"bounded values"}) || len(lastValue.Dependencies) != 1 || lastValue.Dependencies[0].String() != "/shared/imported" {
+		t.Fatalf("canonical resolution history = %#v", lastValue)
+	}
+
+	schema, issue := buildSchema(reflect.TypeFor[layeredConfig]())
+	if issue != nil {
+		t.Fatal(issue)
+	}
+	projected, projectionIssue := projectConfiguration(schema, reflect.ValueOf(result.Config), defaultProjectionLimits())
+	if projectionIssue != nil || !configurationValuesEqual(projected, result.final) {
+		t.Fatalf("typed projection and private snapshot disagree: %#v", projectionIssue)
+	}
+
+	metadata.history[Keys("values").Index(0).String()][0].Origin.Layer = "mutated"
+	sources[1].ImportedFrom.SourceID = "mutated"
+	diagnostics := result.Diagnostics()
+	diagnostics[0].Related[0].SourceID = "mutated"
+	result.Config.Values[0] = 99
+	freshHistory := result.Metadata().History(Keys("values").Index(0))
+	if freshHistory[0].Origin.Layer == "mutated" || result.Sources()[1].ImportedFrom.SourceID == "mutated" ||
+		result.Diagnostics()[0].Related[0].SourceID == "mutated" {
+		t.Fatal("returned inspection records alias private state")
+	}
+	snapshotValue, ok := documentAtPath(result.final, Keys("values").Index(0))
+	if !ok || snapshotValue.Text != "5" {
+		t.Fatal("caller configuration mutation changed the private final snapshot")
+	}
+}
+
+func TestSelectedProfileLoadsRemainIndependentConcurrently(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", "values: [1, 2]\n")
+	writeImportFile(t, directory, "profiles/testing.yml", "values: [3, 4]\n")
+	options := Options[layeredConfig]{Profile: "testing"}
+
+	const loads = 16
+	results := make(chan *Result[layeredConfig], loads)
+	errors := make(chan error, loads)
+	var group sync.WaitGroup
+	for index := 0; index < loads; index++ {
+		group.Add(1)
+		go func() {
+			defer group.Done()
+			result, err := Load[layeredConfig](File(mainPath), options)
+			if err != nil {
+				errors <- err
+				return
+			}
+			results <- result
+		}()
+	}
+	group.Wait()
+	close(results)
+	close(errors)
+	for err := range errors {
+		t.Fatal(err)
+	}
+	var collected []*Result[layeredConfig]
+	for result := range results {
+		collected = append(collected, result)
+	}
+	if len(collected) != loads {
+		t.Fatalf("completed loads = %d", len(collected))
+	}
+	collected[0].Config.Values[0] = 99
+	collected[0].metadata.fields[Keys("values").String()] = FieldInfo{}
+	for _, result := range collected[1:] {
+		if !reflect.DeepEqual(result.Config.Values, []int{3, 4}) {
+			t.Fatalf("concurrent result contaminated = %#v", result.Config.Values)
+		}
+		if info, ok := result.Metadata().Lookup(Keys("values")); !ok || info.Origin.Layer != LayerProfile {
+			t.Fatalf("concurrent metadata contaminated = %#v, %v", info, ok)
+		}
+	}
+}
+
+func assertHistoryLayersAndPaths(t *testing.T, history []Change, layers []Layer, paths []string) {
+	t.Helper()
+	if len(history) != len(layers) || len(history) != len(paths) {
+		t.Fatalf("history = %#v", history)
+	}
+	for index, change := range history {
+		if change.Origin.Layer != layers[index] || change.Path.String() != paths[index] {
+			t.Fatalf("history[%d] = %#v", index, change)
+		}
+	}
+}

records.go

@@ -0,0 +1,286 @@
+package configkit
+
+// Layer identifies the precedence layer responsible for a configuration value.
+type Layer string
+
+const (
+	LayerDefault    Layer = "default"
+	LayerBase       Layer = "base"
+	LayerProfile    Layer = "profile"
+	LayerCLI        Layer = "cli"
+	LayerResolution Layer = "resolution"
+)
+
+// ChangeKind identifies a recorded operation. The zero value is
+// used for input and default origins.
+type ChangeKind string
+
+const (
+	// ChangeDecoding records the built-in transition to canonical typed values.
+	// It is inspection metadata, not an operation accepted by Resolution.Record.
+	ChangeDecoding       ChangeKind = "decoding"
+	ChangeDerivedDefault ChangeKind = "derived_default"
+	ChangeNormalization  ChangeKind = "normalization"
+)
+
+// Severity classifies a diagnostic.
+type Severity string
+
+const (
+	SeverityError   Severity = "error"
+	SeverityWarning Severity = "warning"
+)
+
+// Code is a stable, machine-readable diagnostic classification.
+type Code string
+
+const (
+	CodeSourceInvalid       Code = "source.invalid"
+	CodeSourceMissing       Code = "source.missing"
+	CodeSourcePermission    Code = "source.permission"
+	CodeSourceRead          Code = "source.read"
+	CodeSourceNotRegular    Code = "source.not_regular"
+	CodeSourceLimitExceeded Code = "source.limit_exceeded"
+	CodeSourceReaderUsed    Code = "source.reader_used"
+
+	CodeYAMLInvalid           Code = "yaml.invalid"
+	CodeYAMLEmpty             Code = "yaml.empty"
+	CodeYAMLMultipleDocuments Code = "yaml.multiple_documents"
+	CodeYAMLRootType          Code = "yaml.root_type"
+	CodeYAMLDuplicateKey      Code = "yaml.duplicate_key"
+	CodeYAMLTag               Code = "yaml.tag"
+	CodeYAMLAnchor            Code = "yaml.anchor"
+	CodeYAMLAlias             Code = "yaml.alias"
+	CodeYAMLMergeKey          Code = "yaml.merge_key"
+	CodeYAMLScalar            Code = "yaml.scalar"
+	CodeYAMLDepthLimit        Code = "yaml.depth_limit"
+	CodeYAMLNodeLimit         Code = "yaml.node_limit"
+
+	CodeSchemaRoot            Code = "schema.root"
+	CodeSchemaUnsupportedType Code = "schema.unsupported_type"
+	CodeSchemaAmbiguousField  Code = "schema.ambiguous_field"
+	CodeSchemaTag             Code = "schema.tag"
+	CodeSchemaTextCodec       Code = "schema.text_codec"
+	CodeSchemaInline          Code = "schema.inline"
+	CodeSchemaCycle           Code = "schema.cycle"
+	CodeSchemaProjectionLimit Code = "schema.projection_limit"
+	CodeSchemaCodec           Code = "schema.codec"
+	CodeSchemaNonFinite       Code = "schema.non_finite"
+	CodeSchemaDynamicType     Code = "schema.dynamic_type"
+
+	CodeDecodeType         Code = "decode.type"
+	CodeDecodeNull         Code = "decode.null"
+	CodeDecodeRange        Code = "decode.range"
+	CodeDecodeDuration     Code = "decode.duration"
+	CodeDecodeText         Code = "decode.text"
+	CodeDecodeUnknownField Code = "decode.unknown_field"
+	CodeDecodeArrayLength  Code = "decode.array_length"
+
+	CodeOptionsInvalid     Code = "options.invalid"
+	CodeOptionsUnavailable Code = "options.unavailable"
+
+	CodeImportDisabled      Code = "import.disabled"
+	CodeImportInvalid       Code = "import.invalid"
+	CodeImportMissing       Code = "import.missing"
+	CodeImportPermission    Code = "import.permission"
+	CodeImportRead          Code = "import.read"
+	CodeImportOutsideRoot   Code = "import.outside_root"
+	CodeImportCycle         Code = "import.cycle"
+	CodeImportRepeated      Code = "import.repeated"
+	CodeImportLimitExceeded Code = "import.limit_exceeded"
+	CodeImportVersion       Code = "import.version"
+
+	CodeProfileInvalid       Code = "profile.invalid"
+	CodeProfileMissing       Code = "profile.missing"
+	CodeProfileOutsideRoot   Code = "profile.outside_root"
+	CodeProfileRepeated      Code = "profile.repeated"
+	CodeProfileDirective     Code = "profile.directive"
+	CodeProfileVersion       Code = "profile.version"
+	CodeProfileLimitExceeded Code = "profile.limit_exceeded"
+
+	CodeComposeInvalid  Code = "compose.invalid"
+	CodeComposeConflict Code = "compose.conflict"
+
+	CodeVersionType       Code = "version.type"
+	CodeVersionError      Code = "version.error"
+	CodeVersionWarning    Code = "version.warning"
+	CodeVersionHookFailed Code = "version.hook_failed"
+
+	CodeResolveError            Code = "resolve.error"
+	CodeResolveWarning          Code = "resolve.warning"
+	CodeResolveHookFailed       Code = "resolve.hook_failed"
+	CodeResolveInvalidPath      Code = "resolve.invalid_path"
+	CodeResolveInvalidKind      Code = "resolve.invalid_kind"
+	CodeResolveInvalidRecord    Code = "resolve.invalid_record"
+	CodeResolveInvalidTarget    Code = "resolve.invalid_target"
+	CodeResolveInvalidValue     Code = "resolve.invalid_value"
+	CodeResolveUnrecordedChange Code = "resolve.unrecorded_change"
+
+	CodeValidateError       Code = "validate.error"
+	CodeValidateWarning     Code = "validate.warning"
+	CodeValidateInvalidPath Code = "validate.invalid_path"
+	CodeValidateMutation    Code = "validate.mutation"
+
+	CodeSecretsInvalidPath   Code = "secrets.invalid_path"
+	CodeSecretsMissing       Code = "secrets.missing"
+	CodeSecretsPermission    Code = "secrets.permission"
+	CodeSecretsRead          Code = "secrets.read"
+	CodeSecretsNotRegular    Code = "secrets.not_regular"
+	CodeSecretsInvalidName   Code = "secrets.invalid_name"
+	CodeSecretsLimitExceeded Code = "secrets.limit_exceeded"
+	CodeSecretsInvalidValue  Code = "secrets.invalid_value"
+	CodeSecretsApplyFailed   Code = "secrets.apply_failed"
+
+	CodeSecretRequired    Code = "secret.required"
+	CodeSecretInvalidName Code = "secret.invalid_name"
+	CodeSecretUnset       Code = "secret.unset"
+	CodeSecretEmpty       Code = "secret.empty"
+
+	CodeOverrideInvalidPath   Code = "override.invalid_path"
+	CodeOverrideInvalidLabel  Code = "override.invalid_label"
+	CodeOverrideInvalidValue  Code = "override.invalid_value"
+	CodeOverrideDuplicate     Code = "override.duplicate"
+	CodeOverrideOverlap       Code = "override.overlap"
+	CodeOverrideDirective     Code = "override.directive"
+	CodeOverrideVersion       Code = "override.version"
+	CodeOverrideLimitExceeded Code = "override.limit_exceeded"
+
+	CodeExplainLimitExceeded Code = "explain.limit_exceeded"
+)
+
+// Location identifies a position in a configuration source. Zero line and column
+// values mean that coordinates are unavailable or synthetic.
+type Location struct {
+	SourceID string
+	Line     int
+	Column   int
+}
+
+// SourceInfo describes a source discovered during a load. Path is a canonical
+// local path when one exists. ImportedFrom identifies the declaration that
+// included this source.
+type SourceInfo struct {
+	ID           string
+	Label        string
+	Path         string
+	Layer        Layer
+	ImportedFrom *Location
+}
+
+// Diagnostic is a safe structured problem report. Message and source labels must
+// never contain configuration values or other secrets.
+type Diagnostic struct {
+	Code     Code
+	Severity Severity
+	Path     Path
+	Message  string
+	Primary  *Location
+	Related  []Location
+}
+
+// Origin describes where an effective definition came from. Multiple locations
+// represent a composite origin; resolution origins are synthetic and identify their
+// operation instead of a file location.
+type Origin struct {
+	Layer     Layer
+	Locations []Location
+	Operation ChangeKind
+}
+
+// FieldInfo describes an effective configuration path.
+type FieldInfo struct {
+	Explicit bool
+	Null     bool
+	Origin   Origin
+}
+
+// Change describes a definition or application-resolution operation. Path can name
+// an ancestor operation affecting the queried descendant. Historical values are
+// intentionally not exposed by this record.
+type Change struct {
+	Path         Path
+	Origin       Origin
+	Previous     []Origin
+	Reasons      []string
+	Dependencies []Path
+}
+
+func cloneLocation(location *Location) *Location {
+	if location == nil {
+		return nil
+	}
+	cloned := *location
+	return &cloned
+}
+
+func cloneLocations(locations []Location) []Location {
+	if locations == nil {
+		return nil
+	}
+	return append([]Location(nil), locations...)
+}
+
+func cloneSourceInfo(source SourceInfo) SourceInfo {
+	source.ImportedFrom = cloneLocation(source.ImportedFrom)
+	return source
+}
+
+func cloneSourceInfos(sources []SourceInfo) []SourceInfo {
+	if sources == nil {
+		return nil
+	}
+	cloned := make([]SourceInfo, len(sources))
+	for i, source := range sources {
+		cloned[i] = cloneSourceInfo(source)
+	}
+	return cloned
+}
+
+func cloneOrigin(origin Origin) Origin {
+	origin.Locations = cloneLocations(origin.Locations)
+	return origin
+}
+
+func cloneFieldInfo(info FieldInfo) FieldInfo {
+	info.Origin = cloneOrigin(info.Origin)
+	return info
+}
+
+func cloneChange(change Change) Change {
+	change.Path = change.Path.clone()
+	change.Origin = cloneOrigin(change.Origin)
+	if change.Previous != nil {
+		previous := make([]Origin, len(change.Previous))
+		for i, origin := range change.Previous {
+			previous[i] = cloneOrigin(origin)
+		}
+		change.Previous = previous
+	}
+	change.Reasons = append([]string(nil), change.Reasons...)
+	if change.Dependencies != nil {
+		dependencies := make([]Path, len(change.Dependencies))
+		for i, path := range change.Dependencies {
+			dependencies[i] = path.clone()
+		}
+		change.Dependencies = dependencies
+	}
+	return change
+}
+
+func cloneDiagnostic(diagnostic Diagnostic) Diagnostic {
+	diagnostic.Path = diagnostic.Path.clone()
+	diagnostic.Primary = cloneLocation(diagnostic.Primary)
+	diagnostic.Related = cloneLocations(diagnostic.Related)
+	return diagnostic
+}
+
+func cloneDiagnostics(diagnostics []Diagnostic) []Diagnostic {
+	if diagnostics == nil {
+		return nil
+	}
+	cloned := make([]Diagnostic, len(diagnostics))
+	for i, diagnostic := range diagnostics {
+		cloned[i] = cloneDiagnostic(diagnostic)
+	}
+	return cloned
+}

release_check_test.go

@@ -0,0 +1,108 @@
+package configkit
+
+import (
+	"bytes"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestCheckScriptsHandleFilenamesAndStopOnFailure(t *testing.T) {
+	for _, scriptName := range []string{"release-check.sh", "check.sh"} {
+		t.Run(scriptName, func(t *testing.T) {
+			script, err := os.ReadFile("scripts/" + scriptName)
+			if err != nil {
+				t.Fatal(err)
+			}
+			for _, name := range []string{"success", "failed-go-check", "untracked-whitespace"} {
+				fail := name != "success"
+				t.Run(name, func(t *testing.T) {
+					root := t.TempDir()
+					bin := t.TempDir()
+					log := filepath.Join(bin, "gofmt.args")
+					calls := filepath.Join(bin, "go.calls")
+					for _, directory := range []string{"scripts", "nested"} {
+						if err := os.Mkdir(filepath.Join(root, directory), 0700); err != nil {
+							t.Fatal(err)
+						}
+					}
+					write := func(path string, data []byte, mode os.FileMode) {
+						t.Helper()
+						if err := os.WriteFile(path, data, mode); err != nil {
+							t.Fatal(err)
+						}
+					}
+					write(filepath.Join(root, "scripts", scriptName), script, 0700)
+					write(filepath.Join(root, "go.mod"), []byte("module example.invalid/check\n"), 0600)
+					write(filepath.Join(root, "go.sum"), nil, 0600)
+					write(filepath.Join(root, "obsolete.go"), []byte("package obsolete\n"), 0600)
+					files := []string{"nested/a space.go", "nested/a\nnewline.go"}
+					for _, file := range files {
+						write(filepath.Join(root, file), []byte("package example\n"), 0600)
+					}
+					if name == "untracked-whitespace" {
+						write(filepath.Join(root, "notes.md"), []byte("trailing space \n"), 0600)
+					}
+					// Isolate expensive toolchain commands: this test owns shell orchestration,
+					// not Go's compiler or the production tests exercised by the real checker.
+					write(filepath.Join(bin, "go"), []byte("#!/bin/sh\nprintf '%s\\n' \"$*\" >> \"$CONFIGKIT_TEST_GO_LOG\"\nif [ \"${CONFIGKIT_TEST_FAIL-}\" = yes ] && [ \"${1-}\" = test ]; then exit 37; fi\n"), 0700)
+					write(filepath.Join(bin, "gofmt"), []byte("#!/bin/sh\nprintf '%s\\000' \"$@\" > \"$CONFIGKIT_TEST_FMT_LOG\"\n"), 0700)
+					init := exec.Command("git", "init", "-q")
+					init.Dir = root
+					if out, err := init.CombinedOutput(); err != nil {
+						t.Fatalf("git init: %v %s", err, out)
+					}
+					// HEAD is required by the whitespace gate. Commit only the module baseline.
+					for _, args := range [][]string{{"add", "go.mod", "go.sum", "obsolete.go"}, {"-c", "user.name=Test", "-c", "user.email=test@example.invalid", "-c", "commit.gpgsign=false", "commit", "-qm", "Test fixture"}} {
+						cmd := exec.Command("git", args...)
+						cmd.Dir = root
+						if out, err := cmd.CombinedOutput(); err != nil {
+							t.Fatalf("git fixture: %v %s", err, out)
+						}
+					}
+					if err := os.Remove(filepath.Join(root, "obsolete.go")); err != nil {
+						t.Fatal(err)
+					}
+					cmd := exec.Command("/bin/bash", filepath.Join(root, "scripts", scriptName))
+					cmd.Env = append(os.Environ(), "PATH="+bin+string(os.PathListSeparator)+os.Getenv("PATH"), "CONFIGKIT_BASELINE_TOOLCHAIN=test-baseline", "CONFIGKIT_NEWER_TOOLCHAIN=test-newer", "CONFIGKIT_TEST_FMT_LOG="+log, "CONFIGKIT_TEST_GO_LOG="+calls)
+					if name == "failed-go-check" {
+						cmd.Env = append(cmd.Env, "CONFIGKIT_TEST_FAIL=yes")
+					}
+					out, err := cmd.CombinedOutput()
+					if fail {
+						if err == nil {
+							t.Fatal("checker ignored failed Go test")
+						}
+					} else if err != nil {
+						t.Fatalf("checker: %v %s", err, out)
+					}
+					arguments, err := os.ReadFile(log)
+					if err != nil {
+						t.Fatal(err)
+					}
+					parts := bytes.Split(arguments, []byte{0})
+					for _, file := range files {
+						found := false
+						for _, arg := range parts {
+							if string(arg) == file {
+								found = true
+							}
+						}
+						if !found {
+							t.Fatalf("filename was split: %q", file)
+						}
+					}
+					invoked, err := os.ReadFile(calls)
+					if err != nil {
+						t.Fatal(err)
+					}
+					if fail && strings.Contains(string(invoked), "vet ./...") {
+						t.Fatal("checker continued after failed test")
+					}
+				})
+			}
+		})
+	}
+}

render.go

@@ -0,0 +1,46 @@
+package configkit
+
+import (
+	"errors"
+	"io"
+	"strings"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+var errEffectiveUnavailable = errors.New("configkit: effective configuration is unavailable")
+
+// WriteEffective writes deterministic redacted YAML from the frozen resolved
+// snapshot. The output is intended for inspection and is not promised reloadable.
+// Writer errors, including short writes, are returned and may leave partial output.
+func (r *Result[T]) WriteEffective(output io.Writer) error {
+	if r == nil || r.final == nil || output == nil {
+		return errEffectiveUnavailable
+	}
+	safe := redactedDocument(r.final, r.secrets, Path{})
+	return yamladapter.Write(output, safe)
+}
+
+func redactedDocument(node *document.Node, secret map[string]bool, path Path) *document.Node {
+	result, _ := safeSnapshotDocument(node, secret, path)
+	return result
+}
+
+func pathIsSecret(secret map[string]bool, path Path) bool {
+	if secret[""] {
+		return true
+	}
+	pointer := path.String()
+	for pointer != "" {
+		if secret[pointer] {
+			return true
+		}
+		position := strings.LastIndexByte(pointer, '/')
+		if position <= 0 {
+			break
+		}
+		pointer = pointer[:position]
+	}
+	return false
+}