Correct mounted mapping diagnostic paths

AGENTS.md

@@ -1,35 +1,25 @@
-Please review `docs/development.md` for initial orientation in this repository
-and follow its task-specific reading guide.
+<!-- codebase-memory-mcp:start -->
+# Codebase Knowledge Graph (codebase-memory-mcp)
 
-## Implementation Discipline
+This project uses codebase-memory-mcp to maintain a knowledge graph of the codebase.
+ALWAYS prefer MCP graph tools over grep/glob/file-search for code discovery.
 
-Simplify the mechanism, not the user's choices.
+## Priority Order
+1. `search_graph` — find functions, classes, routes, variables by pattern
+2. `trace_path` — trace who calls a function or what it calls
+3. `get_code_snippet` — read specific function/class source code
+4. `query_graph` — run Cypher queries for complex patterns
+5. `get_architecture` — high-level project summary
 
-**Keep implementation machinery proportionate.** Use straightforward,
-idiomatic code and introduce only the machinery needed to support the requested
-behavior and flexibility. Optimize for conceptual simplicity and
-maintainability, not for the fewest lines, fewest options, or narrowest product
-surface.
+## When to fall back to grep/glob
+- Searching for string literals, error messages, config values
+- Searching non-code files (Dockerfiles, shell scripts, configs)
+- When MCP tools return insufficient results
+<!-- codebase-memory-mcp:end -->
 
-**Do not invent restrictions.** Do not reject, prohibit, constrain, cap,
-harden, or otherwise narrow behavior unless required by the user, an explicit
-repository contract, or an unavoidable correctness constraint. Preserve
-operator and caller choice where policy has not been established. If an
-additional restriction appears advisable, present it as a decision rather than
-silently implementing it.
+Start with `docs/development.md` for orientation and use its policy reading guide
+to select the policies applicable to the task. Do not read the entire suite by
+default.
 
-A correctness constraint must follow from the requested behavior or an
-established invariant, not from the convenience of the chosen implementation.
-Explain the specific failure it prevents and why a less restrictive
-implementation cannot satisfy the contract.
-
-**An implementation detail is not repository policy.** Tests and documentation
-must not elevate incidental behavior into a durable guarantee or restriction
-unless that behavior was expressly selected as part of the contract. Existing
-explicit contracts remain authoritative; changing or weakening one requires a
-deliberate decision rather than being inferred from this principle.
-
-Agent-authored plans, tests, documentation, and prior implementations are not
-evidence that an additional restriction was deliberately approved. Identify the
-authorizing requirement or decision; when that authority is unclear, surface
-the ambiguity rather than treating repetition as approval.
+Read and follow `docs/policy/agents.md` before recommending, planning, or
+implementing repository changes.

COPYRIGHT

@@ -0,0 +1,25 @@
+ConfigKit
+
+Copyright (C) 2026 Eric Rakestraw
+
+SPDX-License-Identifier: LGPL-3.0-or-later
+
+This library is free software: you can redistribute it and/or modify it
+under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+The GNU Lesser General Public License version 3 and its incorporated
+GNU General Public License version 3 text are provided in LICENSE.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU Lesser General Public License for more details.
+
+Third-party code and assets are subject to their respective license terms.
+
+Policy documents adapted from Standard Repository Policies, the root AGENTS.md
+orientation, and the Policy Reading Guide in docs/development.md remain licensed
+under CC-BY-SA-4.0. See docs/oss/attribution.md for the affected material,
+upstream credit, and license links.

README.md

@@ -5,18 +5,26 @@ An opinionated Go library for applications that use YAML configuration files.
 Linux is release-validated; macOS support is best-effort. See the
 [platform support policy](docs/policy/release.md#platform-support) for details.
 
-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.
+ConfigKit's simple `Load` accepts one strict YAML source (or explicit
+defaults-only input). `LoadLayered` additionally combines explicit lower-priority
+source groups with an independently selected main input. Sources can be host
+files, native `fs.FS` files, labeled bytes or readers, or snapshotted mappings.
+Enabled path and glob imports compose strictly within their group; precedence
+between groups combines mappings recursively. Both entry points return a complete,
+validated configuration with 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.
+CLI override records apply above overlays. Value explanation is available through
+detached structured records and deterministic redacted output. Built-in
+`extends` relationships let a mapping inherit from a named sibling during the
+same load, retaining explicit presence and source/derivation history. The root
+`Diff` API compares frozen effective values from two successful same-type loads;
+the [configuration contract](docs/config.md#effective-configuration-diffs) and
+[consumer guide](docs/consumers/README.md#effective-configuration-diffs) describe
+its safe changed-region records and maintained workflows.
 
 ```go
 type Config struct {
@@ -33,10 +41,42 @@ func ReadConfig(path string) (Config, error) {
 }
 ```
 
-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.
+## Design philosophy
+
+ConfigKit favors one coherent, predictable configuration model over flexibility
+for reproducing each consumer's conventions. Its goal is to let applications
+load once, inspect where final values came from, and distinguish explicit values
+from omissions with minimal configuration bookkeeping. Applications retain their
+domain rules and execution responsibilities.
+
+Loading is synchronous and complete on return. Applications normally load once
+near startup and then use the final configuration and metadata; ConfigKit stays
+inactive until explicitly called again. Applications may initiate additional
+loads as needed. See the [loading lifecycle](docs/config.md#completion-and-resource-lifetime)
+for completion and ownership details.
+
+Within a configuration tier, independent contributions compose strictly: shared
+mappings combine, while overlapping non-mapping definitions are errors. Between
+precedence tiers, mappings combine recursively, retaining omitted lower-priority
+children; other values replace lower-priority values. The main document and its
+imports form one strictly composed tier. Explicit underlay groups sit below it
+and above typed defaults; a selected overlay and explicit CLI overrides sit
+above it. The [configuration contract](docs/config.md#strict-document-composition)
+owns the detailed combination rules and input-specific constraints.
+
+The [architecture goals](docs/policy/architecture.md#success-criteria) explain
+these priorities and how genuine new needs can justify extending the shared
+model. The [deferred scope](docs/roadmap/deferred.md) records deliberate exclusions.
+
+The [configuration contract](docs/config.md) describes implemented behavior,
+including [input destinations](docs/config.md#input-destinations). The
+[consumer adoption guide](docs/consumers/README.md) provides maintained simple,
+layered, and mounted-collection examples alongside the migration shapes. The
+accepted placement decision is recorded in [ADR 0011](docs/adr/0011-input-destinations.md).
+See the [development guide](docs/development.md) for repository orientation and
+the [release procedure](docs/release.md) for maintainer-only candidate and
+publication steps.
+
+ConfigKit code is licensed under the [LGPL v3.0 or later](COPYRIGHT).
+
+[Open Source attribution notices](docs/oss/attribution.md) are available.

conversion.go

@@ -43,16 +43,36 @@ func toEngineLocations(values []Location) []engine.Location {
 	return result
 }
 func fromEngineSourceInfo(value engine.SourceInfo) SourceInfo {
-	return SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: Layer(value.Layer), ImportedFrom: fromEngineLocationPointer(value.ImportedFrom)}
+	return SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Destination: fromEnginePath(value.Destination), Layer: Layer(value.Layer), Kind: SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: fromEngineLocationPointer(value.ImportedFrom)}
 }
 func toEngineSourceInfo(value SourceInfo) engine.SourceInfo {
-	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: engine.Layer(value.Layer), ImportedFrom: toEngineLocationPointer(value.ImportedFrom)}
+	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Destination: toEnginePath(value.Destination), Layer: engine.Layer(value.Layer), Kind: engine.SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: toEngineLocationPointer(value.ImportedFrom)}
 }
 func fromEngineOrigin(value engine.Origin) Origin {
-	return Origin{Layer: Layer(value.Layer), Locations: fromEngineLocations(value.Locations), Operation: ChangeKind(value.Operation)}
+	result := Origin{Layer: Layer(value.Layer), Locations: fromEngineLocations(value.Locations), Operation: ChangeKind(value.Operation)}
+	if value.Inheritance != nil {
+		result.Inheritance = make([]InheritanceStep, len(value.Inheritance))
+		for index, step := range value.Inheritance {
+			result.Inheritance[index] = InheritanceStep{
+				From: fromEnginePath(step.From), To: fromEnginePath(step.To), Reference: fromEnginePath(step.Reference),
+				Locations: fromEngineLocations(step.Locations),
+			}
+		}
+	}
+	return result
 }
 func toEngineOrigin(value Origin) engine.Origin {
-	return engine.Origin{Layer: engine.Layer(value.Layer), Locations: toEngineLocations(value.Locations), Operation: engine.ChangeKind(value.Operation)}
+	result := engine.Origin{Layer: engine.Layer(value.Layer), Locations: toEngineLocations(value.Locations), Operation: engine.ChangeKind(value.Operation)}
+	if value.Inheritance != nil {
+		result.Inheritance = make([]engine.InheritanceStep, len(value.Inheritance))
+		for index, step := range value.Inheritance {
+			result.Inheritance[index] = engine.InheritanceStep{
+				From: toEnginePath(step.From), To: toEnginePath(step.To), Reference: toEnginePath(step.Reference),
+				Locations: toEngineLocations(step.Locations),
+			}
+		}
+	}
+	return result
 }
 func fromEngineDiagnostic(value engine.Diagnostic) Diagnostic {
 	return Diagnostic{Code: Code(value.Code), Severity: Severity(value.Severity), Path: fromEnginePath(value.Path), Message: value.Message, Primary: fromEngineLocationPointer(value.Primary), Related: fromEngineLocations(value.Related)}

development_check_test.go

@@ -9,8 +9,8 @@ import (
 	"testing"
 )
 
-func TestCheckScriptsHandleFilenamesAndStopOnFailure(t *testing.T) {
-	for _, scriptName := range []string{"release-check.sh", "check.sh"} {
+func TestDevelopmentCheckHandlesFilenamesAndStopsOnFailure(t *testing.T) {
+	for _, scriptName := range []string{"check.sh"} {
 		t.Run(scriptName, func(t *testing.T) {
 			script, err := os.ReadFile("scripts/" + scriptName)
 			if err != nil {
@@ -66,7 +66,7 @@ func TestCheckScriptsHandleFilenamesAndStopOnFailure(t *testing.T) {
 						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)
+					cmd.Env = append(os.Environ(), "PATH="+bin+string(os.PathListSeparator)+os.Getenv("PATH"), "CONFIGKIT_TEST_FMT_LOG="+log, "CONFIGKIT_TEST_GO_LOG="+calls)
 					// Log only targets selected by the script, not inherited host settings.
 					cmd.Env = append(cmd.Env, "GOOS=", "GOARCH=")
 					if name == "failed-go-check" {
@@ -103,17 +103,6 @@ func TestCheckScriptsHandleFilenamesAndStopOnFailure(t *testing.T) {
 					if fail && strings.Contains(string(invoked), "vet ./...") {
 						t.Fatal("checker continued after failed test")
 					}
-					if scriptName == "release-check.sh" && !fail {
-						// These are selected release gates, not incidental loop details.
-						for _, target := range []string{"linux/amd64", "linux/arm64"} {
-							if !strings.Contains(string(invoked), target+" build ./...") {
-								t.Errorf("checker omitted required target %s", target)
-							}
-						}
-						if strings.Contains(string(invoked), "darwin/") {
-							t.Fatal("checker requires best-effort macOS validation")
-						}
-					}
 				})
 			}
 		})

diff.go

@@ -0,0 +1,53 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+
+// Difference describes one canonical changed region between two successful
+// loads. Left is the earlier value and Right is the later value. Each snapshot
+// is detached; Present distinguishes absence from an explicit null, and
+// Redacted reports whether combined sensitivity affected that side. Mappings
+// are compared recursively, while sequences are reported as atomic values.
+// Records are ordered by canonical JSON pointer. HiddenChanged indicates that
+// a value concealed by combined sensitivity changed, without revealing which
+// concealed member changed.
+type Difference struct {
+	Path          Path
+	Left          ValueSnapshot
+	Right         ValueSnapshot
+	HiddenChanged bool
+}
+
+// Diff compares the frozen effective values of two successful loads of the
+// same configuration type. It does not reread sources, environment variables,
+// or rerun defaults or resolvers. Equal values produce no records even when
+// their sources, defaults, or resolution histories differ. Credential contents
+// are not compared: typed secret references compare by reference name only.
+//
+// Both results must be non-nil successful loads with inspection state; nil or
+// zero results return an error and no records. Load errors are returned by
+// Load, so callers must handle those before calling Diff. Snapshots are safe
+// for presentation but are not a complete reloadable configuration.
+func Diff[T any](left, right *Result[T]) ([]Difference, error) {
+	var leftInspection, rightInspection *engine.Inspection
+	if left != nil {
+		leftInspection = left.inspection
+	}
+	if right != nil {
+		rightInspection = right.inspection
+	}
+
+	values, err := engine.Diff(leftInspection, rightInspection)
+	if err != nil {
+		return nil, err
+	}
+	differences := make([]Difference, len(values))
+	for index, value := range values {
+		differences[index] = Difference{
+			Path:          fromEnginePath(value.Path),
+			Left:          fromEngineValueSnapshot(value.Left),
+			Right:         fromEngineValueSnapshot(value.Right),
+			HiddenChanged: value.HiddenChanged,
+		}
+	}
+	return differences, nil
+}

doc.go

@@ -2,7 +2,7 @@
 // loading for Go programs.
 //
 // Load supports file, byte, reader, and explicit no-input sources with strict YAML,
-// typed defaults, imports, profiles, CLI overrides, schema-version checks, detached
+// typed defaults, imports, overlays, CLI overrides, schema-version checks, detached
 // metadata, recorded application resolution, typed secret references, safe effective
 // rendering, read-only semantic validation, and structured load errors. The
 // configuration contract is documented in docs/config.md.

docs/adr/0007-source-backed-underlays.md

@@ -0,0 +1,118 @@
+# ADR 0007: Add Source-Backed Underlays with Consistent Recursive Precedence
+
+## Status
+
+Proposed draft for review. Completed planning and remediation records are
+preserved in Git history at `b604aa1` under `docs/roadmap/`; the
+[configuration contract](../config.md#layered-loading) owns current behavior.
+
+This extends [ADR 0001](0001-separate-composition-and-overlay.md) and preserves
+its distinction between strict composition and intentional overlay.
+
+## Date
+
+2026-09-29
+
+## Context
+
+Configuration-backed defaults provide lower-priority settings that applications
+can override. These sources carry imports, locations, and relative-resource
+contexts. Integrating them into ConfigKit's loading lifecycle gives their values
+the same presence tracking and inspectable provenance as other configuration
+inputs.
+
+The [architecture goals](../policy/architecture.md#success-criteria) prioritize
+one coherent, predictable configuration model with consistent rules across
+source representations and loading interfaces. Consumers should load once,
+receive a complete validated configuration, and inspect its final values' origins.
+Reducing integration code is important subject to those principles.
+
+## Decision
+
+### Add underlays to the existing precedence model
+
+Introduce explicitly supplied source-backed underlays between typed defaults
+and main configuration. The proposed precedence, from lowest to highest, is:
+
+**Typed application defaults < source underlays < strictly composed main
+configuration < selected overlay < explicit CLI overrides.**
+
+Typed defaults remain the Go-value baseline. Underlays supply lower-priority
+configuration with source provenance and, where applicable, imports and resource
+context. This ordering describes value precedence; typed defaults fill fields
+absent from the assembled input under the existing defaulting rules.
+
+Preserve strict composition of the main document and its imports before applying
+the resulting configuration over underlays. Independent contributions within an
+underlay tier also compose strictly. Explicitly ordered tiers express intentional
+precedence. Strict composition reports conflicting contributions within each tier.
+
+Across precedence tiers, reuse the existing recursive mapping-overlay semantics.
+Omitted children retain lower-priority input; existing scalar, sequence, null,
+and cross-kind replacement semantics remain unchanged. Reusing these operations
+keeps the combination rules consistent across the loading model.
+
+### Keep source representation separate from composition semantics
+
+Native `fs.FS` and programmatic inputs join the shared loading model.
+Equivalent inputs using the same
+operation have the same presence and precedence semantics regardless of medium.
+Programmatic values enter directly as labeled configuration sources.
+
+Retain the contributing source's identity and resource context through layering.
+A retained underlay value keeps its originating source's resource context.
+Consumers remain responsible for opening and interpreting their domain resources
+under the supplied filesystem's lifetime and access semantics. Exact
+context-access and input APIs remain roadmap decisions.
+
+### Deliver one final configuration and its inspection metadata
+
+Assemble effective input, then run the typed loading lifecycle once for the load.
+Preserve explicit zero,
+false, empty, and null input versus omission through configuration and metadata
+together.
+
+Extend existing provenance and history through underlays, defaults, and recorded
+application resolution. The returned result includes the completed combination,
+defaulting, and provenance work. Domain resolution and validation
+remain application-owned within the recorded lifecycle, consistent with
+[ADR 0003](0003-preserve-presence-and-resolution-history.md).
+
+Preserve the existing opt-in schema-version ownership, path protections, and
+check timing before precedence
+assembly and typed decoding. The roadmap owns how layered input identifies that
+owner when version checking is configured.
+
+Favor near-linear processing in input, retained history, and output size through
+incremental assembly of load-owned state. This is an engineering objective
+subordinate to clarity and semantic consistency. Preserve the root-owned facade and
+private implementation boundary established by [ADR 0006](0006-public-facade.md).
+
+## Alternatives considered
+
+- **Leave bundled defaults to consumers.** Avoids extending loading, but makes
+  consumers reproduce merging, presence tracking, and provenance reconciliation.
+  It does not meet the goal of obtaining a complete configuration in one load.
+- **Offer recursive or atomic combination per layer.** Makes both behaviors
+  explicit and could serve more consumers, but expands the semantic surface and
+  its interactions with presence and provenance without a demonstrated need that
+  outweighs a single consistent rule. A future concrete requirement may justify
+  reconsideration; this feature does not add the option.
+- **Flatten all documents into last-wins layers.** Simplifies ordering but turns
+  accidental main/import conflicts into overrides, weakening the intentional
+  distinction established by ADR 0001.
+
+## Consequences
+
+Consumers gain source-backed defaults within the shared loading lifecycle.
+Strict contributions and intentional precedence remain distinct, with consistent
+combination rules across source media. Presence and provenance remain
+part of the same loading lifecycle as the final values.
+
+Higher-priority mappings retain omitted lower-priority children, so application
+validation assesses the final combined configuration.
+
+Implementation must integrate source context and underlay history into the shared
+engine with attention to assembly and history-retention costs. Current behavior
+documentation will change when the feature lands. The roadmap retains ownership
+of the remaining API decisions.

docs/adr/0008-complete-on-return-loading.md

@@ -0,0 +1,82 @@
+# ADR 0008: Synchronous, Complete-on-Return Configuration Loading
+
+## Status
+
+Proposed draft for review. The
+[configuration contract](../config.md#completion-and-resource-lifetime) describes
+the implemented loading lifecycle and
+[source resource contexts](../config.md#source-resource-contexts).
+
+## Date
+
+2026-09-29
+
+## Context
+
+Applications need a clear point at which configuration is ready for use.
+Completing configuration processing and inspection metadata together gives
+applications a coherent result whose values and origins agree. A bounded loading
+lifecycle also makes resource ownership and startup failures straightforward to
+reason about.
+
+ConfigKit's [architecture goals](../policy/architecture.md#success-criteria)
+emphasize complete configuration, inspectable provenance, predictable behavior,
+and minimal consumer bookkeeping. Applications own their execution lifecycle and
+the timing of configuration acquisition.
+
+## Decision
+
+Make loading a synchronous, complete-on-return operation. Within each load,
+ConfigKit completes input assembly, defaulting, typed decoding, configured
+application resolution, secret lookup, validation, and snapshotting before
+returning the final configuration and metadata. Reported failures return
+diagnostics and a nil result.
+
+Release all library-owned loading handles before returning on success or failure.
+The resulting configuration and metadata are independently usable, caller-owned
+data with a retained inspection snapshot. Caller-supplied readers and filesystems
+remain under caller ownership and lifetime control.
+
+After the call completes, ConfigKit remains inactive until explicitly invoked
+again. Applications normally load once at or near startup and may initiate
+additional loads when needed. Each load follows the same lifecycle and source
+reuse contracts. Inspection, rendering, and separately requested operations run
+in response to explicit application calls.
+
+For the planned resource-context capability, retain source context as passive
+information or a borrowed caller-supplied filesystem. Subsequent resource access
+acquires its own handles with explicit ownership. This preserves a clear boundary
+between configuration completion and application resource use.
+
+These choices give consumers one completion boundary for configuration and
+provenance, predictable resource lifetimes, and control over when ConfigKit runs.
+Application callbacks remain responsible for their own effects and resources.
+
+## Alternatives considered
+
+- **Background watching or refresh.** Can automate responses to source changes,
+  but adds scheduling, shutdown, publication, and resource-lifetime concerns to
+  the library. Explicit calls keep those responsibilities with the application.
+- **Lazy completion after return.** Can defer work until particular values are
+  used, but moves configuration failures into application execution and makes
+  the readiness of values and metadata harder to establish.
+- **Transactional atomic loading.** Could imply simultaneous observation of all
+  external inputs and rollback of callback effects. Filesystems, environment
+  variables, and application callbacks do not share a transaction mechanism.
+  Synchronous completion provides the required usable-result boundary without
+  promising those external guarantees.
+- **Retain loading handles in results.** Can preserve open filesystem access,
+  but gives configuration results an ongoing resource lifecycle and cleanup
+  obligation. Passive source context supports later explicit resource access
+  while keeping loading resources scoped to the load call.
+
+## Consequences
+
+Applications can treat successful return as the point at which the final
+configuration and its metadata are ready for use. Startup loading remains simple,
+and additional loads have an explicit application-owned trigger.
+
+Configuration inspection operates on retained data. Resource access observes the
+backing resource when explicitly requested and has its own failure and ownership
+boundary. Implementations of new source kinds and layering features preserve the
+same completion boundary, including cleanup on reported load failures.

docs/adr/0009-sibling-inheritance.md

@@ -0,0 +1,76 @@
+# ADR 0009: Resolve Sibling Inheritance After Source Assembly
+
+## Status
+
+Accepted and implemented. The [configuration contract](../config.md#sibling-inheritance)
+owns current behavior. The completed implementation plan is preserved in Git
+history at `83c59ea`.
+
+## Date
+
+2026-09-29
+
+## Context
+
+Named configuration mappings often share settings while retaining individual
+overrides. ConfigKit already distinguishes strict composition of independent
+source contributions from recursive overlay between explicit precedence tiers.
+Doing another untracked merge in applications would lose omission, source
+context, and historical explanation information. A shared structural operation
+must preserve the same presence and provenance model as the rest of loading.
+
+Inheritance also interacts with source precedence, nested namespaces, version
+ownership, and secrets. Resolving it before source assembly would validate
+superseded references and copy values that a later tier should replace. Fully
+resolving nested relationships inside a reusable parent would bind them to the
+wrong sibling scope when copied into a child.
+
+## Decision
+
+Reserve `extends` as a built-in input key on mappings. A string names one exact
+sibling mapping; null cancels the relationship. Complete source acquisition,
+strict composition, selected overlay, and CLI precedence first. Then resolve
+each effective relationship in the parser-independent document tree, before
+typed defaults and decoding. A child recursively overlays its own payload on
+its resolved parent using the existing mapping-combination rules. This creates
+neither a source tier nor a configurable merge strategy.
+
+Resolve enclosing sibling chains before nested relationships. Reuse memoized
+parent templates with unresolved nested instructions; evaluate those instructions
+at each destination. Preserve original source applications separately from
+structural transfer/control evidence. Effective origins carry concrete
+source-to-destination derivation, and inspection projects relevant inheritance
+events between source replay and typed reconciliation. Inherited input counts as
+explicit presence, while direct replacement retains its own effective origin.
+
+The existing main-input version check remains at its original point. Reject
+structural contributions or directive consumption affecting its protected path.
+Classify sensitivity at corresponding source and destination paths and retain
+historical redaction. Secret references remain unresolved input until the
+existing lookup point. Keep all machinery behind the root-owned public facade.
+
+## Alternatives considered
+
+- **Application-owned inheritance after `Load`.** Leaves every consumer to
+  recreate ordering, presence, provenance, error reporting, and redaction after
+  ConfigKit has already declared a result complete.
+- **Resolve each source before precedence assembly.** Makes superseded
+  references active and can copy a parent definition before higher-priority
+  input finishes changing it.
+- **Resolve nested relationships in a cached parent subtree.** Shares more
+  ready-made output but binds nested sibling lookups to the parent's scope,
+  giving incorrect values at a child destination.
+- **Registered collections, configurable keywords, or multiple-parent merge
+  modes.** Increase public setup and semantic branches without a requirement
+  that warrants diverging from one structural rule.
+
+## Consequences
+
+Consumers can load a configuration once and inspect direct, inherited,
+defaulted, and application-derived values through the existing result surfaces.
+`extends` is no longer an ordinary application field, so existing configurations
+using that key as data must migrate. Expanding many children can materialize
+substantially more output than the input contains; projection and retained-history
+budgets bound that work without charging copies as newly acquired sources.
+Applications still own domain validation, entry selection, and runtime resource
+use.

docs/adr/0010-effective-configuration-diff.md

@@ -0,0 +1,99 @@
+# ADR 0010: Compare Frozen Effective Configuration Snapshots
+
+## Status
+
+Accepted and implemented. The
+[configuration contract](../config.md#effective-configuration-diffs) owns current
+behavior. Completed planning and remediation records are preserved in Git
+history at `1c2610c` under `docs/roadmap/`.
+
+## Date
+
+2026-10-09
+
+## Context
+
+Applications need to inspect how two completed configurations differ, including
+the effective impact of an overlay after defaults and application resolution.
+ConfigKit already retains canonical typed inspection snapshots, semantic paths,
+presence information, and sensitivity classification for successful loads.
+
+Extending this inspection model lets applications share configuration comparison
+and safe disclosure while keeping their presentation and domain interpretation.
+The operation should follow ConfigKit's existing equality, ownership, and secret
+boundaries so consumers can reason about loading and inspection consistently.
+
+## Decision
+
+Provide structured effective-value comparison between successful results of the
+same configuration type. Compare their retained canonical inspection snapshots
+through the root-owned public facade. Each comparison uses the completed load
+state and returns detached records with a semantic path and safe left and right
+value snapshots.
+
+Use canonical value equality and the existing structural comparison rules.
+Mappings compare recursively, sequences report atomic changes, and scalar or
+kind replacements report the affected region. Presence distinguishes additions
+and removals from explicit null. Equal effective values produce an empty diff;
+source attribution, explicit-input status, and history remain separately
+inspectable through each result. Canonical pointer ordering makes reporting
+deterministic.
+
+Detect changes before masking values. Apply sensitivity from both loads to both
+returned sides, including historical and dependency-derived classification.
+Preserve public content where the shapes permit corresponding sensitive regions
+to be identified. When a structural replacement prevents safe correspondence,
+mask the complete reported replacement region on both present sides. Absence
+retains its own representation.
+
+Keep masking and hidden-change information distinct. Each record indicates
+whether an actual change is withheld by redaction, allowing applications to
+display a changed-sensitive-value indicator even when safe snapshots render
+identically. Public-only changes alongside unchanged masked content retain that
+distinction.
+
+Typed secret references participate through their configured environment names.
+Resolved credential bytes remain behind the existing explicit secret accessor.
+The comparison uses the same frozen inspection boundary as effective rendering
+and explanation.
+
+ConfigKit owns comparison and paired safe disclosure. Applications own labels,
+layout, and rendering of the returned changed regions. Reusing the existing
+snapshot representation and engine boundary keeps the extension consistent with
+the library's established inspection model.
+
+## Alternatives considered
+
+- **Compare input text or separately rendered YAML.** Supports familiar textual
+  displays, but input text precedes composition and resolution, and independently
+  redacted output can hide changes or apply different disclosure rules. Canonical
+  comparison with paired masking represents the completed configuration.
+- **Leave comparison to applications using returned Go values.** Offers direct
+  access to application types, but duplicates structural and redaction machinery
+  and observes caller mutation. Retained inspection snapshots preserve the
+  completed load's values and established ownership boundary.
+- **Align sequence elements and return context-rich rendering data.** Enables
+  finer-grained displays, but requires additional correspondence and disclosure
+  rules. Atomic sequence records and existing value snapshots satisfy the
+  selected changed-region workflows with a smaller public model.
+- **Compare resolved credentials or retain credential-derived tokens.** Could
+  detect rotation behind an unchanged reference, but expands retained sensitive
+  state and introduces a separate trusted comparison mechanism. Reference-name
+  comparison preserves the existing inspection boundary.
+
+## Consequences
+
+Applications can compare independently loaded configurations using ConfigKit's
+shared equality, presence, and disclosure rules. Existing composition and load
+semantics continue to determine the values being compared. Caller mutation of a
+returned configuration leaves its retained comparison snapshot unchanged.
+
+Paired redaction can conceal public content within a structurally incompatible
+replacement. The changed region and hidden-change indicator still communicate
+its effective impact. Credential rotation behind an unchanged environment name
+produces equal reference values for comparison.
+
+The implementation requires operation-local disclosure preparation and focused
+coverage for presence, shape changes, historical sensitivity, and detached
+results. Applications retain control of presentation; changed-region records
+supply the safe data needed for that responsibility.

docs/adr/0011-input-destinations.md

@@ -0,0 +1,70 @@
+# ADR 0011: Place Configuration Inputs at Mapping Destinations
+
+## Status
+
+Accepted. The [input-destinations roadmap](../roadmap/input-destinations.md)
+owns implementation status and the [implementation plan](../roadmap/implementation.md)
+owns sequencing and completion evidence.
+
+## Date
+
+2026-10-14
+
+## Context
+
+Consumers assembling collections of independently authored configuration
+fragments need each source to contribute beneath a selected application path.
+Requiring consumers to rewrite source documents or merge completed values would
+separate placement from ConfigKit's existing composition, precedence, source
+identity, and inspection behavior. Placement should remain part of the ordinary
+loading lifecycle and preserve the root-owned public facade.
+
+## Decision
+
+Add an optional `Destination Path` to root-owned `Input` and expose the effective
+destination on existing `SourceInfo` records. The empty path means the existing
+application root. A non-empty destination is a sequence of literal mapping keys;
+explicit sequence-index segments are invalid. Existing reserved structural keys
+remain reserved, while keys that merely contain their spelling remain ordinary
+keys.
+
+Acquire and interpret each source using its original physical identity and
+source-local directives, then place its application body before strict
+composition and the existing final lifecycle. Imported application bodies
+inherit the input destination. Placement applies to main and underlay inputs;
+ordinary root loading, selected overlays, and CLI inputs retain root placement.
+The main-owned version rule continues to check its effective application path
+at its existing lifecycle point.
+
+Placement preserves authored node locations and source identity. Introduced
+mapping structure belongs to the contributing source and uses that document's
+root location as its source-coordinate fallback. A present empty mapping
+materializes its destination; an absent `NoSource` contributes nothing. The
+result continues through the existing strict composition, overlay, inheritance,
+typed loading, diagnostics, provenance, history, sensitivity, and resource
+context mechanisms.
+
+The added exported fields preserve keyed and zero-value struct use. Callers
+using unkeyed composite literals for `Input` or `SourceInfo` must migrate to
+keyed literals. Treat this feature as minor-release work; select the version and
+document the compatibility impact during separately authorized release
+preparation, consistent with the release policy.
+
+## Alternatives considered
+
+- **Rewrite or merge sources in consumers.** This makes each consumer reproduce
+  placement, strict composition, source attribution, and inspection semantics.
+- **Add a separate loading entry point or public wrapper type.** This creates a
+  parallel API for a behavior that belongs to existing per-input loading.
+- **Load each fragment as a complete application before placement.** This runs
+  defaults, decoding, resolution, and validation on incomplete fragments rather
+  than once on the assembled application.
+
+## Consequences
+
+Consumers can assemble contributions at effective mapping paths in one ordinary
+layered load and inspect each source's placement through existing source
+records. Paths, conflicts, and later application operations use the placed
+structure while source coordinates and relative-resource contexts continue to
+identify their original sources. Public field additions require release-note
+guidance for callers using unkeyed literals.

docs/config.md

@@ -12,20 +12,30 @@ 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. |
+| `NewFilesystem(label, filesystem).File(name)` | Borrows a caller-owned `fs.FS` under a safe nonempty display label. `name` must be a non-root slash-relative `fs.ValidPath`. Each load reopens the selected regular file; copies of the `Filesystem` preserve namespace identity. Separate constructors create separate namespaces even for the same label or filesystem. |
 | `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. |
+| `Mapping(label, values)` | Requires a safe nonempty label and a non-nil root `map[string]any`. Snapshots supported built-in values into an owned mapping document at construction; `{}` is valid. Constructor conversion failures become safe deferred diagnostics. |
 | `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.
+File and filesystem sources can observe changes between consumptions; byte and
+mapping sources cannot. Mapping accepts nil, built-in string, boolean, signed or
+unsigned integer, finite float, `[]any`, and `map[string]any` values recursively.
+Typed nil maps and slices represent explicit null; empty collections remain empty.
+Keys are sorted for stable provenance, integer decimal spelling stays exact, and
+cycles or unsupported types are rejected without exposing values. Mapping input
+uses the same directive and version rules as YAML, not CLI override restrictions.
 
-Each source is limited to 8 MiB. A load using imports or an active profile is limited
+Each source is limited to 8 MiB. A load using imports or an active overlay 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.
+For mapping sources, key and string bytes use the same accounting as programmatic
+overrides, and the converted tree uses the same node and depth bounds.
 
 ## Single-source loading
 
@@ -33,7 +43,7 @@ truncated. Data returned together with a read error is not accepted as partial i
 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
+schema-version checking, and selected-overlay 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
@@ -43,34 +53,217 @@ 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
+  → build schema → check the optional application version → apply a selected overlay
+  → apply explicit CLI overrides → expand sibling inheritance
+  → 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
+`File`, `Filesystem.File`, `Bytes`, `Reader`, and `Mapping` 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
+semantic validator, explicit path and glob imports, one selected overlay, and
 filesystem context. It also accepts explicit CLI override records, static secret
-tags, and a dynamic path predicate. `NoSource` rejects imports, active profiles,
+tags, and a dynamic path predicate. `NoSource` rejects imports, active overlays,
 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
+`Metadata`, a defensive source list, and defensive nonfatal diagnostics. Each source
+record's `Kind` identifies file, filesystem, bytes, reader, mapping, or CLI acquisition
+independently of its precedence `Layer`. Host file paths remain canonical;
+filesystem paths are slash-relative within their namespace; synthetic inputs have
+no path. 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.
 
+### Completion and resource lifetime
+
+`Load` is synchronous and complete on return. Success means input assembly,
+defaulting, typed decoding, configured application resolution, secret lookup,
+validation, and configuration/metadata snapshotting have completed. A reported
+load failure returns diagnostics and a nil result, as described above.
+
+Before returning on success or failure, ConfigKit closes the filesystem handles
+it opened for loading, including files and directory/root handles. Caller-owned
+filesystems are borrowed, never closed. The returned
+configuration and inspection metadata require no close operation and remain
+usable independently of the source files. A supplied `Reader` remains
+caller-owned and is never closed by ConfigKit.
+
+ConfigKit performs no background work after returning: it does not watch inputs,
+refresh secrets, or mutate published results. Inspection and rendering run only
+when explicitly called and use the retained snapshot. Applications normally load
+once near startup, but may explicitly load again using sources that satisfy their
+documented reuse rules. Additional loads produce independently owned results.
+
+Completion is not a transaction over external state. Files and environment
+variables are observed during processing rather than as one simultaneous external
+snapshot. ConfigKit does not roll back application callback effects or manage work
+or resources created by application callbacks. The explicit secrets-directory
+bootstrap remains a separate operation invoked by the application.
+
+## Layered loading
+
+`LoadLayered[T](main, options...)` accepts zero or one `LayeredOptions[T]` value.
+Its `main` is an `Input` containing a source, import opt-in, and optional
+filesystem context. A zero source is invalid; `Input{Source: NoSource()}` omits
+the main document. Each `Underlay` contains inputs that compose strictly as
+one tier, including each input's enabled imports. Empty groups contribute
+nothing. Underlay groups are listed from lowest to highest priority. Repeating
+a file within one group remains an import/repetition error; the same reusable
+source may be selected again in another tier.
+
+ConfigKit acquires and composes the main input first, constructs the schema,
+and checks the optional application version from the main tier only. Underlay
+roots cannot claim that version path (`underlay.version`); imported fragments
+retain `import.version` protection. It then acquires and composes underlay
+groups in order, recursively combines the groups, and applies the main tier,
+one selected overlay, and explicit CLI overrides. Scalar, sequence, and null
+definitions replace lower values between tiers; mappings combine recursively.
+Syntax and strict-composition errors inside a tier cannot be masked by a
+higher-priority tier. Defaults, typed decoding, resolution, secret lookup,
+validation, and publication run once on the final assembled input.
+
+An active layered overlay requires `LayeredOptions.OverlayFilesystem` explicitly,
+independent of the main source's namespace. Its name and root-relative lookup
+follow the selected-overlay rules below; inactive overlays ignore the context.
+The same path in a different filesystem namespace is not a repeated file.
+Ordinary `Load` continues to infer its selected-overlay context from its source.
+
+Source IDs follow acquisition order—main and its depth-first imports, then each
+underlay tier's inputs and depth-first imports, then overlay and CLI—even though
+history follows precedence order. Source `Layer` describes precedence while
+`Kind` describes acquisition. Underlay records use `LayerUnderlay` and a
+one-based `UnderlayIndex` giving their original tier position; all other records
+have index zero. Explicit empty groups and `NoSource` consume no source ID.
+Byte, source, node, and glob-entry budgets are shared across the load, while
+the existing per-source and import-depth bounds still apply.
+
+#### Input destinations
+
+`Input.Destination` places the source's application mapping beneath a path
+relative to the configuration root. Its zero or empty value means root, so
+existing inputs keep their behavior. Main inputs and underlay inputs use the
+same destination model. Destination segments are literal mapping keys: dots,
+slashes, and numeric-looking text are not split or interpreted as sequence
+indexes. Explicit index segments and paths that introduce the reserved
+`configkit` or mapping-valued `extends` structure are invalid input options and
+produce a structured `LoadError` before source acquisition.
+
+Source-root `configkit.imports` directives are processed before placement, and
+each imported application contribution inherits its input's destination,
+including transitive imports. A valid destination on `NoSource()` contributes
+nothing; an actual empty mapping materializes its destination containers. Once
+placed, contributions follow the same strict composition within a tier and
+recursive precedence behavior described above. A selected overlay and CLI
+overrides remain rooted at the application root and are not moved beneath the
+main input's destination.
+
+`SourceInfo.Destination` reports the placement for each acquired input source,
+including imports. Placement does not change source IDs, acquisition identity,
+physical source locations, or passive resource contexts. Introduced ancestor
+mappings use the contributing document's root location as a source fallback;
+authored content retains its original source coordinates. Returned source
+records are detached copies.
+
+The maintained [layered consumer example](consumers/README.md#native-filesystem-and-layered-input)
+shows a main host file, two underlay tiers, embedded defaults, a mapping source,
+and explicit resource access without a second configuration pass.
+
+### Sibling inheritance
+
+Any input mapping may declare `extends: name` to inherit from the mapping named
+`name` among its immediate siblings. Names are exact keys, including the empty
+string; lookup does not search ancestor or unrelated scopes. The document root
+and a sequence-element mapping have no sibling namespace for their own string
+relationship, though mappings nested inside them can have one. An `extends`
+value of null cancels a relationship without looking up a parent. Other value
+types, missing or non-mapping parents, and cycles fail the load with value-free
+diagnostics.
+
+`extends` is reserved structural input across ordinary and layered loads, every
+source kind, imports, selected overlays, and explicit CLI overrides. It is
+consumed before typed decoding and need not appear in the application's Go
+types. Applications previously using `extends` as an ordinary YAML field must
+rename that key and its field. No collection registration or keyword setup is
+required.
+
+ConfigKit completes strict composition within tiers and precedence assembly
+between tiers before resolving relationships. An omitted `extends` retains a
+lower-priority relationship; `extends: null` cancels it while preserving the
+child's own settings. Superseded references are historical evidence, not active
+relationships to validate. Each child recursively overlays its own settings
+over its resolved parent: explicit zero, false, empty string, sequence, and
+null values replace inherited values, while mappings combine recursively.
+The child retains its key. All effective relationships resolve, including
+entries the application may never select for execution. Partial parent
+definitions undergo normal typed decoding and application-owned validation,
+not a special structural exemption.
+
+Enclosing relationships resolve first. Nested instructions carried by a parent
+are then resolved separately at each destination, so a child's nested sibling
+override can change an inherited nested relationship. Typed defaults and
+application-derived values do not start another inheritance pass. Inherited
+input counts as explicit presence for `Metadata.IsExplicit`,
+`Resolution.IsExplicit`, and absence-based resolver defaults, even for false,
+zero, empty, or null input. `Origin.Inheritance` distinguishes it from a direct
+declaration with concrete source, destination, and controlling directive paths.
+A direct child replacement has its own effective origin; displaced ancestry
+remains inspectable in history. An inherited relative resource keeps the
+declaring source's passive `SourceContext`, not the child's directory.
+
+The optional schema-version checker still runs once against the main input at
+its original point. Inheritance cannot add, replace, or consume the protected
+version path; an unrelated relationship inside an enclosing mapping does not
+invalidate an unchanged version. Expansion uses the decoded projection
+node/depth limits without charging copied values as acquired sources;
+retained evidence uses the existing history-node limit. See the
+[consumer example](consumers/README.md#named-sibling-inheritance) for one-load
+usage.
+
+### Source resource contexts
+
+`Result.SourceContext(id)` and callback-scoped `Resolution.SourceContext(id)`
+return a passive `ResourceContext` for a source with filesystem context, or
+false for unknown and contextless IDs. The result query is nil-safe; the
+resolution query follows the callback's lifetime guard. Queries perform no
+filesystem access. Obtain IDs from exact origin locations rather than guessing
+one context for a composite origin or an application-derived value.
+
+`ResourceContext.FS` is rooted at the source's configured root, and `BaseDir`
+is its slash-relative directory inside that root. This includes imported
+sources, selected overlays, host files even without enabled imports, native
+files, and byte, reader, or mapping inputs with an explicit context. CLI and
+contextless synthetic inputs have no resource context. To open a relative
+resource, join `BaseDir` with the application resource name using slash-path
+operations and open that path through `FS`.
+
+For a resource-valued field, query `result.Metadata().Lookup(fieldPath)`, inspect
+its origin locations, choose a source ID only when the application's policy can
+identify the intended contributor, then call `result.SourceContext(id)`. The
+context belongs to that source even when the field survived from a lower-priority
+underlay. A composite origin has no automatically selected resource base.
+
+Native contexts borrow the caller's filesystem within the configured root.
+Host contexts retain a canonical root path, not an open root handle; each
+resource `Open` checks containment and opens a fresh handle that the returned
+file's `Close` releases. Applications own and close the files they open.
+Resources are application data, not configuration input, and do not consume
+configuration byte or node budgets. The loaded configuration and metadata stay
+usable if source files later change or disappear. Later resource operations
+observe the backing filesystem then and may fail if files move, disappear, or
+a caller-owned filesystem closes. This is not a snapshot or universal sandbox.
+
 ### Application schema versions
 
 `VersionRule` declares a semantic path and non-nil checker. Before defaults or typed
@@ -93,7 +286,7 @@ 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
+the normal decoding reconciliation event. Fragment, overlay, and CLI ownership
 protections apply before this conversion.
 
 The checker reports safe, value-free errors and warnings through `Validation.Error`
@@ -108,12 +301,13 @@ successful result. Callback panics remain programming failures.
 `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.
+existed is not historical merely because an atomic ancestor was replaced. Exact
+surviving direct or inherited input paths are explicit and retain their source
+layer and derivation; 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
+default baseline followed by exact base or overlay definitions, including
+equal-valued input. Overlay 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.
 
@@ -124,22 +318,32 @@ 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
+History is chronological: a path can have a synthetic default record, its exact
+base definition, an overlay replacement, structural inheritance, 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
+whose locations retain source traversal order. A mapping also visited by an overlay
+has an overlay-layer composite origin with the lower contributing locations followed
+by the overlay 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.
+selected overlay, then synthetic CLI records in supplied order.
+
+Inheritance adds no acquired source or precedence layer. `Origin.Inheritance`
+lists concrete transfers from contributor paths through destinations, with
+each step's controlling `extends` path and location. `History` retains source
+applications at their original paths and projects parent contributions, child
+replacements, and consumed controls at destination paths after source assembly
+and before typed decoding. `HistoricalPaths` includes consumed directives and
+displaced inherited descendants. A control can have history even when `Lookup`
+reports it absent. Queries return detached records.
 
 ### Secret classification and effective rendering
 
@@ -156,6 +360,11 @@ Propagation reaches a fixed point, so record/map iteration order does not affect
 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.
+Inherited value and control paths link source and destination classifications
+through chains and nested copies; unrelated sibling fields are not classified
+merely because another field is secret. Displaced historical evidence remains
+redacted. Secret references are copied only as unresolved names; credential
+lookup stays after decoding and application resolution.
 
 `Result.WriteEffective` writes the frozen resolved snapshot as deterministic redacted
 YAML. Mappings sort by decoded key string, sequences retain order, indentation is two
@@ -189,8 +398,11 @@ historical path. Its effective value and each chronological before/after value a
 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
+traversal order, the selected overlay, supplied CLI records, and audited net
+resolution operations. Structural inheritance events appear after source replay
+and before typed reconciliation, including consumed or canceled `extends` input
+and equal-valued child replacements. 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
@@ -199,7 +411,7 @@ 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,
+Base, overlay, 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
@@ -238,6 +450,46 @@ an explicit `LoadSecretsDir` bootstrap, then use `WriteDiagnostics`,
 ConfigKit does not infer bootstrap outcomes from a secret reference and does not
 bundle these independent reports into a command framework.
 
+## Effective configuration diffs
+
+`configkit.Diff(left, right)` compares two completed successful loads of the same
+configuration type. `left` is the earlier state and `right` is the later state.
+Applications load both sides with the sources and options they choose; to inspect
+an overlay's impact, load the same base once without the overlay and once with it
+selected. The [consumer guide](consumers/README.md#effective-configuration-diffs)
+links to maintained examples. The public API returns safe changed-region records;
+applications own their presentation.
+
+Comparison uses the frozen canonical effective inspection values, not mutable
+`Result.Config`, source text, provenance, or history. It does not reopen sources,
+reread environment variables, or rerun defaults or callbacks. Equal effective
+values produce no records even when sources, explicit-input status, defaults, or
+resolution histories differ; for example, explicit zero and defaulted zero are
+equal. Mappings compare recursively, sequences compare atomically, and scalar,
+kind, or null/container replacements report their affected region. Records use
+the existing comparator's canonical pointer ordering. `Present=false` with empty
+YAML means absence; explicit null is present and renders as `null`.
+
+Before either side is returned, sensitivity from both loads is applied to both
+snapshots, including historical and dependency-derived classification. A
+classification on either side protects the corresponding values on both sides.
+`Redacted` reports masking, while `HiddenChanged` reports an actual concealed
+value or presence change; it is not implied merely by redaction. A structural
+replacement that prevents safe correspondence masks the complete reported
+region, and an absent side remains absent. Record membership establishes that a
+change occurred even when safe snapshots render identically. Literal
+marker-looking input is ordinary input; consumers use the record fields rather
+than interpreting marker text.
+
+Typed secret references contribute their configured environment names to the
+frozen comparison, never resolved credential bytes. Credential rotation behind
+an unchanged reference is therefore not detected. The records contain changed
+regions only, not paired full-document context; independently calling
+`WriteEffective` on each result does not create cross-protected context. Nil or
+uninitialized results return an error with no records. A failed `Load` retains
+its normal nil-result/`LoadError` behavior, which callers handle before invoking
+`Diff`.
+
 ## Typed secret references
 
 Credential-bearing fields use the opaque `Secret` type and store an environment
@@ -338,7 +590,7 @@ found entry path. Resolution changes, additions, removals, and moves do not alte
 this entry view.
 
 `Resolution.Source(id)` returns a detached record for an already acquired source in
-the same load: the main source, imports, selected profile, and supplied CLI records
+the same load: the main source, imports, selected overlay, and supplied CLI records
 where applicable. Empty and unknown IDs return false. It never reloads input, reads
 the filesystem, resolves symlinks, or looks up credentials. A source path exists
 only when the acquired source has one; defaults and byte, reader, and CLI sources do
@@ -436,41 +688,41 @@ traversal order. Traversal order never authorizes replacement. Composition clone
 the resulting tree and its ordered claim records rather than mutating or aliasing an
 input document.
 
-## Selected profiles
+## Selected overlays
 
-`Options.Profile` selects at most one overlay by name. Names must match
+`Options.Overlay` 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.
+`default` select no overlay, and `overlays/default.yml` is never loaded implicitly.
+Every other name resolves only to `<configuration-root>/overlays/<name>.yml`, with
+no extension fallback, case normalization, inheritance between overlay files,
+or overlay stack. A selected missing overlay is an error.
 
-The profile is a separately policy-validated YAML mapping. It is loaded after the
+The overlay 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
+and typed decoding. Overlay 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 |
+| Existing lower node | Incoming overlay node | Result |
 | --- | --- | --- |
-| Absent | Value | Retain the profile value |
+| Absent | Value | Retain the overlay 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
+Scalars, sequences, and null are atomic. Thus an overlay 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
+replacement. Surviving overlay paths have overlay origins, while recursively shared
 mapping containers retain their contributing locations.
 
-A profile may not contain `configkit` anywhere, contribute the configured schema
+An overlay 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
+apply even if the supplied version text would be unchanged. Overlays cannot mask a
 base parse, composition, or version failure because those checks run first.
 
 ## Explicit CLI overrides
@@ -483,8 +735,8 @@ 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 <
+Overrides apply after the selected overlay and before typed defaults/decoding, so
+the input precedence is application defaults < composed base < selected overlay <
 CLI. Recorded resolution, typed secret lookup, and semantic validation follow the
 effective override tree. Overrides also work with `NoSource`.
 
@@ -506,7 +758,7 @@ configured schema-version path. Unknown fields and effective type mismatches rem
 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
+source after file/overlay 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
@@ -514,7 +766,7 @@ 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
+Override origins are explicit CLI origins. History retains replaced base/overlay
 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
@@ -557,18 +809,25 @@ walk failures, matched directories, invalid documents, escapes, and repeated tar
 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;
+For host files, the configuration root defaults to the selected file's canonical
+directory. A host file may supply `FilesystemContext.Root` to widen that root but
+may not supply `BaseDir` or a native `FS`. Byte, reader, and mapping inputs with host context
+must provide both `BaseDir` and `Root` as absolute existing directories.
+For `Filesystem.File`, the default root is `.` in its namespace and the base is
+the selected file's directory. An explicit context must use the same `Filesystem`
+namespace and may set `Root` to contain the file and its base; it may not supply
+`BaseDir`. Byte, reader, and mapping inputs with native context provide both `BaseDir` and
+`Root` as slash-relative `fs.ValidPath` directories in `FS` (`.` means its root),
+with the base inside the root. Context paths are checked even when imports and
+overlays are inactive. Diagnostic labels, including URI-looking labels, never
+imply filesystem paths. 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.
+distinct paths remain distinct sources. Native filesystems use slash-path operations
+and their own symlink behavior; imports stay in their selected namespace and root.
 
 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

docs/consumers/README.md

@@ -1,13 +1,13 @@
 # Consumer Adoption Guide
 
 ConfigKit removes parser setup, strict decoding, presence bookkeeping, import and
-profile traversal, precedence, provenance, secret lookup, and safe inspection from
+selected-overlay 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:
+sanitized shapes using only ConfigKit and the Go standard library:
 
 | Shape | Demonstrated fit | Application code deliberately retained |
 | --- | --- | --- |
@@ -15,6 +15,11 @@ four sanitized shapes using only ConfigKit and the Go standard library:
 | 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 |
+| Bundled file | Simple native `fs.FS` source through ordinary `Load` | Selection and lifetime of the application-owned embedded filesystem |
+| Layered catalog | Embedded defaults with an import, a mapping underlay, an independent host main file, explicit false/zero input, and source-relative resource access | Domain validation and opening/interpreting the template resource |
+| Mounted collections | Literal input destinations, peer prompt/profile fragments, local inheritance, imported collection metadata, and a version-only higher-tier override | Collection IDs, profile meaning, and opening/interpreting prompt resources |
+| Named presets | One-load sibling inheritance, explicit false, nested destination resolution, and source/derivation inspection | Choosing and executing a preset |
+| Effective configuration diff | Same-type frozen effective comparisons for two selected files or base-versus-overlay impact | Source/options selection and consumer-owned safe presentation |
 
 The YAML under [`testdata`](../../examples/consumers/testdata) is loaded by the
 executable examples and ordinary tests. It contains no private application data,
@@ -34,7 +39,7 @@ features appear only when the application needs them:
 result, err := configkit.Load[Config](configkit.File(path), configkit.Options[Config]{
     Definition: rules,
     Imports:    useDeclaredImports,
-    Profile:    selectedConfigProfile,
+    Overlay:    selectedConfigOverlay,
     Overrides:  explicitlySetFlags,
 })
 if err != nil {
@@ -43,11 +48,98 @@ if err != nil {
 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;
+Simple consumers use neither imports nor selected overlays. A complex consumer can opt into
+both without redefining their syntax. ConfigKit overlays are configuration overlays;
 they do not select or replace an application's LLM, model, pipeline, report, or
 execution profile concepts.
 
+## Effective configuration diffs
+
+[`CompareFiles`](../../examples/consumers/diff.go) loads two caller-selected files
+with one application definition, handles each load error, calls `configkit.Diff`,
+and presents the safe records through consumer-owned `FormatDifferences`.
+[`CompareRolloutOverlay`](../../examples/consumers/diff.go) loads the same base
+once without an overlay and once with the selected overlay, so omitted lower
+priority values remain present. Its resolver records a derived endpoint URL after
+overlay application, and its nested typed secret reference is classified before
+the resulting records are presented.
+
+The maintained [behavior tests and executable examples](../../examples/consumers/diff_test.go)
+use temporary files and disposable secret values. Presenters should use
+`ValueSnapshot.Present` for absence, record membership for change, and
+`HiddenChanged` for a withheld change indicator. Do not parse the redaction marker
+or add separately rendered full contexts that lack combined sensitivity. The
+[configuration contract](../config.md#effective-configuration-diffs) owns shared
+equality, granularity, and disclosure semantics.
+
+## Named sibling inheritance
+
+[`LoadPresets`](../../examples/consumers/inheritance.go) loads the maintained
+[`presets.yml`](../../examples/consumers/testdata/inheritance/presets.yml) in one
+call. `custom` inherits `enabled: false` from `base`, while its nested
+`settings.primary` changes the inherited `settings.secondary` result at the
+custom destination. The application struct has no `extends` field; ConfigKit
+consumes the relationship before strict typed decoding.
+
+The [executable example](../../examples/consumers/consumers_test.go) reads
+`Metadata.IsExplicit` for the inherited false value, then follows
+`Origin.Inheritance` back to the parent path while `Origin.Locations` identifies
+the declaring source. Applications still choose which preset to execute and
+validate domain meaning. The [configuration contract](../config.md#sibling-inheritance)
+owns exact lookup, precedence, cancellation, and migration rules.
+
+## Native filesystem and layered input
+
+[`LoadBundled`](../../examples/consumers/layered.go) keeps the simple `Load` shape:
+it selects a file in an application-owned embedded filesystem and needs no
+underlay setup. [`LoadCatalog`](../../examples/consumers/layered.go) uses
+`LoadLayered` with an imported embedded-defaults group, a separate
+`Mapping` group, and a host main file supplied by its caller. The groups
+compose strictly within themselves and recursively across precedence boundaries;
+the main file's explicit `false` and `0` replace lower values without being
+mistaken for omissions. Its validator owns allowed modes and resource-name policy.
+
+[`ReadWelcome`](../../examples/consumers/layered.go) follows the effective field's
+origin location to a source ID, queries `Result.SourceContext`, and opens the
+resource relative to that context's `BaseDir`. It does not assume the main file
+owns a retained underlay value. A composite origin needs an application choice;
+ConfigKit does not guess a directory. The context query is passive, while later
+resource reads observe the caller-owned filesystem or a freshly opened host root
+and may fail independently of the completed configuration load. The
+[configuration contract](../config.md#source-resource-contexts) owns exact
+lifetime and precedence rules.
+
+## Input destinations and mounted collections
+
+Main and underlay inputs use the same destination field; imported application
+contributions inherit the declaring input's destination. A destination is an
+application mapping path, independent of filesystem context. The configuration
+contract owns the complete behavior and validation rules.
+
+The small [`Example_inputDestination`](../../examples/consumers/destinations_test.go)
+mounts one mapping source beneath literal collection/prompt keys and loads a
+second input without a destination to show that root placement remains the
+default. Consumers pass paths, not filenames or namespace strings for ConfigKit
+to interpret.
+
+[`LoadCollectionBundle`](../../examples/consumers/destinations.go) assembles an
+embedded collection manifest with a source-root import, separate prompt and
+profile peers, and a higher-priority input that changes only one prompt's
+ordinary `version` field. The main document alone supplies the application
+schema version. A prompt's local `extends` relationship resolves after all
+tiers, so `custom` inherits the final `brief` content and fields while retaining
+its own profile selection.
+
+The resolver follows each effective `content_file` origin to its declaring
+source context. This also works for `custom`'s inherited resource. The
+application-owned [`ReadCollectionPromptContent`](../../examples/consumers/destinations.go)
+demonstrates the same lookup after loading; ConfigKit does not open or interpret
+the resource. [`destinations_test.go`](../../examples/consumers/destinations_test.go)
+checks source destinations and authored coordinates, presence, inherited values,
+relative resources, and strict peer conflicts. The
+[configuration contract](../config.md#input-destinations) owns shared placement,
+import, source-location, and ownership behavior.
+
 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-
@@ -100,6 +192,16 @@ The examples preserve domain meaning while adopting one configuration convention
   blocked empty variables remain blocked, and ordinary `Load` never changes the
   process environment.
 
+### Selected-overlay migration
+
+To adopt the current selected-overlay API, rename `Options.Profile`,
+`LayerProfile`, and `CodeProfile*` references to their `Overlay` equivalents; move
+selected files from `profiles/<name>.yml` to `overlays/<name>.yml`; update any
+handling of the `overlay` layer and `overlay.*` diagnostic strings; then rebuild.
+The [configuration contract](../config.md#selected-overlays) is the canonical
+behavioral reference. Application-owned profile concepts, such as `model_profile`,
+are unchanged, and any CLI flag rename remains the application's decision.
+
 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
@@ -113,9 +215,13 @@ 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 |
+| Source/import/selected-overlay 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` |
+| Native filesystem, grouped underlays, programmatic mapping, retained resource context | `Example_nativeFilesystem`, `Example_layeredCatalog`, and their focused consumer tests |
+| Literal input destinations, root-default compatibility, and mounted peer collections | `Example_inputDestination`, `LoadCollectionBundle`, and `destinations_test.go` |
+| Named sibling inheritance, presence, and derivation | `Example_inheritedPresets` and `TestPresetCatalogOneLoadAndDerivation` |
+| Effective configuration diffs and consumer-owned presentation | `ExampleDiff`, `ExampleCompareFiles`, and `ExampleCompareRolloutOverlay` |
 | 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

docs/development.md

@@ -8,20 +8,44 @@ public behavior.
 ## Initial orientation
 
 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.
+2. Use the Policy Reading Guide below to select applicable policies.
+3. Read the contract and internal documents applicable to the task.
 4. Inspect the relevant implementation and behavioral tests before changing them.
 
+## Policy Reading Guide
+
+| Policy | Read before… |
+| --- | --- |
+| [Policy set provenance](policy/meta.md) | Adopting policies, checking upstream revisions, or updating the adopted policy set. |
+| [Agents](policy/agents.md) | Recommending, planning, or implementing repository changes. |
+| [Architecture](policy/architecture.md) | Planning or changing Go library code or its structure. |
+| [Testing](policy/testing.md) | Selecting validation for code changes, or adding, changing, removing, or evaluating tests. |
+| [Documentation](policy/documentation.md) | Writing, revising, or reorganizing documentation. |
+| [OSS](policy/oss.md) | Adding or updating dependencies or incorporated third-party material, or changing licensing or attribution. |
+| [Release](policy/release.md) | Preparing or publishing a release, changing release procedures, or making versioning and compatibility decisions. |
+
+- Read applicable policies before making decisions they govern; multiple policies
+  may apply.
+- Select policies using the task triggers above. Follow cross-references to
+  relevant sections when needed to resolve a concrete question.
+- Reuse policies already available in context; reread when changed or no longer
+  available.
+- Expand reading when the task's scope expands.
+- When revising a policy itself, read that policy and any provisions elsewhere
+  affected by the revision.
+
 ## Task-specific reading guide
 
 | Task | Read |
 | --- | --- |
-| 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 |
+| Repository orientation or component responsibility | [Component overview](internal/overview.md) |
+| Input destinations | [Configuration contract](config.md#input-destinations), [ADR 0011](adr/0011-input-destinations.md), and the destination tests below |
+| Documentation changes | The owning document |
+| Licensing, dependencies, attribution, or release-document changes | [Project grant](../COPYRIGHT) and [attribution](oss/attribution.md); release changes also follow the release procedure |
+| Architecture decisions or future work | [Deferred scope](roadmap/deferred.md) and relevant ADRs listed below |
+| Code and test changes | 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) |
+| Release preparation | [Release procedure](release.md) |
 
 Accepted decisions:
 
@@ -31,6 +55,8 @@ Accepted decisions:
 - [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)
 - [Keep the public facade root-owned](adr/0006-public-facade.md)
+- [Compare frozen effective configuration snapshots](adr/0010-effective-configuration-diff.md)
+- [Place inputs at literal application mapping paths](adr/0011-input-destinations.md)
 
 For cross-cutting changes, follow every applicable row. Completed implementation
 and remediation plans are preserved in Git history at `cb5b435`; use current
@@ -41,7 +67,9 @@ git show cb5b435:docs/roadmap/remediation.md
 ```
 
 Release-candidate, publication, and verification work follows the operational
-[release procedure](release.md).
+[release procedure](release.md). Its module-selected ReleaseKit `publish`
+invocation validates the committed candidate and publishes only the intended
+tag; those release checks remain distinct from the everyday development check.
 
 The completed string-and-integer schema-version implementation and remediation
 records are preserved in Git history at `244104f`. The
@@ -56,9 +84,40 @@ and current facade ownership belongs to the [component overview](internal/overvi
 
 ## 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:
+The completed filesystem-source and underlay roadmap, original feature request,
+implementation plan, and resolved remediation record are preserved in Git history
+at `b604aa1` under `docs/roadmap/`. Current behavior belongs to the
+[configuration contract](config.md#layered-loading). Built-in sibling
+inheritance is now part of the [configuration contract](config.md#sibling-inheritance).
+The completed request, roadmap, plan, and resolved review findings are preserved
+in Git history at `83c59ea` under `docs/roadmap/`.
+
+The completed overlay rename plan and release-workflow simplification record are
+preserved in Git history at `73ce6e9` under `docs/roadmap/`. Current overlay
+behavior belongs to the [configuration contract](config.md#selected-overlays),
+and release operations belong to the [release procedure](release.md).
+
+The completed ReleaseKit adoption, startup performance request and experiments,
+benchmark evidence, performance audit, implementation plan, and remediation
+record are preserved in Git history at `8e91a2b` under `docs/roadmap/`. For example:
+
+```sh
+git show 8e91a2b:docs/roadmap/startup-benchmark.md
+```
+
+Remaining optimization candidates belong to
+[deferred scope](roadmap/deferred.md#deferred-performance-experiments).
+
+The completed effective-configuration diff roadmap, implementation plan, and
+resolved review findings are preserved in Git history at `1c2610c` under
+`docs/roadmap/`. The
+[configuration contract](config.md#effective-configuration-diffs) owns current
+comparison and disclosure behavior, and
+[ADR 0010](adr/0010-effective-configuration-diff.md) records the decision.
+
+Install the Go version required by `go.mod` or a compatible newer toolchain.
+Acquire declared dependencies explicitly before entering an offline
+development or validation environment:
 
 ```sh
 go mod download
@@ -88,8 +147,10 @@ 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.
+The [release procedure](release.md#4-publish-the-committed-candidate) performs
+its selected validation through one `publish` invocation. The everyday checker
+remains a development gate and is not a separate release prerequisite. CI never
+gates a release.
 
 ## Task-to-test map
 
@@ -101,14 +162,23 @@ code changes. Use the component overview to find the corresponding implementatio
 | --- | --- | --- |
 | YAML syntax, diagnostics, parser limits | [Adapter tests](../internal/yaml/parse_test.go), [source translation](../internal/engine/parse_test.go) | `go test ./internal/yaml` and `go test ./internal/engine -run '^TestParseSource'` |
 | File/reader access, imports, globs, containment | [Sources](../internal/engine/source_test.go), [FIFO](../internal/engine/source_fifo_test.go), [imports](../internal/engine/imports_test.go), [globs](../internal/engine/glob_test.go) | `go test ./internal/engine -run 'Source\|Import\|Glob\|Filesystem\|Nonregular\|RegularFile'` |
-| Composition, profiles, CLI precedence | [Composition](../internal/engine/compose_test.go), [overlay](../internal/engine/overlay_test.go), [profiles](../internal/engine/profile_test.go), [overrides](../internal/engine/override_test.go) | `go test ./internal/engine -run 'Compose\|Overlay\|Profile\|Override'` |
+| Native filesystem and mapping sources | [Native acquisition](../internal/engine/native_filesystem_test.go), [filesystem facade](../facade_filesystem_test.go), [mapping facade](../facade_mapping_test.go) | `go test ./internal/engine -run 'NativeFilesystem'` and `go test . -run 'TestFacade'` |
+| Composition, selected overlays, CLI precedence | [Composition](../internal/engine/compose_test.go), [overlay](../internal/engine/overlay_test.go), [selected overlays](../internal/engine/selected_overlay_test.go), [overrides](../internal/engine/override_test.go) | `go test ./internal/engine -run 'Compose\|Overlay\|Override'` |
+| Sibling inheritance, provenance, history, sensitivity | [Expansion](../internal/engine/inheritance_test.go), [destination scopes](../internal/engine/inheritance_walk_test.go), [origins](../internal/engine/inheritance_origin_test.go), [history](../internal/engine/inheritance_history_test.go), [secrets](../internal/engine/inheritance_secret_test.go), [public loading](../facade_inheritance_test.go) | `go test ./internal/engine -run 'Inheritance\|Inherited'` and `go test . -run '^TestFacadeInheritance\|^TestFacadeInheritedSecrets'` |
+| Layered loading and source equivalence | [Grouped lifecycle](../internal/engine/load_request_test.go), [layered facade](../facade_layered_test.go), [equivalence](../facade_equivalence_test.go) | `go test ./internal/engine -run '^TestGrouped'`, `go test . -run '^TestFacadeLayered'`, and `go test . -run '^TestEquivalentSourceForms'` |
+| Input destinations, mounted sources, and source ownership | [Destination engine tests](../internal/engine/destination_test.go), [tier and import tests](../internal/engine/destination_tiers_test.go), [integration and provenance tests](../internal/engine/destination_integration_test.go), [public facade tests](../facade_destination_test.go), and [consumer examples](../examples/consumers/destinations_test.go) | `go test ./internal/engine -run 'Destination|Mounted'`, `go test . -run 'Destination|FacadeMounted'`, and `go test ./examples/consumers -run 'CollectionBundle|CollectionPeer'` |
 | Schema, defaults, projection, cloning | [Schema](../internal/engine/schema_test.go), [decoding](../internal/engine/decode_test.go), [projection](../internal/engine/projection_test.go) | `go test ./internal/engine -run 'Schema\|Decode\|Projection\|Clone\|Inline\|CompareDocument'` |
 | Lifecycle, resolution, validation, callback ownership | [Loading](../internal/engine/load_test.go), [resolution](../internal/engine/resolution_test.go), [validation](../internal/engine/validation_test.go), [ownership](../internal/engine/callback_ownership_test.go), [independence](../internal/engine/independence_test.go) | `go test ./internal/engine -run 'Load\|Resolution\|Validation\|Validator\|Hook\|Callback\|Parallel'` |
 | Paths, metadata, provenance | [Paths](../internal/engine/path_test.go), [metadata](../internal/engine/metadata_test.go), [provenance](../internal/engine/provenance_test.go) | `go test ./internal/engine -run 'Path\|Metadata\|Provenance\|Queries'` |
+| Passive resource contexts | [Engine contexts](../internal/engine/resource_context_test.go), [public contexts](../facade_resource_context_test.go) | `go test ./internal/engine -run 'Resource'` and `go test . -run 'SourceContext'` |
 | Secrets, bootstrap, redaction, explanation | [References](../internal/engine/secret_reference_test.go), [bootstrap](../internal/engine/secrets_test.go), [rendering](../internal/engine/render_test.go), [explanation](../internal/engine/explain_test.go) | `go test ./internal/engine -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'` |
+| Effective configuration diff | [Engine comparison and masking tests](../internal/engine/diff_test.go), [engine operation tests](../internal/engine/diff_operation_test.go), [root facade tests](../facade_diff_test.go), and [consumer examples](../examples/consumers/diff_test.go) | `go test ./internal/engine -run 'Diff|Explain|History|Secret|Independent'`, `go test . -run 'Diff|FacadeInspection|PublicFacadeBoundary|DependencyImportBoundary'`, `go test ./examples/consumers`, and `go test . -run '^ExampleDiff$'` |
+| Consumer examples and migration fixtures | [Consumer tests](../examples/consumers/consumers_test.go), [native and layered examples](../examples/consumers/layered_test.go), [API examples](../example_test.go) | `go test ./examples/consumers` and `go test . -run '^Example'` |
+| Assembly and layered scaling evidence | [Assembly benchmarks](../internal/engine/assembly_bench_test.go), [layered benchmarks](../internal/engine/underlay_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkUnderlay' -benchmem`; repeat with `^BenchmarkCompose` and `^BenchmarkOverlay` |
+| Inheritance scaling evidence | [Expansion benchmarks](../internal/engine/inheritance_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkInheritance' -benchmem` |
 | Documentation links and dependency isolation | [Link checks](../documentation_test.go), [import boundary](../import_boundary_test.go), [public boundary](../public_boundary_test.go) | `go test . -run 'TestLocalDocumentationLinks\|TestDependencyImportBoundary\|TestPublicFacadeBoundary'` |
-| Check-command behavior | [Shell regression](../release_check_test.go) | `go test . -run '^TestCheckScripts'` |
+| Development check behavior | [Development checker test](../development_check_test.go) | `go test . -run '^TestDevelopmentCheckHandlesFilenamesAndStopsOnFailure$'` |
+| Release profile wiring | [Release profile test](../releasekit_profile_test.go) | `go test . -run '^TestReleaseKitProfile$'` |
 
 ## Fuzz validation
 

docs/internal/extraction.md

@@ -137,9 +137,11 @@ permission failures, explicit `conf.d` use, and entry/candidate budgets. This wo
 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
+### Upstream profiles and ConfigKit selected overlays
 
-Assigned to selected-profile overlay:
+The following upstream symbols and case names retain their original profile
+terminology. ConfigKit adapts their selected-overlay behavior without renaming the
+upstream evidence:
 
 - Production: `mergeOverlayComposition`, `mergeOverlayNodes`,
   `selectPipelineProfile`, `loadPipelineProfileOverlays`, and
@@ -153,19 +155,21 @@ Assigned to selected-profile overlay:
   `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
+- Adapt: load only the selected fixed overlay path; lowercase `default` means no
+  selection. ConfigKit permits null and cross-kind replacement and does not support
+  declared defaults, inheritance, arbitrary overlay 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
+Implementation record (2026-09-06, updated 2026-09-23): `overlayDocuments`,
+`mergeOverlayMappings`, `loadSelectedOverlayDocument`, and
+`overlayDirectiveDiagnostics` adapt the assigned merge, selection, loading, and
+ownership mechanics. `TestOverlayDocumentsRecursesMappingsAndReplacesAtomicSubtrees`,
+`TestLoadAppliesSelectedOverlayAfterBaseAndBeforeDefaults`, selected-overlay
+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
+retains displaced descendant history, rejects any overlay directive or version-path
+contribution, and counts the overlay in aggregate budgets. Narratio's declared
 default, multi-overlay validation, inheritance-like application conventions, and
 null/kind restrictions were not retained.
 
@@ -196,7 +200,7 @@ 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
+inspection cases across one default/import/overlay/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

docs/internal/overview.md

@@ -7,23 +7,34 @@ private state; public signatures and fields do not expose engine types.
 
 Implemented component owners:
 
-- `internal/engine` owns source acquisition, composition, schema inspection,
-  decoding, lifecycle coordination, callback auditing, provenance/history,
-  secret resolution and bootstrap, and safe inspection rendering. It never
-  imports the root package.
-- Root `Source`, `Path`, `Secret`, records, lifecycle callbacks, results, and
-  bootstrap reports are root-defined wrappers or records. Callback wrappers pass
-  only live engine context during a callback; `Result.Config` is detached and its
-  private inspection remains frozen independently.
+- `internal/engine` owns host and native-filesystem source acquisition,
+  grouped strict composition, recursive precedence assembly, per-input mapping
+  placement within that assembly, and destination-local sibling inheritance
+  before typed decoding. It retains source placement alongside physical source
+  identity and owns schema
+  inspection, lifecycle coordination, callback auditing,
+  provenance/history, passive source-context retention, secret resolution and
+  bootstrap, safe inspection rendering, and effective-value comparison over
+  frozen inspection snapshots. It never imports the root package.
+- Root `Source`, `Filesystem`, `Input`, `Underlay`, `Path`, `Secret`, records,
+  lifecycle callbacks, results, and bootstrap reports are root-defined wrappers
+  or records. Callback wrappers pass only live engine context during a callback;
+  `Result.Config` is detached and its private inspection remains frozen
+  independently.
 - Root conversion files explicitly cross the path, record, callback, secret, and
   error boundaries. There are no public aliases, anonymous engine embeddings, or
   public engine bridge methods.
+- Root `Diff` and `Difference` provide the public comparison boundary and
+  convert detached engine records to root-owned paths and snapshots; consumer
+  examples own presentation of the safe changed regions.
 - `internal/document` owns the parser-independent ordered tree and cloning.
   `internal/yaml` is the sole upstream YAML adapter and parser/dumper boundary.
-- Engine mechanism tests own source, composition, decoding, callback, provenance,
-  secret, history, rendering, and bootstrap behavior. Root tests own public API
-  compatibility, facade integration, documentation links, and dependency/public
-  boundary checks. Consumer examples remain under `examples/consumers`.
+- Engine mechanism tests own source, composition, inheritance, decoding,
+  callback, provenance,
+  source-context, secret, history, rendering, and bootstrap behavior. Root tests
+  own public API compatibility, facade integration, documentation links, and
+  dependency/public boundary checks. Consumer examples remain under
+  `examples/consumers`.
 
 Dependency direction is root to engine to document/YAML support. The root may
 import only `internal/engine` among project internal production packages; engine
@@ -32,8 +43,14 @@ these constraints and rejects exported aliases, fields, or signatures involving
 internal types.
 
 The [configuration contract](../config.md) owns implemented YAML and lifecycle
-semantics. The [consumer adoption guide](../consumers/README.md) owns application
-guidance. The [architecture policy](../policy/architecture.md) owns contributor
-boundaries, and [ADR 0006](../adr/0006-public-facade.md) records the facade
+semantics, including [input destinations](../config.md#input-destinations) and
+[effective configuration diffs](../config.md#effective-configuration-diffs).
+The [consumer adoption guide](../consumers/README.md) owns application guidance.
+The [architecture policy](../policy/architecture.md) owns contributor
+boundaries; [ADR 0006](../adr/0006-public-facade.md) records the facade decision
+and [ADR 0010](../adr/0010-effective-configuration-diff.md) records the diff
+decision; [ADR 0011](../adr/0011-input-destinations.md) records the placement
 decision. See the [development guide](../development.md) for validation commands
-and [release procedure](../release.md) for source-candidate gates.
+and [release procedure](../release.md) for source-candidate gates. [ADR
+0009](../adr/0009-sibling-inheritance.md) records why inheritance is a built-in
+post-assembly structural operation.

docs/oss/attribution.md

@@ -0,0 +1,43 @@
+# Open Source Attribution Notices
+
+## Standard Repository Policies
+
+Material: `docs/policy/release.md`, adapted from
+`e833faddf120c789a9b45348edaf75720de16348`; `docs/policy/meta.md`, `docs/policy/oss.md`,
+`docs/policy/documentation.md`, `docs/policy/agents.md`,
+`docs/policy/architecture.md`, `docs/policy/testing.md`; root `AGENTS.md`; and the
+Policy Reading Guide in `docs/development.md`, adapted from
+`d84a38ebab26f048cafbe59a2246700c35d29843` for this repository.
+
+Copyright (C) 2026 Eric Rakestraw
+
+SPDX-License-Identifier: CC-BY-SA-4.0
+
+License and notices: [CC BY-SA 4.0](licenses/CC-BY-SA-4.0.txt) |
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/COPYRIGHT) |
+[LICENSE](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/LICENSE) |
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/COPYRIGHT) |
+[LICENSE](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/LICENSE).
+
+[Source](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843).
+
+[Source](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348).
+
+## go-yaml v4
+
+Selected module: `go.yaml.in/yaml/v4` at `v4.0.0-rc.6`.
+
+Copyright 2011-2019 Canonical Ltd
+
+Copyright 2025 The go-yaml Project Contributors
+
+Copyright 2006-2010 Kirill Simonov
+
+SPDX-License-Identifier: Apache-2.0 AND MIT
+
+License and notices: [Apache 2.0](licenses/Apache-2.0.txt) |
+[MIT](licenses/MIT.txt) |
+[NOTICE](https://github.com/yaml/go-yaml/blob/1c17b9cee72bf81ae73a8be50fbd7dd3a9985125/NOTICE) |
+[LICENSE](https://github.com/yaml/go-yaml/blob/1c17b9cee72bf81ae73a8be50fbd7dd3a9985125/LICENSE).
+
+[Source](https://github.com/yaml/go-yaml/tree/1c17b9cee72bf81ae73a8be50fbd7dd3a9985125).

docs/oss/licenses/Apache-2.0.txt

@@ -0,0 +1,202 @@
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@@ -0,0 +1,170 @@
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+Creative Commons public licenses provide a standard set of terms and conditions that creators and other rights holders may use to share original works of authorship and other material subject to copyright and certain other rights specified in the public license below. The following considerations are for informational purposes only, are not exhaustive, and do not form part of our licenses.
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+Section 1 – Definitions.
+
+     a.	Adapted Material means material subject to Copyright and Similar Rights that is derived from or based upon the Licensed Material and in which the Licensed Material is translated, altered, arranged, transformed, or otherwise modified in a manner requiring permission under the Copyright and Similar Rights held by the Licensor. For purposes of this Public License, where the Licensed Material is a musical work, performance, or sound recording, Adapted Material is always produced where the Licensed Material is synched in timed relation with a moving image.
+
+     b.	Adapter's License means the license You apply to Your Copyright and Similar Rights in Your contributions to Adapted Material in accordance with the terms and conditions of this Public License.
+
+     c.	BY-SA Compatible License means a license listed at creativecommons.org/compatiblelicenses, approved by Creative Commons as essentially the equivalent of this Public License.
+
+     d.	Copyright and Similar Rights means copyright and/or similar rights closely related to copyright including, without limitation, performance, broadcast, sound recording, and Sui Generis Database Rights, without regard to how the rights are labeled or categorized. For purposes of this Public License, the rights specified in Section 2(b)(1)-(2) are not Copyright and Similar Rights.
+
+     e.	Effective Technological Measures means those measures that, in the absence of proper authority, may not be circumvented under laws fulfilling obligations under Article 11 of the WIPO Copyright Treaty adopted on December 20, 1996, and/or similar international agreements.
+
+     f.	Exceptions and Limitations means fair use, fair dealing, and/or any other exception or limitation to Copyright and Similar Rights that applies to Your use of the Licensed Material.
+
+     g.	License Elements means the license attributes listed in the name of a Creative Commons Public License. The License Elements of this Public License are Attribution and ShareAlike.
+
+     h.	Licensed Material means the artistic or literary work, database, or other material to which the Licensor applied this Public License.
+
+     i.	Licensed Rights means the rights granted to You subject to the terms and conditions of this Public License, which are limited to all Copyright and Similar Rights that apply to Your use of the Licensed Material and that the Licensor has authority to license.
+
+     j.	Licensor means the individual(s) or entity(ies) granting rights under this Public License.
+
+     k.	Share means to provide material to the public by any means or process that requires permission under the Licensed Rights, such as reproduction, public display, public performance, distribution, dissemination, communication, or importation, and to make material available to the public including in ways that members of the public may access the material from a place and at a time individually chosen by them.
+
+     l.	Sui Generis Database Rights means rights other than copyright resulting from Directive 96/9/EC of the European Parliament and of the Council of 11 March 1996 on the legal protection of databases, as amended and/or succeeded, as well as other essentially equivalent rights anywhere in the world.
+
+     m.	You means the individual or entity exercising the Licensed Rights under this Public License. Your has a corresponding meaning.
+
+Section 2 – Scope.
+
+     a.	License grant.
+
+          1. Subject to the terms and conditions of this Public License, the Licensor hereby grants You a worldwide, royalty-free, non-sublicensable, non-exclusive, irrevocable license to exercise the Licensed Rights in the Licensed Material to:
+
+               A. reproduce and Share the Licensed Material, in whole or in part; and
+
+               B. produce, reproduce, and Share Adapted Material.
+
+          2. Exceptions and Limitations. For the avoidance of doubt, where Exceptions and Limitations apply to Your use, this Public License does not apply, and You do not need to comply with its terms and conditions.
+
+          3. Term. The term of this Public License is specified in Section 6(a).
+
+          4. Media and formats; technical modifications allowed. The Licensor authorizes You to exercise the Licensed Rights in all media and formats whether now known or hereafter created, and to make technical modifications necessary to do so. The Licensor waives and/or agrees not to assert any right or authority to forbid You from making technical modifications necessary to exercise the Licensed Rights, including technical modifications necessary to circumvent Effective Technological Measures. For purposes of this Public License, simply making modifications authorized by this Section 2(a)(4) never produces Adapted Material.
+
+          5. Downstream recipients.
+
+               A. Offer from the Licensor – Licensed Material. Every recipient of the Licensed Material automatically receives an offer from the Licensor to exercise the Licensed Rights under the terms and conditions of this Public License.
+
+               B. Additional offer from the Licensor – Adapted Material. Every recipient of Adapted Material from You automatically receives an offer from the Licensor to exercise the Licensed Rights in the Adapted Material under the conditions of the Adapter’s License You apply.
+
+               C. No downstream restrictions. You may not offer or impose any additional or different terms or conditions on, or apply any Effective Technological Measures to, the Licensed Material if doing so restricts exercise of the Licensed Rights by any recipient of the Licensed Material.
+
+          6. No endorsement. Nothing in this Public License constitutes or may be construed as permission to assert or imply that You are, or that Your use of the Licensed Material is, connected with, or sponsored, endorsed, or granted official status by, the Licensor or others designated to receive attribution as provided in Section 3(a)(1)(A)(i).
+
+     b.	Other rights.
+
+          1. Moral rights, such as the right of integrity, are not licensed under this Public License, nor are publicity, privacy, and/or other similar personality rights; however, to the extent possible, the Licensor waives and/or agrees not to assert any such rights held by the Licensor to the limited extent necessary to allow You to exercise the Licensed Rights, but not otherwise.
+
+          2. Patent and trademark rights are not licensed under this Public License.
+
+          3. To the extent possible, the Licensor waives any right to collect royalties from You for the exercise of the Licensed Rights, whether directly or through a collecting society under any voluntary or waivable statutory or compulsory licensing scheme. In all other cases the Licensor expressly reserves any right to collect such royalties.
+
+Section 3 – License Conditions.
+
+Your exercise of the Licensed Rights is expressly made subject to the following conditions.
+
+     a.	Attribution.
+
+          1. If You Share the Licensed Material (including in modified form), You must:
+
+               A. retain the following if it is supplied by the Licensor with the Licensed Material:
+
+                    i.	identification of the creator(s) of the Licensed Material and any others designated to receive attribution, in any reasonable manner requested by the Licensor (including by pseudonym if designated);
+
+                    ii.	a copyright notice;
+
+                    iii. a notice that refers to this Public License;
+
+                    iv.	a notice that refers to the disclaimer of warranties;
+
+                    v.	a URI or hyperlink to the Licensed Material to the extent reasonably practicable;
+
+               B. indicate if You modified the Licensed Material and retain an indication of any previous modifications; and
+
+               C. indicate the Licensed Material is licensed under this Public License, and include the text of, or the URI or hyperlink to, this Public License.
+
+          2. You may satisfy the conditions in Section 3(a)(1) in any reasonable manner based on the medium, means, and context in which You Share the Licensed Material. For example, it may be reasonable to satisfy the conditions by providing a URI or hyperlink to a resource that includes the required information.
+
+          3. If requested by the Licensor, You must remove any of the information required by Section 3(a)(1)(A) to the extent reasonably practicable.
+
+     b.	ShareAlike.In addition to the conditions in Section 3(a), if You Share Adapted Material You produce, the following conditions also apply.
+
+          1. The Adapter’s License You apply must be a Creative Commons license with the same License Elements, this version or later, or a BY-SA Compatible License.
+
+          2. You must include the text of, or the URI or hyperlink to, the Adapter's License You apply. You may satisfy this condition in any reasonable manner based on the medium, means, and context in which You Share Adapted Material.
+
+          3. You may not offer or impose any additional or different terms or conditions on, or apply any Effective Technological Measures to, Adapted Material that restrict exercise of the rights granted under the Adapter's License You apply.
+
+Section 4 – Sui Generis Database Rights.
+
+Where the Licensed Rights include Sui Generis Database Rights that apply to Your use of the Licensed Material:
+
+     a.	for the avoidance of doubt, Section 2(a)(1) grants You the right to extract, reuse, reproduce, and Share all or a substantial portion of the contents of the database;
+
+     b.	if You include all or a substantial portion of the database contents in a database in which You have Sui Generis Database Rights, then the database in which You have Sui Generis Database Rights (but not its individual contents) is Adapted Material, including for purposes of Section 3(b); and
+
+     c.	You must comply with the conditions in Section 3(a) if You Share all or a substantial portion of the contents of the database.
+For the avoidance of doubt, this Section 4 supplements and does not replace Your obligations under this Public License where the Licensed Rights include other Copyright and Similar Rights.
+
+Section 5 – Disclaimer of Warranties and Limitation of Liability.
+
+     a.	Unless otherwise separately undertaken by the Licensor, to the extent possible, the Licensor offers the Licensed Material as-is and as-available, and makes no representations or warranties of any kind concerning the Licensed Material, whether express, implied, statutory, or other. This includes, without limitation, warranties of title, merchantability, fitness for a particular purpose, non-infringement, absence of latent or other defects, accuracy, or the presence or absence of errors, whether or not known or discoverable. Where disclaimers of warranties are not allowed in full or in part, this disclaimer may not apply to You.
+
+     b.	To the extent possible, in no event will the Licensor be liable to You on any legal theory (including, without limitation, negligence) or otherwise for any direct, special, indirect, incidental, consequential, punitive, exemplary, or other losses, costs, expenses, or damages arising out of this Public License or use of the Licensed Material, even if the Licensor has been advised of the possibility of such losses, costs, expenses, or damages. Where a limitation of liability is not allowed in full or in part, this limitation may not apply to You.
+
+     c.	The disclaimer of warranties and limitation of liability provided above shall be interpreted in a manner that, to the extent possible, most closely approximates an absolute disclaimer and waiver of all liability.
+
+Section 6 – Term and Termination.
+
+     a.	This Public License applies for the term of the Copyright and Similar Rights licensed here. However, if You fail to comply with this Public License, then Your rights under this Public License terminate automatically.
+
+     b.	Where Your right to use the Licensed Material has terminated under Section 6(a), it reinstates:
+
+          1. automatically as of the date the violation is cured, provided it is cured within 30 days of Your discovery of the violation; or
+
+          2. upon express reinstatement by the Licensor.
+
+     c.	For the avoidance of doubt, this Section 6(b) does not affect any right the Licensor may have to seek remedies for Your violations of this Public License.
+
+     d.	For the avoidance of doubt, the Licensor may also offer the Licensed Material under separate terms or conditions or stop distributing the Licensed Material at any time; however, doing so will not terminate this Public License.
+
+     e.	Sections 1, 5, 6, 7, and 8 survive termination of this Public License.
+
+Section 7 – Other Terms and Conditions.
+
+     a.	The Licensor shall not be bound by any additional or different terms or conditions communicated by You unless expressly agreed.
+
+     b.	Any arrangements, understandings, or agreements regarding the Licensed Material not stated herein are separate from and independent of the terms and conditions of this Public License.
+
+Section 8 – Interpretation.
+
+     a.	For the avoidance of doubt, this Public License does not, and shall not be interpreted to, reduce, limit, restrict, or impose conditions on any use of the Licensed Material that could lawfully be made without permission under this Public License.
+
+     b.	To the extent possible, if any provision of this Public License is deemed unenforceable, it shall be automatically reformed to the minimum extent necessary to make it enforceable. If the provision cannot be reformed, it shall be severed from this Public License without affecting the enforceability of the remaining terms and conditions.
+
+     c.	No term or condition of this Public License will be waived and no failure to comply consented to unless expressly agreed to by the Licensor.
+
+     d.	Nothing in this Public License constitutes or may be interpreted as a limitation upon, or waiver of, any privileges and immunities that apply to the Licensor or You, including from the legal processes of any jurisdiction or authority.
+
+Creative Commons is not a party to its public licenses. Notwithstanding, Creative Commons may elect to apply one of its public licenses to material it publishes and in those instances will be considered the “Licensor.” Except for the limited purpose of indicating that material is shared under a Creative Commons public license or as otherwise permitted by the Creative Commons policies published at creativecommons.org/policies, Creative Commons does not authorize the use of the trademark “Creative Commons” or any other trademark or logo of Creative Commons without its prior written consent including, without limitation, in connection with any unauthorized modifications to any of its public licenses or any other arrangements, understandings, or agreements concerning use of licensed material. For the avoidance of doubt, this paragraph does not form part of the public licenses.
+
+Creative Commons may be contacted at creativecommons.org.

docs/oss/licenses/MIT.txt

@@ -0,0 +1,18 @@
+MIT License
+
+Copyright (c) <year> <copyright holders>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and
+associated documentation files (the "Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the
+following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial
+portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
+LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO
+EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+USE OR OTHER DEALINGS IN THE SOFTWARE.

docs/policy/agents.md

@@ -0,0 +1,91 @@
+# Agent Policy
+
+**Base Policy Version:** 1.3
+
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/agents.md)
+
+## Purpose
+
+Agents must read and follow these rules.
+
+## Implementation Discipline
+
+**KEY PRINCIPLES:** Write the simplest idiomatic code that satisfies the user's
+request. Do not implicitly strengthen requirements, restrictions, or guarantees
+beyond what the user actually requested.
+
+**Keep implementation machinery proportionate.** Use straightforward,
+idiomatic code and introduce only the machinery needed to support the requested
+behavior and flexibility. Optimize for conceptual simplicity and
+maintainability, not for the fewest lines, fewest options, or narrowest product
+surface.
+
+**Do not invent restrictions.** Do not reject, prohibit, constrain, cap,
+harden, or otherwise narrow behavior unless required by the user, an explicit
+repository contract, or an unavoidable correctness constraint. Preserve
+operator and caller choice where policy has not been established. If an
+additional restriction appears advisable, present it as a decision rather than
+silently implementing it.
+
+A correctness constraint must follow from the requested behavior or an
+established invariant, not from the convenience of the chosen implementation.
+Explain the specific failure it prevents and why a less restrictive
+implementation cannot satisfy the contract.
+
+**Do not invent guarantees.** Do not strengthen public contracts, guarantees,
+or invariants beyond those expressly requested by the user.
+
+**Use the standard library.** For example: in almost every case, the best
+approach to copying files is to use the standard library. Writing custom code to
+implement power-loss protection at the application level, or to guard against
+bad actors with privileged and concurrent access to the host's filesystem,
+is wasteful, unnecessary, and not idiomatic. Stick to the simple, standard, and
+idiomatic approach unless another path is expressly requested by the user.
+
+**An implementation detail is not repository policy.** Tests and documentation
+must not elevate incidental behavior into a durable guarantee or restriction
+unless that behavior was expressly selected as part of the contract. Existing
+explicit contracts remain authoritative; changing or weakening one requires a
+deliberate decision rather than being inferred from this principle.
+
+Agent-authored plans, tests, documentation, and prior implementations are not
+evidence that an additional restriction was deliberately approved. Identify the
+authorizing requirement or decision; when that authority is unclear, surface
+the ambiguity rather than treating repetition as approval.
+
+## Git Commit Policy
+
+By default, commit each completed, nonempty unit of work using a
+concise, plain-English message describing what changed. This policy is standing
+authorization to create those local commits; a separate user request is not
+required. An explicit user instruction to leave work uncommitted takes precedence.
+
+A unit of work is a focused, reviewable outcome, including its relevant tests and
+documentation.
+Complete the appropriate validation before committing. If work is incomplete or
+validation is blocked, report that state accurately.
+
+Before committing:
+
+1. Run `git status --short` and review the relevant diff, including any staged
+   changes. Identify changes that predate the current unit of work.
+2. Exclude secrets, local environment files, generated junk, and unrelated changes.
+3. Stage only the files or hunks belonging to the completed unit. Use `git add .`
+   only when every included change belongs to that unit. Preserve unrelated
+   staged and unstaged work.
+4. Review `git diff --staged` and run appropriate checks for the change. Reuse
+   successful checks for unchanged content.
+5. Review `git log --oneline -n 10` and use
+   `git commit -m "<concise plain-English commit message>"`.
+6. Report the commit identifier and any remaining uncommitted work.
+
+If the unit cannot be safely separated from pre-existing work, leave it uncommitted
+and explain the overlap rather than silently including that work. If a commit
+fails, preserve the changes and report the failure; do not bypass hooks.
+
+Default commit authorization does not authorize pushing, publishing, amending,
+resetting, reverting, or rewriting history. Those actions require a separate user
+instruction. Do not use `git commit -a` or `git commit --no-verify` to bypass the
+staging and validation discipline above.

docs/policy/architecture.md

@@ -1,18 +1,55 @@
 # Architecture Policy
 
-**Base Policy Version:** 1.0
+**Base Policy Version:** 1.3
 
-**Last Updated:** 2026-09-06
+**Last Updated**: 2026-09-29
 
-## Purpose and applicability
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/architecture.md)
+
+## Purpose And Ownership
+
+The adopted policy defines the application's purpose and the architectural
+constraints contributors must follow. It owns system boundaries, responsibility
+and dependency rules, established invariants, and explicit non-goals.
+
+Describe the concrete package inventory and implementation mechanics in internal
+documentation. Link to canonical public contracts for their exact behavior and
+to ADRs for decision rationale.
 
 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.
+and resolution across consuming applications. 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.
 
-## Success criteria
+## Adopted Constraints And Implemented Structure
+
+Architectural constraints govern current and future development. Record
+implemented structure accurately, proposals and implementation status in the
+roadmap, and significant decisions and rationale in ADRs. Describe a target
+structure as implemented when its changes land.
+
+Constraints must reflect deliberately selected requirements and decisions.
+Do not promote incidental implementation choices into architectural invariants.
+
+## Success Criteria
+
+ConfigKit prioritizes a coherent configuration model: consistency and
+predictability take precedence over flexibility for accommodating individual
+downstream conventions. Consumers should learn one set of rules, not different
+combination or presence semantics depending on the source format or consuming
+application. Distinct operations such as strict composition and intentional
+overlay express distinct purposes within that model.
+
+Consumers should be able to load once and receive a complete, validated
+configuration with inspectable provenance for its final values, including
+defaults and recorded application-derived values. Configuration and metadata
+together must distinguish explicit zero, false, empty, and null input from
+omission; ordinary Go values alone need not encode those distinctions.
+Application-owned resolution belongs within the recorded loading lifecycle, so
+consumers need no subsequent generic merging, defaulting, or provenance
+reconstruction. This does not transfer domain execution or runtime resource
+management into ConfigKit.
 
 1. **Reduce application infrastructure:** Can applications delete substantial
    configuration infrastructure while retaining clear ownership of their domain
@@ -21,8 +58,11 @@ The [component overview](../internal/overview.md) owns implementation inventory,
    configuration model, with only the flexibility needed to express genuine
    application requirements?
 
-Both criteria must hold. ConfigKit should not preserve divergent consumer
-conventions merely for compatibility, nor impose restrictions that prevent useful
+Both criteria must hold. Minimize consumer loading and processing code subject
+to the consistency, presence, and provenance principles above, rather than
+trading those principles for consumer-specific shortcuts. 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
@@ -51,7 +91,7 @@ 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
+## Responsibility Boundaries
 
 ConfigKit owns configuration input processing and the coordination of decoding,
 resolution, validation, and inspection. Applications own their configuration
@@ -59,6 +99,13 @@ 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.
 
+The engine applies each input's mapping destination within the existing source
+assembly and retains that placement with source identity and inspection records;
+the root facade owns the public fields and conversions. The
+[configuration contract](../config.md#input-destinations) defines the observable
+placement behavior, and [ADR 0011](../adr/0011-input-destinations.md) records the
+accepted decision.
+
 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`.
@@ -68,10 +115,26 @@ 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
+## Loading Lifecycle
+
+Configuration loading is a synchronous, complete-on-return operation. A successful
+load provides the final configuration and inspection metadata after the configured
+processing and validation phases complete. A failed load reports diagnostics and
+returns no usable partial result. The normal application pattern is one load near
+startup; applications may explicitly initiate additional loads.
+
+ConfigKit releases its library-owned loading handles before returning and remains
+inactive until the application explicitly invokes it again. It performs no
+background watching, refresh, or mutation of published results. Caller-owned
+resources and application callback effects remain the caller's responsibility.
+The [configuration contract](../config.md#completion-and-resource-lifetime) owns
+the precise completion and ownership behavior. The proposed decision record is
+[ADR 0008](../adr/0008-complete-on-return-loading.md).
+
+## 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.
+single-document consumers must not need import, overlay, 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).
@@ -94,20 +157,43 @@ 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
+## 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).
+Within a configuration tier, independent contributions use strict composition:
+shared mappings combine recursively, while overlapping non-mapping definitions
+are conflicts, including equal values. Between precedence tiers, intentional
+overlay combines mappings recursively, retains omitted lower-priority children,
+and replaces other present values according to the existing overlay rules.
+Tier precedence is explicit; traversal order within a tier establishes stable
+reporting, not permission to override. The distinction and its rationale are
+recorded in [ADR 0001](../adr/0001-separate-composition-and-overlay.md).
+
+The main document and its imports form a strictly composed tier. Explicit
+underlay groups compose strictly within each group, then combine recursively
+from lowest to highest beneath the main tier. Source acquisition kind does not
+determine its precedence role. Input-specific constraints, including CLI path
+rules and opt-in version ownership, remain part of the configuration contract. Consistent combination
+semantics do not require identical acquisition or lifetime behavior from every
+source kind.
+
+After source precedence is complete, built-in sibling `extends` relationships
+resolve structurally in the same parser-independent representation, using the
+existing recursive overlay rules. This adds neither an acquired source nor a
+consumer registration mechanism. Enclosing relationships precede nested
+destination-local resolution; typed defaults, application resolution, and
+secret lookup retain their existing lifecycle roles. The [configuration
+contract](../config.md#sibling-inheritance) owns the exact syntax and presence
+rules, and [ADR 0009](../adr/0009-sibling-inheritance.md) records the rationale.
 
 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
+User-controlled precedence is application defaults, ordered underlays, composed
+main input, selected overlay, 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
@@ -122,7 +208,14 @@ 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
+The frozen inspection boundary also supports comparing completed same-type
+results without reopening sources or rerunning the load lifecycle. The
+[configuration contract](../config.md#effective-configuration-diffs) owns
+comparison and disclosure behavior, the [component overview](../internal/overview.md)
+owns implementation responsibility, and [ADR 0010](../adr/0010-effective-configuration-diff.md)
+records the accepted decision.
+
+## 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
@@ -143,7 +236,7 @@ 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
+## Scope Boundaries
 
 Do not introduce automatic directory discovery, network providers, arbitrary
 merge strategies, additional configuration languages, or application runtime

docs/policy/documentation.md

@@ -1,125 +1,176 @@
 # Documentation Policy
 
-**Base Policy Version:** 1.0
+**Base Policy Version:** 1.11
 
-**Last Updated:** 2026-09-06
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/documentation.md)
 
 ## Purpose
 
-Assign each documentation topic one canonical owner so ConfigKit's documentation
-remains accurate and useful to consumers, maintainers, and implementation agents.
+This policy assigns each documentation topic to one canonical owner. Its goal is
+to keep ConfigKit's documentation accurate, concise, discoverable, and resistant
+to drift for consumers, maintainers, developers, and coding agents.
 
-## Core rules
+## Core Rules
 
-### One canonical owner
+### One Canonical Owner
 
-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.
+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
+for volatile details.
 
-### Policies, current behavior, and future work
+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 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.
+### Current And Future Behavior
 
-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.
+Outside `docs/roadmap/`, documentation describes implemented behavior only.
+Partial features may be described only to their implemented boundary.
 
-### Audience and examples
+Architectural constraints may govern future changes. Describe current structure
+from implemented behavior.
 
-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.
+ADRs may record an accepted architectural decision before implementation. 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.
 
-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.
+`docs/roadmap/deferred.md` owns deliberate exclusions; task-specific plans
+own execution order and progress. Completed plans retire during release
+preparation.
 
-### Security and privacy
+### Audience And Detail
 
-Do not include real credentials, private keys, environment dumps, sensitive input,
-or private infrastructure details unless intentionally public. Document secret
-handling, not secret values.
+Write for the document's stated audience and include only the detail needed for
+its owned topic. Keep user and operator docs focused on using and operating the
+application. Developer docs should link to user-facing and external contracts
+for their definitions.
 
-## Canonical ownership
+### Examples
 
-| 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 |
+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 for complete copyable files.
 
-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.
+Executable Go API examples may live beside the relevant package.
 
-ConfigKit has no owned HTTP service, application CLI, or runtime operations
-interface. Do not create those reference documents for consuming applications.
+Examples must be valid, secret-free, and tested where practical. Commands and
+configuration used in documentation should match the application.
 
-## Boundary rules
+### Security And Privacy
 
-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.
+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.
 
-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.
+## Canonical Ownership
 
-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.
+| Topic | Canonical owner | Owned content | Content owned elsewhere |
+| --- | --- | --- | --- |
+| Product orientation and minimal example | `README.md` | Purpose, current status, shortest useful Go example, and navigation, including licensing and attribution. | Full Go API and configuration contracts, release procedure, and implementation detail. |
+| Contributor entry point | `docs/development.md` | Task-oriented reading guide, contributor orientation, baseline validation commands, and canonical links. | Package inventory, architecture rules, subsystem behavior, detailed change recipes. |
+| Application architecture policy | `docs/policy/architecture.md` | System shape, normative ownership, dependency direction, boundaries, invariants, safety properties, and non-goals. | Concrete package inventory, implementation mechanics, contributor procedures, decision history, future work. |
+| Policy-set provenance and upstream navigation | `docs/policy/meta.md` | Policy-set origin, local authority, adoption baselines, and upstream adoption and update links. | Task-specific reading triggers, detailed adoption procedures, and formal attribution notices. |
+| Agent policy | `docs/policy/agents.md` | Proportionate, idiomatic implementation; rules against inventing requirements, restrictions, guarantees, or policy. | Application contracts, architecture, testing policy, and contributor procedures. |
+| 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 sufficiency, boundaries, doubles, coverage, regressions, and test maintenance. | Subsystem behavior, application contracts, subsystem-specific test inventories, and implementation plans. |
+| Release policy | `docs/policy/release.md` | Versioning, compatibility, release integrity and immutability, release documentation, proportionate gates, publication controls, completion, and required versus optional checks. | Repository-specific commands, product installation guidance, CLI version semantics, historical release summaries, and CI implementation detail. |
+| Open source licensing and attribution policy | `docs/policy/oss.md` | License defaults and author choices, COPYRIGHT and attribution formats, license/notice links, discoverability, maintenance, and concrete exceptions. | Component entries, license texts, release commands, and dependency security review. |
+| Project copyright and license grant | `COPYRIGHT` | Project identity, copyright holders and years, SPDX expression, code grant, separately licensed document scope, warranty disclaimer, and third-party statement. | Complete license texts and upstream component attribution. |
+| Project license texts | `LICENSE` | Complete project and incorporated companion license texts, with required project-specific fields. | General licensing policy and upstream component attribution. |
+| Upstream component attribution | `docs/oss/attribution.md` | House Format entries for dependencies and adapted documents: identity, material scope, copyright, license, notices, and source links. | General policy, exception reasoning, and complete standard license texts. |
+| Standard license texts | `docs/oss/licenses/` | Complete, verbatim standard license texts shared by applicable entries, including copied documentation. | Project-specific upstream notices, attribution prose, and policy interpretation. |
+| Repository-specific release procedure | `docs/release.md` | Maintainer prerequisites, candidate preparation, validation and publication commands and automated guards, asynchronous deliverables, delivery status, optional consumer verification, and recovery. | General release requirements, product installation guidance, historical release summaries, and current compatibility contracts. |
+| Exported Go API | Go documentation beside declarations | Symbol contracts, errors, ownership, and lifecycle behavior. | Shared YAML semantics and complete examples. |
+| Configuration contract | `docs/config.md` | YAML syntax, composition, precedence, schema, defaults, resolution, validation, secrets, and explanation semantics. | Go symbol contracts, complete examples, and implementation detail. |
+| Consumer guidance for the public Go API | `docs/consumers/` | Task-oriented public Go API usage, minimal examples, and consumer responsibilities. | Go symbol contracts, configuration semantics, and internal implementation detail. |
+| Inventory of implemented internal packages | `docs/internal/overview.md` | Implemented packages and components, 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, collaborators, 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. |
+| Deferred scope | `docs/roadmap/deferred.md` | Deliberate exclusions and conditions for reconsideration. | Task execution and current behavior. |
+| Future implementation plans | Task-specific files under `docs/roadmap/` | Unfinished work, sequencing, acceptance, and progress; completed plans retire during release preparation. | Implemented behavior and ADR rationale. |
+| Release-note history | `docs/releases/` | One checked-in summary per source release under the procedure; the note at its immutable tag is the release record. | Current commands, behavior, contracts, and compatibility definitions. |
+| Complete examples | `examples/` and executable Go examples | Maintained copyable artifacts and Go API examples. | Configuration reference, symbol contracts, and prose explanation. |
 
-## Architectural decision records
+Create supporting documents for existing interfaces and responsibilities when
+substantive content is available.
 
-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.
+Do not create empty owners.
 
-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.
+## Boundary Rules
 
-## Maintenance and verification
+### Orientation
 
-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.
+Link among orientation, architecture, and internal documents; maintain each
+package or behavior description in its canonical owner.
+
+### Go API And Configuration
+
+Go declaration documentation owns exported symbol behavior. The configuration
+contract owns shared YAML and resolution semantics. Consumer guidance explains
+tasks and links to those contracts.
+
+### Releases
+
+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 record history
+at immutable tags. Maintain current behavior in its canonical documentation,
+update it with the corresponding behavior changes, and keep release procedures
+consistent with adopted policy.
+
+### Project Licensing And Upstream Attribution
+
+README and release documents link to the canonical licensing and attribution
+materials under the [OSS policy](oss.md). The release procedure owns URL
+construction and publication mechanics.
+
+### Contracts And Implementation
+
+Go API and configuration documents define externally observable behavior.
+Internal documents explain implementation and link to the owning contract.
+
+### Security Topics
+
+This policy owns what documentation and examples may contain. Architecture owns
+application security invariants. The configuration and public API contracts own
+secret-classification, bootstrap, and rendering behavior. 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. Routine choices need no ADR.
+
+## 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.
 
 Before completing documentation work:
 
-- 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.
+- 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.

docs/policy/meta.md

@@ -0,0 +1,27 @@
+# Policy Set Provenance
+
+**Base Policy Version:** 1.0
+
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/meta.md)
+
+## Origin And Local Authority
+
+This repository's policy set is adapted from
+[Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy).
+The adopted local policies govern work in this repository. Upstream revisions
+become local policy through deliberate adoption, preserving local decisions.
+Each policy's `Upstream Source` identifies its adopted baseline.
+
+## Adoption And Updates
+
+For policy adoption, upstream revision reviews, and updates, consult the
+[upstream adoption and update guide](https://gitea.maximumdirect.net/eric/policy/src/branch/main/docs/adoption.md).
+That guide and the reference example collection are maintained upstream and
+accessed through links. Keep discovery links pointed at the current upstream
+project and guide; keep adopted-source and example links pinned to commits.
+
+The local [developer guide](../development.md) routes tasks to applicable
+policies. Formal credit and licensing notices for the copied policy material
+belong in the local [attribution document](../oss/attribution.md).

docs/policy/oss.md

@@ -0,0 +1,217 @@
+# Open Source Licensing And Attribution Policy
+
+**Base Policy Version:** 1.8
+
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/oss.md)
+
+## ConfigKit Applicability
+
+ConfigKit's selected grant is LGPL-3.0-or-later, recorded in
+[COPYRIGHT](../../COPYRIGHT), with the complete LGPLv3 and incorporated GPLv3
+texts in [LICENSE](../../LICENSE). The [release procedure](../release.md)
+owns source-only Go module publication through Gitea. No separate hosted release
+object or license asset is required for this channel.
+
+## Purpose And Scope
+
+This policy defines project licensing conventions and a repeatable House Format
+for upstream attribution and license notices. It covers the project's own code
+and upstream components incorporated into or distributed with the application
+or library, including relevant transitive dependencies and copied code,
+documentation, or assets.
+Determine treatment from the component's terms and actual use.
+
+Consulting development guidance or using tooling alone does not require an
+attribution entry. Copying or adapting documentation for inclusion in the
+repository is different: preserve its applicable attribution and license terms.
+A material's role as development guidance does not exempt copied documents.
+Apply any express permissions for attribution-free reuse of example formatting.
+
+The baseline is a publicly available source repository with releases published
+alongside it. The format is independent of language, forge, and build system.
+It does not require attribution for every development tool, establish license
+compatibility, or resolve every obligation arising from use or distribution.
+
+## Canonical Locations
+
+The [documentation policy](documentation.md) assigns ownership:
+
+- `COPYRIGHT`: project name, copyright statements, SPDX expression, grant,
+  warranty disclaimer, and third-party statement.
+- `LICENSE` contains the complete, unmodified project license text, including
+  incorporated companion text. Dependency attribution does not relicense the project.
+- `docs/oss/attribution.md`: canonical component entries.
+- `docs/oss/licenses/`: complete, verbatim standard and applicable companion texts.
+
+Use recognizable license filenames, such as `Apache-2.0.txt` or `LGPL-3.0.txt`.
+A shared standard text may serve multiple entries. Keep explanations outside
+license texts and preserve distinct versions and exceptions.
+
+## Upstream Reference Examples
+
+Use the commit-pinned upstream examples to create actual project notices.
+Retain their links and keep the example collection upstream; keep applicable
+license texts locally as specified above. The
+[standard license-text collection](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/oss/licenses/README.md)
+identifies the available reference texts.
+
+## Project Licensing
+
+Unless the author specifies otherwise, use **GPL version 3 or later**
+(`GPL-3.0-or-later`) for applications and **LGPL version 3 or later**
+(`LGPL-3.0-or-later`) for libraries. An author's explicit choice takes precedence:
+for example, a one-off script may use MIT. Such a choice requires no exception
+rationale, additional approval, or compensating controls under this policy.
+
+These defaults guide new license selection. Adopting this policy does not
+relicense existing code or override third-party terms. Preserve an existing
+license unless the author deliberately changes it. For mixed-purpose projects,
+follow the author's selected scope rather than inferring different licenses for
+individual packages.
+
+Use the upstream [application COPYRIGHT example](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/examples/licensing/application/COPYRIGHT)
+or [library COPYRIGHT example](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/examples/licensing/library/COPYRIGHT)
+to create the project’s root `COPYRIGHT` file.
+Use verified project names, contribution years, and copyright holders.
+For another selected license, adapt the grant, SPDX expression, disclaimer, and
+text references together to reflect the author's selected grant.
+
+For GPLv3 projects, copy the complete GPLv3 text into `LICENSE`. For LGPLv3
+projects, place the complete LGPLv3 text followed by the complete GPLv3 text in
+`LICENSE`; the LGPL incorporates the GPL terms. Keep the texts unmodified and
+place project-specific statements in `COPYRIGHT`. For the default grants, the
+grant and SPDX expression in `COPYRIGHT` must expressly identify `or-later`;
+follow the author’s choice if a different grant is selected.
+
+For other licenses, provide their complete text and any required project-specific
+fields; the standard reference collection remains unchanged. Keep `COPYRIGHT`,
+`LICENSE`, SPDX expressions, and README descriptions consistent with the actual
+grant. This convention introduces no blanket requirement for per-file headers;
+preserve notices or add them where the applicable terms require them.
+
+Identify separately licensed documentation or assets in the project grant, with
+links to their attribution and license information. An application's default
+code license does not override the terms of incorporated documentation.
+
+## Upstream Attribution House Format
+
+Begin `docs/oss/attribution.md` with `# Open Source Attribution Notices`,
+followed directly by component entries. These comprise the complete document.
+
+Each entry presents these five fields in order, with blank lines between them:
+
+1. **Project name** as a level-two heading. An optional identification paragraph
+   may follow, such as `Selected module: MODULE at VERSION.` with the module and
+   version in backticks. For copied documentation, a concise `Material:` sentence
+   may identify the affected files and whether they were adapted or copied
+   without modification. A factual qualifier such as `(transitive dependency)`
+   is permitted. Limit optional identification prose to factual component and
+   material scope.
+2. **Upstream copyright statement or statements**, preserving their wording.
+   Put multiple statements in separate paragraphs. Include this field when the
+   selected upstream source supplies an applicable statement; otherwise, omit it.
+3. **SPDX license identifier or expression**, when applicable, using
+   `SPDX-License-Identifier: EXPRESSION`.
+4. **License and notice links**, beginning `License and notices:`, separated
+   by ` | `, and ending with a period. Label local standard texts by license
+   name, such as `MIT` or `Apache 2.0`. Label upstream files by their actual
+   filenames, such as `COPYRIGHT`, `LICENSE`, `NOTICE`, or `PATENTS`.
+5. **Source repository link**, rendered exactly as `[Source](COMMIT_URL).`.
+
+Use the upstream [attribution example](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/examples/licensing/attribution.md)
+to create the project’s `docs/oss/attribution.md`.
+The example illustrates omitted fields and multiple copyrights and licenses.
+Replace the uppercase placeholders and illustrative URLs with actual component
+information. Local license links are relative to the destination
+`docs/oss/attribution.md`; copy the applicable texts into its `licenses/`
+directory. Do not copy example entries for components the project does not use.
+
+These are five fields, not a line limit. Omit the SPDX field when no applicable
+identifier exists; identify nonstandard terms through the linked text. Preserve
+licensing alternatives and exceptions, and record a selected option when
+relevant through the factual license identification. Do not invent an identifier
+or silently resolve an unsettled licensing choice.
+
+Keep entries factual. Do not add agent-authored explanations of license
+expressions, file-level license scope, research performed, retained source
+notices, or compliance conclusions. Avoid filler such as "none is invented
+here" and embellished link labels such as "upstream LICENSE", "additional
+patent grant", or "at the imported commit". Preserve actual required upstream
+content; this rule excludes editorial additions, not required notices. Policy
+interpretation and exception reasoning belong in the adopted OSS policy or an
+appropriate decision record, not the attribution document.
+
+## License Texts And Upstream Links
+
+Link standard license texts to their local copies. Link project-specific
+notices to their upstream copies. Every upstream repository or notice link must
+identify the exact commit corresponding to the
+imported version, including the selected source revision for copied material.
+
+Classify material according to its contents. An upstream `LICENSE`,
+`COPYING`, or similar file may contain project-specific notices or exceptions
+in addition to standard wording. Link both the local standard text and the
+commit-specific upstream file when needed. Make companion license texts directly
+reachable from the entry.
+
+For project-specific license and attribution material, this policy accepts
+direct links to the applicable upstream texts as satisfying reproduction,
+inclusion, or copy requirements. This is the adopted house interpretation.
+Agents must apply it without adding local-copy requirements or seeking repeated
+approval solely because a license uses those terms.
+
+This interpretation concerns notice reproduction. Concrete obligations such as
+marking modified files or supplying corresponding source receive their own
+applicable treatment. Preserve required notices already present in incorporated
+source files; the central attribution entry does not authorize stripping them.
+
+## Discoverability
+
+End the project's README with two friendly statements linking to its actual
+grant in `COPYRIGHT` and the attribution document. ConfigKit's
+[README](../../README.md) contains the two required statements: one linking to
+the project grant and one linking to attribution.
+
+Each new checked-in release document and its hosted download page must end
+with links whose visible text is `COPYRIGHT | LICENSE | ATTRIBUTION`. Use absolute
+URLs to those files at the release's immutable tag or corresponding immutable source
+revision so the links work in both locations. This release navigation is distinct
+from upstream component links, which must use the imported commit.
+
+Reuse the checked-in release document as the hosted release body where supported.
+The release procedure owns concrete URLs and publication mechanics for delivering
+the links through the selected channel. Do not maintain duplicate notice lists.
+These conventions do not by themselves require separate downloadable license
+assets, source bundles, or a different binary package format.
+
+## Maintenance And Exceptions
+
+Update project licensing statements together when the author changes the grant
+or its scope. Update upstream notices in the change that adds, removes, upgrades,
+copies, or changes the distribution of a relevant component. Check the selected source's license
+and notices rather than assuming they are unchanged. Update affected entries,
+commit links, and texts; retain historical versions through Git.
+
+Check affected statements, faithful standard-text copies, and link destinations
+as part of ordinary change review. For a future release tag, check intended file
+paths against the candidate source; the tag need not resolve remotely before
+publication. Do not add publication-time network checks, CI polling, recurring
+legal sign-offs, mandatory scanners, or a full software bill of materials merely
+to implement this policy.
+
+Use the House Format routinely. Raise an exception only for a concrete obligation
+or unresolved rights question it does not address, including any arising from a
+particular distribution channel. Record the issue and selected treatment in the
+adopted OSS policy or an appropriate decision record. Attribution entries
+contain the resulting factual identification and required material, without
+repeating that reasoning. Resolve the issue before the affected use or
+distribution without reopening settled choices.
+Common ownership of projects is neither required nor a blanket exception.
+
+Apply the formatting convention prospectively. Do not rewrite published tags,
+notes, binaries, or checksums to retrofit it. Address actual omissions deliberately
+under the applicable terms and the [release correction policy](release.md#immutability-and-correction).
+Ordinary attribution maintenance and release links are preparation work, not new
+release approval gates.

docs/policy/release.md

@@ -1,41 +1,39 @@
 # Release Policy
 
-**Base Policy Version:** 0.1.0
+**Base Policy Version:** 1.6
 
-**Last Updated:** 2026-09-12
+**Last Updated**: 2026-10-09
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/docs/policy/release.md)
 
 ## Purpose
 
-This policy defines the minimum requirements for selecting, validating,
-publishing, verifying, and correcting releases. Its goal is to make every
-release intentional, reproducible from an exact source revision, documented
-for consumers, and safe from retrospective replacement.
+This policy defines a proportionate workflow for validating and publishing an
+intended source revision under a fresh version. Releases should require little
+routine intervention, avoid repeated work, and preserve published contents.
 
-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.
+Adopt stronger verification or reproducibility requirements through an explicit
+project-specific decision justified by the intended audience and support needs.
 
-## ConfigKit applicability
+The repository-specific commands belong in `docs/release.md`. Historical release
+summaries belong in `docs/releases/`. The procedure must agree with the adopted
+policy and link to it rather than introduce additional release requirements.
 
-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.
+## ConfigKit Applicability
 
-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.
+ConfigKit publishes a source-only Go module. Its declared deliverables are the
+immutable annotated source tag and version-matched checked-in release note.
+Standalone binaries, container images, hosted release objects, and separate
+registry publication are not required. Additional channels require a deliberate
+update to the policy and procedure.
 
-The Go API, documented YAML/resolution semantics, and structured diagnostic
-contracts all participate in compatibility review. A signature-preserving change
+The Go API, documented YAML and resolution semantics, and structured diagnostic
+contracts 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.
+contract expressly promises it. Minimum Go version and platform support
+changes also require an explicit compatibility assessment.
 
-### Platform support
+### Platform Support
 
 Linux is the release-validated platform. macOS has best-effort support: keeping
 it working remains desirable, but neither native macOS validation nor Darwin
@@ -44,49 +42,67 @@ does not by itself block a release. Voluntary checks and fixes are welcome; ther
 is no guarantee that each release has been validated on macOS.
 
 The [release procedure](../release.md#release-identity-and-support) owns target
-architectures and required Linux checks. This support change applies to upcoming
-releases; historical release notes and their validation records remain unchanged.
-The v0.3.0 release note must explain the change in Compatibility, with version
-selection following this policy's existing compatibility rules.
+architectures and required Linux checks. Historical release notes and their
+validation records remain unchanged.
 
-## Release Model
+## Release Model And Completion
 
-A release consists of:
+A source release identifies one exact committed source revision through an
+immutable version tag and a version-matched, checked-in release note. The
+primary release branch is `main` unless the adopted policy specifies otherwise.
+The procedure must name the authoritative publication remote.
 
-- one exact source commit reachable from the repository's primary branch;
-- one immutable version tag identifying that commit;
-- one version-matched, checked-in release note; and
-- every artifact, image, package, registry entry, or other deliverable the
-  repository declares for that release.
+A successful push of the intended tag, with its source commit available on the
+authoritative primary branch, completes the release. The source commit may be
+pushed before the tag or published together with it through an atomic push.
+The procedure must choose and document the supported approach.
 
-The repository's primary branch is `main` unless an adopted policy documents
-an exception. A release candidate must be committed and pushed to that branch
-before its release tag is published.
+ConfigKit uses sequential publication: first push the committed candidate to
+authoritative `main`, then invoke ReleaseKit once to validate that candidate and
+publish its annotated tag. The [release procedure](../release.md) names
+`origin`; ReleaseKit checks its exact effective push URL before validation and
+tag publication.
 
-The source tag is the authoritative release identity. Published deliverables
-must be traceable to the tagged source and must agree with its version. A
-repository may publish source only. When it publishes additional deliverables,
-its release procedure must enumerate them and define how each is built,
-identified, published, and verified.
+Release commands must finish at successful publication. They must not wait for
+or poll CI, require successful CI status, inspect downloadable artifacts, or
+check consumer installation or Go module availability as completion gates.
+A failed or ambiguous push requires resolving the authoritative remote state
+before declaring success or retrying; successful pushes do not require a
+separate remote read-back gate.
 
-Release automation may validate or publish a release only when its exact
-responsibility is documented. Automation must not silently change the source
-candidate, move a published tag, replace a published deliverable, or repair a
-release in place.
+Remote tag read-back and fresh consumer retrieval are optional and non-gating
+after successful publication. A failed push receives one bounded check of the
+authoritative ref; do not poll or blindly retry.
+
+Repositories may publish source only. Additional binaries, images, packages,
+checksums, or other declared deliverables may be published asynchronously from
+the tagged source. The procedure must identify those deliverables and the
+responsibility of their automation. Published deliverables must be traceable
+to that source and identify the same version.
+
+ConfigKit currently declares no additional deliverables. Confirmed publication
+of the intended source tag is the completion point. CI, macOS validation,
+hosted delivery, and consumer retrieval are outside the release gates.
 
 ## Versioning And Compatibility
 
 Stable releases use Semantic Versioning in the form `vMAJOR.MINOR.PATCH`, with
-no leading zeroes. Each version must be unused locally and in every configured
-remote or publication channel before publication begins.
+no leading zeroes. Prereleases require an explicit adopted policy defining
+their format and use.
 
-Unless an adopted policy explicitly supports and defines prereleases,
-prerelease versions are not permitted.
+ConfigKit does not support prereleases.
+
+The selected version must be unused locally and on the authoritative remote.
+Do not require queries to every configured remote or publication service.
+A version known to have already been published must not be reused, even if the
+normal guards do not inspect that channel. Adding a distribution channel
+requires a deliberate update to the policy and procedure where needed.
 
 For releases at or after `v1.0.0`:
 
 - a major release may make intentional incompatible changes;
-- a minor release may add backward-compatible behavior; and
+- a minor release may add backward-compatible behavior or raise the Go consumer
+  minimum as described below; and
 - a patch release may make backward-compatible fixes.
 
 Before `v1.0.0`, a minor release may change a documented public, operational,
@@ -94,146 +110,211 @@ configuration, data, integration, or compatibility contract when the release
 note explains the impact and required consumer action. A patch release must
 not intentionally break those contracts within its minor line.
 
-A repository with additional compatibility promises must define them in its
-adopted policy or canonical contract documentation. A release note may explain
-a compatibility change, but it must not redefine the current compatibility
-contract.
+Additional compatibility promises and deliberate exceptions belong in the
+adopted policy or canonical contract documentation. Release notes explain
+changes; they do not redefine current compatibility contracts.
 
-## Release Documentation
+## Consumer Minimum And Release Toolchain
 
-Every release made under this policy must include
-`docs/releases/<version>.md` in the tagged source tree. The filename must use
-the complete version tag, and the document must begin with a title containing
-the product name and that exact version.
+Distinguish the minimum toolchain supported for consumers from the toolchain
+maintainers use to build and validate a release. Record the consumer minimum in
+the language's standard metadata and explain its support meaning in canonical
+documentation. Define the selected release toolchain and its setup commands in
+`docs/release.md`. Toolchain selection and upgrade cadence are repository-specific
+choices.
 
-Every new release note must contain these sections:
+For Go repositories, normally declare the initial stable release of the selected
+series as the consumer minimum, such as `go 1.26.0`. Use a higher patch minimum
+when a dependency or an identified correctness fix requires it, and document
+the reason.
 
-```markdown
-# <Product> vMAJOR.MINOR.PATCH
+For release-policy purposes, successful build and release validation on the
+selected Go patch release are sufficient to presume support across that Go
+series, subject to the declared consumer minimum and concrete evidence of
+incompatibility. Investigate specific compatibility concerns as they arise.
 
-This release ...
+Use the selected release toolchain for candidate validation and release builds,
+including CI jobs performing those tasks. A newer validation patch version alone
+leaves the consumer minimum unchanged.
 
-## Summary
+A minor release may raise the Go consumer minimum, including from one Go series
+to the next, both before and after `v1.0.0`. This is an explicit exception to the
+backward-compatibility requirement for minor releases. Patch releases preserve
+the consumer minimum. Explain an increase's impact and required consumer action
+in the release note. Other incompatible changes remain subject to the versioning
+rules above.
 
-## Compatibility
+### ConfigKit Toolchain Selection
 
-## Upgrade
+For minor and major releases, select the newest stable Go series available in
+the development environment. For patch releases, retain the series recorded in
+`go.mod` and its consumer minimum. Use the latest locally available compatible
+patch in the selected series, with `GOTOOLCHAIN=local`, to build the
+module-selected ReleaseKit CLI and validate the candidate. The same Go
+executable builds the tool and runs its validation commands.
 
-## Changes
-```
+## Release Documentation And Preparation
 
-Use an explicit statement such as "No action required" when a section has no
-substantive content.
+Every new release must include `docs/releases/<version>.md` in the tagged
+source tree. Use the complete version tag as the filename and begin with a
+title containing the product name and that exact version.
 
-Release notes are immutable historical summaries. They should link to current
-canonical documentation for exact behavior. Every affected canonical document
-must be updated in the same candidate; a release note must not substitute for
-current documentation or defer required documentation changes.
+ConfigKit requires the first line `# ConfigKit <version>`. ReleaseKit's
+publication guard checks the release-note H1 for the product and version.
 
-## Candidate Preparation And Validation
+Include a short summary and any relevant compatibility impact or upgrade
+instructions. Choose sections suited to the release and keep the note
+proportionate to the change.
 
-Each repository must provide one documented release-check command or script
-that exercises the complete set of automated source-candidate gates. The
-repository-specific procedure must state all additional manual gates.
+Keep exact current contracts in their canonical documents and update affected
+documentation with the behavior change. Link to those documents from the note
+where useful. If annotated tags are used, derive their summary from the
+checked-in note.
 
-The required gates are:
+ConfigKit uses the complete checked-in note as the annotated tag message.
 
-- dependency or module hygiene appropriate to the language and ecosystem;
-- the complete deterministic, offline default test suite;
-- race, concurrency, or equivalent safety checks where the implementation can
-  be meaningfully checked for those risks;
-- static analysis and linting used by the repository;
-- production builds for every supported target that can be built in the
-  release environment;
-- formatting and whitespace checks;
-- validation of maintained examples, configuration, schemas, generated
-  sources, and documentation commands where applicable;
-- review of changed local documentation links;
-- 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
-- any additional security, compatibility, migration, packaging, or platform
-  checks required by the product.
+Include the release footer defined by the [OSS policy](oss.md#discoverability)
+in each new release document. The release procedure defines the concrete URLs
+and publication mechanics. Prepare these links with the release note; they do
+not add a network check or approval gate. If a hosted download page is used in
+a future channel, preserve the footer there.
 
-Unavailable tooling or an inapplicable check must be addressed explicitly in
-the adopted policy or release procedure; it must not disappear silently from
-the candidate gates.
+Handle version selection, release-note content, and retirement of completed
+roadmaps during preparation. Preserve planned and deferred work and repair
+affected links when retiring roadmaps. Reuse ordinary change review for
+unintended files, credentials, and private data. Scope release-content review
+to the tagged source.
 
-Run the complete checker while preparing the candidate. Commit the release
-note and all associated code, tests, documentation, and configuration, then
-run the complete checker again against the exact commit intended for release.
-Do not publish a release from uncommitted or unvalidated content.
+## Candidate Validation
+
+Provide one publication command that validates the final committed candidate,
+checks publication state, and publishes the intended refs. A validation-only
+mode or command may be provided for diagnosis, but must not be a required step
+before publication repeats the same checks.
+
+ConfigKit uses the module-selected ReleaseKit CLI's `publish
+vMAJOR.MINOR.PATCH` command. Its `check` command is optional for diagnosis; it
+is not a required preliminary pass to publication.
+
+Run the complete deterministic, offline default test suite once for the final
+candidate, following the [testing policy](testing.md). For repositories that
+publish builds, verify the required build targets. Select those targets for
+the application's actual support commitments; native testing on each platform
+and access to particular hardware are not default release prerequisites.
+
+Prepare dependencies and build the tool before offline validation; the
+publication command is the required validation run, with no separate successful
+test or CI run required before it.
+
+The selected profile runs the complete deterministic, offline default suite
+once with the race detector and test-time vet disabled. The suite already
+includes examples, consumer fixtures, documentation links, and package
+boundaries. One full vet pass adds static checks beyond the test-time subset.
+CGO-disabled Linux amd64 and arm64 package compilation covers supported
+configurations distinct from native race compilation. macOS and newer-toolchain
+checks are optional.
+
+Additional gates must address a concrete risk that the existing checks do not
+adequately cover. Do not automatically require linting, race testing, module
+hygiene audits, documentation audits, credential scans, example validation, or
+expanded platform matrices solely because they are possible. Retain such
+checks when their distinct protection justifies their cost. A one-time migration
+check must not silently become a permanent release requirement.
+
+ReleaseKit runs its selected Go commands with readonly module mode and
+`GOPROXY=off`; acquire dependencies and build the tool before candidate
+validation. Formatting and whitespace remain ordinary development checks in
+`scripts/check.sh`.
+Review module tidiness when dependencies change, not as a release gate.
+
+Release tests must not require real credentials, mutable application services,
+paid APIs, provider probes, or inference calls. The procedure may describe dependency
+preparation separately from offline validation.
+
+Avoid duplicate validation across release phases. Successful checks for the
+same final candidate need not be repeated solely to satisfy another phase;
+any reuse must clearly apply to that exact candidate. Persistent validation
+caches are not required. If the candidate changes, validate the new candidate.
+Cheap publication-state checks may be repeated immediately before pushing
+because local or remote state may have changed during validation.
+
+Automate mechanical guards. Human review gates are strongly disfavored: add one
+only through an explicit project-specific policy decision explaining the
+concrete risk and why ordinary change review or automation is insufficient.
+Do not add routine confirmations for checks the command can perform itself.
+
+Compatibility review, release-note accuracy, completed-roadmap retirement, and
+review for unintended files or credentials remain ordinary preparation. The
+executing agent may perform them without routine human approval or separate
+manual evidence records. Publication itself requires authorization.
 
 ## Publication Guards
 
-Immediately before publication, the maintainer or release automation must
-fetch the current primary branch and tags from the authoritative remote and
-verify all of the following:
+Automate the checks needed to publish the intended source under a fresh version:
 
-- the version has the required format and is not already published;
-- the current branch is the primary release branch;
-- the worktree and index are clean;
-- the current commit is the recorded, validated candidate;
-- the authoritative remote branch points to that same commit;
-- the matching release note exists in that commit and has the required title
-  and sections;
-- all required approvals and automated gates succeeded for that commit; and
-- no local or remote tag already uses the selected version.
+- the version has the adopted format and is unused locally and on the
+  authoritative remote;
+- the worktree and index are clean on the primary release branch;
+- the source being published is the exact candidate that passed validation,
+  including its matching release note H1;
+- the remote primary branch can be advanced to that candidate without
+  discarding remote history, and is published before the tag; and
+- the tag identifies the validated candidate and only the intended refs are
+  published.
 
-Create the version tag against the guarded commit and verify its target before
-publishing it. Publish only the exact intended ref and deliverables. Broad
-operations such as `git push --tags` are discouraged because they can publish
-unreviewed refs unrelated to the release.
+ReleaseKit checks that `origin`'s effective push URL exactly matches the
+authoritative URL before candidate validation. This does not assert the
+configured fetch URL or require the release-note H1 to be the first line; the
+required `# ConfigKit <version>` first-line format remains a content rule. The
+command checks the candidate and unused version again after validation, and
+checks local state before creating the tag. Ignored local files do not block
+publication.
 
-The repository-specific procedure must make these guards executable or give
-copyable commands for performing them.
+Fetch the authoritative branch and tag state as needed for these checks.
+Do not force branch or tag updates to bypass a failed guard. Publish explicit
+refs rather than all local tags. Do not require CI success, publication-service
+availability checks, or a separate manual confirmation for each guard.
 
 ## Immutability And Correction
 
-Published version tags and published deliverables are immutable. Do not move,
-overwrite, recreate, or delete a published tag. Do not replace an artifact,
-image, package, checksum, signature, provenance statement, or release note
-under an already published version.
+Published version tags and published contents are immutable. Do not move,
+overwrite, recreate, or delete a published tag, or replace a published artifact,
+checksum, signature, provenance statement, or release note under that version.
+The checked-in note at the immutable tag is that release's historical record.
 
-If validation fails before publication, correct the candidate, rerun the
-complete gates, and repeat the publication guards. An unpublished local tag
-may be removed after confirming that it was never published or used by a
-release channel.
+If validation fails before publication, correct and commit the candidate and
+rerun the publication command. If a push fails or its outcome is ambiguous,
+inspect the authoritative remote before retrying. Remove an unpublished local
+tag only after confirming it was never published or used by a release channel.
+An unavailable remote leaves publication status unknown; do not overwrite a tag
+to resolve uncertainty.
 
-If publication or post-publication verification reveals a defect, leave the
-published release intact. Correct the defect on the primary branch, choose a
-new patch version or other semantically appropriate new version, write a new
-matching release note, and perform the complete release procedure again.
+Transient network, authentication, CI, or consumer-availability failures do not
+by themselves require a new version.
 
-A distribution channel's supported yank, deprecation, or advisory mechanism
-may warn consumers away from a defective release, but it must not replace that
-release's contents or reuse its version.
+A defect requiring changes to published source or contents requires a corrected
+commit, a new semantically appropriate version, a matching release note, and
+the release procedure again. Leave the original release intact. A channel's
+yank, deprecation, or advisory mechanism may warn consumers away from a defective
+release without replacing its contents or reusing its version.
 
 ## Repository Release Procedure
 
-Every adopting repository must maintain `docs/release.md` as the canonical
-maintainer runbook for implementing this policy. It must define, in execution
-order:
+Maintain `docs/release.md` as a short, executable maintainer runbook. It must
+identify prerequisites, the selected release toolchain, the primary branch and
+authoritative remote, version and note preparation, one publication command with
+local checks and guards, source-only deliverables, the successful-tag-push
+completion point, and partial or unknown-state recovery.
 
-1. maintainer prerequisites and required access;
-2. version selection and compatibility review;
-3. release-note creation;
-4. candidate preparation and the complete validation command;
-5. candidate commit, push, and exact commit recording;
-6. publication guards;
-7. exact tag and deliverable publication commands;
-8. authoritative remote and consumer-facing verification; and
-9. failure handling and correction.
-
-The procedure should use copyable, fail-fast commands where practical. It must
-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.
+Use copyable commands where practical. Keep optional diagnostics and consumer
+verification clearly optional. The procedure must not turn preparation work,
+ordinary change review, or post-publication investigation into additional
+release gates.
 
 ## Policy Exceptions
 
+The following historical exceptions do not change the current release workflow.
+
 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
@@ -246,6 +327,3 @@ For v0.2.0 only, the maintainer accepted the successful native macOS check at
 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

@@ -1,14 +1,16 @@
 # Testing Policy
 
-**Base Policy Version:** 1.0
+**Base Policy Version:** 1.3
 
-**Last Updated:** 2026-09-06
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/testing.md)
 
 ## Purpose
 
 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.
+We optimize for sufficient confidence in important behavior while minimizing friction on future development.
 
 ## ConfigKit-specific priorities
 
@@ -44,32 +46,19 @@ 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.
-
-A test must be:
-
-- written and reviewed;
-- understood by future maintainers and coding agents;
-- executed in local and CI workflows;
-- diagnosed when it fails;
-- updated when legitimate behavior changes;
-- maintained as fixtures, APIs, and dependencies evolve; and
-- removed or rewritten when it becomes redundant, brittle, misleading, or obsolete.
-
-Tests also create cognitive and architectural friction. They can constrain refactoring, duplicate policy, slow feedback loops, add noise to failures, and cause harmless implementation changes to require unrelated edits across the suite.
+Every test has an immediate and continuing cost: writing, review, execution,
+diagnosis, fixture maintenance, and updates when legitimate behavior changes.
+Tests can also constrain refactoring, duplicate policy, and add failure noise.
 
 A test is warranted only when the confidence it provides justifies these costs.
-
-Apply this cost-benefit analysis at two levels:
+Evaluate that tradeoff at two levels:
 
 1. **Per test:** What realistic defect does this test detect, how consequential would that defect be, and is that protection worth the test's lifetime cost?
 2. **Across the suite:** Does this collection provide materially more confidence than a smaller, simpler suite would?
 
-The preferred test suite is a **lean suite that provides sufficient confidence in the risks that matter, without redundant or low-value tests**. We seek sufficient confidence with the least unnecessary testing friction, not the fewest possible tests.
+Some friction is intentional. Tests should make dangerous changes—such as breaking compatibility, corrupting data, violating security boundaries, or reintroducing subtle bugs—require deliberate review. Keep ordinary internal changes inexpensive.
 
-Some friction is intentional. Tests should make dangerous changes—such as breaking compatibility, corrupting data, violating security boundaries, or reintroducing subtle bugs—require deliberate review. They should not make ordinary internal changes needlessly expensive.
-
-The cost of a test is not a reason to omit testing by default. Do not cite maintenance cost abstractly. When omitting a plausible test, be able to state why the protected failure is low-risk, already covered, obvious, reversible, or cheaper to detect elsewhere. For consequential, subtle, or difficult-to-observe behavior, the presumption should favor testing.
+Evaluate maintenance cost against the specific failure being protected. When omitting a plausible test, be able to state why the protected failure is low-risk, already covered, obvious, reversible, or cheaper to detect elsewhere. For consequential, subtle, or difficult-to-observe behavior, the presumption should favor testing.
 
 ## Default testing style
 
@@ -81,6 +70,8 @@ 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.
 
@@ -92,7 +83,7 @@ Control clocks, randomness, environment variables, and other process-global or m
 
 ## What deserves tests
 
-Prioritize tests for:
+To the extent the following features are present in the application, prioritize tests for:
 
 1. Public and package-level contracts.
 2. Domain rules and important invariants.
@@ -102,15 +93,27 @@ Prioritize tests for:
 6. Previously observed or plausible regressions.
 7. Representative integration and end-to-end workflows.
 
-A package-level contract is behavior relied upon by another package or major collaborator, not every observable detail of a package implementation.
+A package-level contract comprises behavior relied upon by another package or major collaborator.
 
-For behavior involving **data integrity, destructive operations, compatibility, security, concurrency, idempotency, or recovery**, presume that durable tests are required unless the behavior is already credibly protected at another layer.
+Choose tests for requested behavior and established contracts. Tests must not
+introduce new restrictions, guarantees, or threat scenarios merely because they
+could be asserted. Follow the [agent policy](agents.md) when deciding whether a
+behavior belongs to the contract; current implementation behavior alone is not
+sufficient authority.
+
+For established behavior involving **data integrity, destructive operations,
+compatibility, security, concurrency, idempotency, or recovery**, favor durable
+tests for realistic failures whose consequences warrant protection. Identify
+the failure and the existing contract it would violate. These categories call
+for attention, not an automatic test inventory or permission to strengthen the
+application's guarantees. Account for protection already provided at other layers.
 
 Do not add tests merely because a function, branch, or line exists. Do not add a test when the same meaningful risk is already adequately protected elsewhere.
 
 ## Choose the right test boundary
 
-Test through the narrowest stable boundary that expresses the behavior clearly.
+Choose the narrowest stable boundary that expresses the behavior clearly and
+provides durable confidence with minimal incidental coupling.
 
 This is often the package API, but it may instead be:
 
@@ -118,28 +121,15 @@ This is often the package API, but it may instead be:
 - a package-level operation when several internal collaborators jointly produce the behavior; or
 - a larger integration boundary when correctness emerges from interaction with a real dependency.
 
-Do not force all behavior through oversized end-to-end tests. Do not test every private helper merely because it exists. Choose the boundary that gives durable confidence with the least incidental coupling.
-
 ## Test behavior, not implementation
 
-A test should protect a decision, contract, or invariant—not memorialize the current implementation.
+A test should protect a selected decision, contract, or invariant.
 
-Before adding or retaining a test, ask:
+A test is suspect when it primarily detects changes to private constants or
+helpers, equivalent operation ordering, incidental formatting, equivalent
+algorithms, or internal structure that preserves behavior.
 
-> What realistic defect would this test catch?
-
-A test is suspect when its main purpose is to detect that someone:
-
-- changed an internal constant;
-- renamed or split a private helper;
-- reordered equivalent internal operations;
-- changed incidental formatting;
-- replaced one correct algorithm with another; or
-- refactored internal object structure without changing behavior.
-
-Refactoring should normally require no test edits unless the refactored structure is itself part of the contract.
-
-A test can be factually correct and still have negative value. Accurately describing current behavior is not enough; the protected behavior must be important enough to justify the future friction.
+Behavioral tests should normally remain stable through internal refactoring that preserves the contract.
 
 ## Expected effects of different changes
 
@@ -156,12 +146,13 @@ A test failing is not the same as a test needing to be edited. Many tests may co
 
 ## Separate mechanism from policy
 
-Configurable thresholds and defaults must not be duplicated throughout the test suite.
+Derive expectations from test-controlled configuration and relationships.
 
-For example, do not encode an internal concurrency limit indirectly:
+For this example, assume the selected contract rejects a request to start more
+processes than a configured limit. Do not encode its production default indirectly:
 
 ```go
-// Production policy:
+// Production default:
 const maxConcurrency = 4
 
 // Brittle test:
@@ -179,25 +170,30 @@ require.NoError(t, runner.Start(limit))
 require.ErrorIs(t, runner.Start(limit+1), ErrTooMuchConcurrency)
 ```
 
-The test should prove:
+For this selected contract, the test should prove:
 
 - the configured limit is accepted; and
 - one beyond the configured limit is rejected.
 
+Rejection is not a general requirement for concurrency control. If the selected
+contract queues excess work or limits active workers while accepting larger
+batches, test that behavior instead. Do not add rejection or a configurable
+limit solely to reproduce this example.
+
 The production default should be tested exactly only when its literal value is itself a public, operational, safety, protocol, or compatibility requirement.
 
-Apply the same rule to limits, timeouts, capacities, retry counts, and ranges: test relationships and behavior, not duplicated literals.
+Apply the same rule to limits, timeouts, capacities, retry counts, and ranges: test relationships and behavior.
 
-For concurrency limits, test both kinds of behavior when relevant:
+For established concurrency limits, distinguish these behaviors where the contract requires them:
 
-1. **Configuration enforcement:** invalid or excessive requested values are handled correctly.
+1. **Configuration handling:** values are accepted or rejected according to the selected contract.
 2. **Runtime enforcement:** observed peak concurrency never exceeds the configured limit.
 
-Use a test-controlled limit and measure the behavior relative to that limit. Do not merely assert today's default value.
+Use a test-controlled limit and measure the behavior relative to that limit.
 
-## Avoid semantic duplication across layers
+## Assign each behavior a test owner
 
-Each behavior should have a clear test owner.
+For ConfigKit, assign ownership as follows:
 
 - YAML boundary tests own syntax and policy enforcement.
 - Composition and overlay tests own their operation semantics.
@@ -209,9 +205,7 @@ Each behavior should have a clear test owner.
 
 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.
-
-Tests that are individually reasonable may still be collectively redundant. Evaluate the marginal value of each additional test in light of the protection already provided by the rest of the suite.
+Give each layer its distinct responsibility and keep expectation updates localized to the contract being changed.
 
 ## Use test doubles deliberately
 
@@ -232,104 +226,77 @@ Mocks are appropriate when the contract includes facts such as:
 - an expensive API is called no more than once; or
 - a security audit event is emitted.
 
-Do not use mocks merely to isolate every object or reproduce the implementation's call graph.
-
 ## Go-specific guidance
 
-Use:
+Select techniques by behavior and risk:
 
 - table-driven tests for meaningful behavioral categories and boundaries;
 - `t.TempDir()` for real filesystem behavior;
-- fuzz tests for parsers, normalization, path handling, and broad input spaces;
+- `httptest.Server` for realistic HTTP interactions;
+- fuzz tests when exploring broad input spaces, such as parsers or path handling, adds useful protection beyond representative cases;
 - golden files only when the complete output is intentionally stable;
 - integration tests where correctness depends on component interaction; and
-- a small number of representative end-to-end tests.
+- a small number of representative end-to-end tests when assembled workflows expose meaningful risks not adequately covered at narrower boundaries.
 
-Avoid exact error-string assertions unless the wording is itself contractual. Prefer `errors.Is`, `errors.As`, typed errors, or structured error fields.
+Prefer `errors.Is`, `errors.As`, typed errors, or structured error fields. Use exact error-wording assertions only when the wording 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.
 
-Keep tests readable and direct. Test helpers and fixture frameworks must earn their own maintenance cost; do not build elaborate test infrastructure for small or isolated needs.
+Keep tests readable and direct. Size test helpers and fixture frameworks to the maintenance benefit they provide.
 
 ## Coverage
 
-Coverage is a diagnostic, not a target.
+Use coverage to locate untested critical branches and unexpectedly weak packages. Evaluate test quality through meaningful behavioral protection and choose coverage according to risk.
 
-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.
-
-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.
+Pure domain logic will often warrant higher coverage than CLI wiring or external adapters. Uneven coverage is acceptable when it reflects risk.
 
 ## Regression tests
 
 A bug fix should normally include a regression test that fails before the fix and passes afterward.
 
-Retain the test when the defect could realistically recur and its consequences justify the ongoing cost. Prefer the narrowest durable test of the violated contract or invariant; do not preserve accidental implementation details from the original bug.
+Retain the test when the defect could realistically recur and its consequences justify the ongoing cost. Prefer the narrowest durable test of the violated contract or invariant.
 
-Not every historical bug requires a permanent test. If the underlying design has made recurrence impossible, the test has become redundant, or a stronger invariant test now subsumes it, remove or consolidate it.
+Remove or consolidate a test when the underlying design has made recurrence impossible, its protection is redundant, or a stronger invariant test subsumes it.
 
 ## Deleting or rewriting tests
 
-Tests are maintained code, not permanent historical artifacts.
-
-Delete or rewrite a test when its maintenance cost exceeds the confidence it provides.
+Delete or rewrite tests when maintenance cost exceeds their continuing
+protective value.
 
 Strong candidates include tests that:
 
 - require updates after harmless internal changes;
-- directly assert private constants without protecting a real contract;
-- duplicate the same policy across several layers;
+- assert incidental private constants or snapshot large amounts of incidental output;
 - verify mock choreography rather than outcomes;
-- snapshot large amounts of incidental output;
-- test trivial private helpers already exercised through stable package behavior;
-- protect risks already covered more effectively elsewhere;
+- duplicate policy across layers, retest trivial helpers covered by package
+  behavior, or protect risks covered more effectively elsewhere;
 - are flaky, misleading, obsolete, or disproportionately expensive to diagnose; or
 - no longer correspond to a plausible failure mode.
 
-Several brittle tests may encode one genuine requirement. Replace them with one durable behavior-level or invariant test rather than preserving all of them.
-
-Deleting a low-value test can improve the quality of the suite by reducing noise, maintenance burden, and friction around legitimate change.
+Several brittle tests may encode one genuine requirement. Consolidate them into one durable behavior-level or invariant test.
 
 ## Reviewing a proposed test
 
-Use the following questions when the value, boundary, or durability of a proposed test is not self-evident. Significant test additions should be reviewable against them, but written answers are not required for every routine test.
+Use the following questions when a proposed test needs closer evaluation. Significant test additions should be reviewable against them; written answers are optional for routine tests.
 
-1. What realistic defect would it catch?
-2. How likely is that defect?
-3. How consequential would it be?
-4. Is the behavior already protected elsewhere?
-5. At which layer should this behavior be owned?
-6. Does the test assert a durable contract or an incidental implementation detail?
-7. Could the implementation be refactored without changing the behavior and without editing this test?
-8. What should cause this test to fail?
-9. What legitimate changes should not cause this test to fail?
-10. What ongoing maintenance, execution, and diagnostic cost will the test impose?
-11. Is there a smaller or more direct test that protects the same risk?
-
-Do not add the test when its expected lifetime cost exceeds its expected protective value.
-
-When deciding not to test plausible behavior, record or be able to explain why the risk is low, already protected, obvious, reversible, or cheaper to detect elsewhere.
+1. Which requested behavior or established contract does it protect, and what realistic defect would violate it?
+2. How likely and consequential is that defect, and is it already protected elsewhere?
+3. What is the narrowest stable boundary that can test it clearly?
+4. Would a correct internal refactor or a change to an incidental default require test edits?
+5. Does its confidence justify its maintenance, execution, and diagnostic cost, or would a smaller test suffice?
 
 ## Definition of sufficient
 
 A test suite is sufficient when:
 
-- important contracts and invariants are protected;
-- meaningful boundaries and failure modes are exercised;
-- realistic and consequential regressions are credibly protected against silent recurrence;
-- behavior involving data integrity, destructive operations, compatibility, security, concurrency, idempotency, and recovery is credibly protected;
-- important external boundaries have realistic integration coverage;
-- representative complete workflows are tested;
-- failures provide useful signal rather than redundant noise;
-- legitimate internal changes usually do not require test edits; and
+- important requested behavior and established contracts are protected against realistic, consequential failures;
+- boundaries and component interactions have coverage where they introduce meaningful risks not protected elsewhere;
+- failures provide useful, distinct signals;
+- tests usually remain stable through legitimate internal changes; 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 library, its consumers, and the consequences of failure evolve.
-
-The governing rule is:
-
-> Test heavily where failure is consequential, subtle, or difficult to detect after the fact. Test lightly where failure is obvious, reversible, and inexpensive—and retain no test whose lifetime cost exceeds the confidence it provides.
+Reassess sufficiency as the application, its users, and the consequences of
+failure evolve.

docs/release.md

@@ -1,244 +1,237 @@
 # 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.
+ConfigKit publishes a source-only Go module. The immutable annotated tag and
+matching checked-in release note are the deliverables; there is no 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
-- Release-gated build targets: Linux on amd64 and arm64
-- Best-effort targets: macOS on amd64 and arm64, under the
+- Authoritative remote: `origin`
+- Authoritative push URL: `https://gitea.maximumdirect.net/eric/ConfigKit.git`
+- Tags: stable Semantic Versions such as `v0.5.0`; no prereleases
+- Minimum Go version: the first stable patch of the selected Go series, or a
+  higher patch when required by a dependency or specific correctness fix, as
+  recorded in `go.mod`; see the
+  [consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain)
+- Release-gated targets: Linux amd64 and arm64
+- Best-effort targets: macOS amd64 and arm64, under the
   [platform support policy](policy/release.md#platform-support)
 
-The automated checker cross-builds the Linux targets. Before release, the maintainer
-must also obtain native test, race, symlink, and permission-fixture evidence on Linux;
-cross-compilation is not native execution evidence. Native macOS checks and Darwin
-cross-builds are optional and do not gate publication.
+Release validation runs one offline race-enabled Go suite, one full vet pass,
+and CGO-disabled package compilation for both Linux targets. macOS and CI are
+not release gates; do not wait for or poll CI.
 
-## 1. Prepare access and tools
+## 1. Select Go and prepare the tool
 
-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:
+Use a clean checkout with Git, Bash 3.2 or newer, and a native compiler for race
+tests. Choose the release type before selecting Go. For a minor or major release,
+select the newest stable Go series available in the development environment.
+For a patch release, retain the series recorded in `go.mod` and leave its consumer
+minimum unchanged. Follow the
+[toolchain selection policy](policy/release.md#configkit-toolchain-selection).
+
+Put the selected series' latest locally available compatible patch first on
+`PATH`, ensuring it meets `go.mod`'s minimum. The same `go` executable must build
+ReleaseKit and run its child validation commands. Set local toolchain selection
+and verify it:
 
 ```sh
-git remote get-url origin
-git ls-remote origin HEAD
+export GOTOOLCHAIN=local
+go version
+go env GOVERSION
+command -v go
 ```
 
-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.
+Only when a minor or major release advances the consumer minimum to a newer Go
+series, derive that series' initial stable release from the selected executable:
 
 ```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
+selected_go=$(GOTOOLCHAIN=local go env GOVERSION)
+selected_series=${selected_go#go}
+selected_series=${selected_series%.*}
+release_minimum="$selected_series.0"
+go mod edit -go="$release_minimum"
 ```
 
-Those commands explicitly acquire toolchains and dependencies. The checker itself
-sets `GOPROXY=off` and fails if its required cache is incomplete.
+If a dependency or identified correctness fix requires a higher patch minimum,
+use that required version instead and document the reason. When the series is
+unchanged, leave the minimum unchanged unless a minor or major release
+deliberately changes it under the
+[consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain).
+For example, validating with Go 1.26.8 leaves `go 1.26.0` unchanged. There is no
+separate requirement to validate with Go 1.26.0.
 
-## 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.
+Prepare dependencies whether or not the minimum changes:
 
 ```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
+go mod download
 ```
 
-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:
+Build the tool selected by `go.mod` into temporary storage outside the checkout.
+Ensure `$TMPDIR` names a system temporary location, not the repository. These
+commands never install another ReleaseKit version:
 
 ```sh
-go list -m -versions go.yaml.in/yaml/v4
-go list -m -json go.yaml.in/yaml/v4@latest
+releasekit_tmp=$(mktemp -d "${TMPDIR:-/tmp}/configkit-releasekit.XXXXXX")
+releasekit_bin="$releasekit_tmp/releasekit"
+go build -o "$releasekit_bin" gitea.maximumdirect.net/eric/ReleaseKit/cmd/releasekit
+go list -m gitea.maximumdirect.net/eric/ReleaseKit
+"$releasekit_bin" version
 ```
 
-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.
+The module-selected executable is ReleaseKit v0.4.1. Use the built executable
+for publication to preserve its exit status and publication-state diagnostics.
+`go run` maps a nonzero child exit status to 1. ReleaseKit's child Go commands
+use the `go` found on `PATH` with `GOTOOLCHAIN=local`; keep the selected
+executable first on `PATH` through the publication command.
 
-## 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 on Linux 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 Linux cross-builds, formatting and whitespace (including untracked files),
-examples, package isolation, and local documentation links.
+Verify access to the authoritative remote without displaying credentials:
 
 ```sh
-./scripts/release-check.sh
+git ls-remote https://gitea.maximumdirect.net/eric/ConfigKit.git HEAD refs/heads/main
 ```
 
-`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.
+Publication requires explicit authorization. Local implementation commits do
+not authorize a push or release.
 
-Complete these manual gates and record their evidence with the candidate:
+## 2. Prepare and commit 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 a successful native checker run on Linux, including
-  filesystem capability tests. Missing required Linux evidence blocks release;
-  no macOS run, sign-off, or cross-build result is required.
-- 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.
+Choose an unused stable version locally and on the authoritative remote. Compare
+the candidate with the previous tag and review exported Go signatures and
+ownership, YAML and resolution semantics, diagnostic contracts, minimum Go
+version, and platform support. Review maintained consumer examples and the
+migration guidance. Explain any consumer-minimum increase and required consumer
+action under the
+[consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain).
 
-## 5. Commit, push, and record the candidate
+Create `docs/releases/<version>.md`. Its first line must be exactly
+`# ConfigKit <version>`, and it must end with the license and attribution footer:
 
-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)
+```markdown
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/docs/oss/attribution.md)
 ```
 
-If review changes the source after this point, create and validate a new candidate;
-do not reuse the recorded commit.
+Replace `VERSION` with the full tag and confirm the three target files exist in
+the candidate. Retire completed roadmaps, preserve unfinished and deferred
+work, and repair affected links. Review the complete candidate for unintended
+files and credentials, then commit it on `main`. These are ordinary preparation
+tasks, not separate approval gates. Any `go.mod` change belongs in this
+candidate.
 
-## 6. Run publication guards
+## 3. Push the candidate to `main`
 
-Immediately before tagging, fetch the authoritative branch and tags, restore the
-recorded variables if this is a new shell, and execute every guard:
+Push only that committed candidate to the literal authoritative URL. This does
+not follow local tags:
 
 ```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
+git push --no-follow-tags https://gitea.maximumdirect.net/eric/ConfigKit.git HEAD:refs/heads/main
 ```
 
-Confirm the recorded automated and manual approvals apply to exactly
-`$candidate_commit`. A successful older run is not evidence for a changed commit.
+ReleaseKit requires authoritative `main` to equal this candidate before it
+validates. A failure before this push means ReleaseKit publication has not
+started. If the push result is uncertain, inspect authoritative `main` and
+continue only when it is confirmed at the candidate. If validation later fails,
+no release tag is published; correct the issue in a new commit, push the new
+candidate to `main`, and invoke validation again.
 
-## 7. Tag and publish the exact ref
+## 4. Publish the committed candidate
 
-Create one annotated tag at the guarded commit, verify its peeled target, and push
-only that ref. Do not use `git push --tags`.
+With the authorized candidate already on authoritative `main`, invoke the
+module-selected native executable exactly once:
 
 ```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/$release_version"
+"$releasekit_bin" publish vMAJOR.MINOR.PATCH
 ```
 
-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.
+Replace the placeholder with the selected version. ReleaseKit checks the
+effective push URL against `expected_push_url`, verifies the candidate on
+`main`, and checks the release-note H1 for the product and version. It then
+runs the selected race-enabled standard suite once with test-time vet disabled,
+one full vet pass, and CGO-disabled package compilation for Linux amd64 and
+arm64. It publishes only the annotated tag for that candidate. Confirmed
+publication of the intended tag completes the release. The tag points to the
+captured commit and uses the complete checked-in note as its annotation message.
+The optional `check`
+command is available for diagnosis but is not a separate required pass:
 
 ```sh
-remote_tag_commit=$(git ls-remote origin "refs/tags/$release_version^{}" | awk '{print $1}')
-test "$remote_tag_commit" = "$candidate_commit"
+"$releasekit_bin" check vMAJOR.MINOR.PATCH
+```
+
+Status 0 confirms publication, status 1 reports a known publication failure or
+conflict, status 2 reports usage, profile, or version errors, and status 3 means
+the push outcome remains unknown. Preserve both the native exit status and its
+diagnostics.
+
+The command does not push or repair `main`, wait for CI, or publish binaries or
+a hosted release page. A successful intended-tag publication does not require
+remote read-back or consumer retrieval; those checks remain optional.
+
+## 5. Recover from a failed or uncertain tag push
+
+The executable reports the publication state, phase, version, candidate commit,
+tag-object ID, effective endpoint, and error on standard error. It performs one
+bounded authoritative reconciliation after a failed tag push. Exit status 3
+means the tag outcome remains unknown; do not blindly retry or move a tag.
+
+For an uncertain result, use the exact endpoint, candidate commit, version, and
+tag-object ID from the report. Copy the captured endpoint into the variable
+below using normal shell quoting. Check that Git still resolves it to itself,
+then query the authoritative refs once:
+
+```sh
+authoritative='COPY_CAPTURED_ENDPOINT'
+release_version='vMAJOR.MINOR.PATCH'
+test "$(git ls-remote --get-url -- "$authoritative")" = "$authoritative" &&
+  git ls-remote --refs -- "$authoritative" refs/heads/main "refs/tags/$release_version"
+```
+
+A failed URL-equality check or remote query leaves the outcome unresolved.
+Correct transport configuration before continuing; a rewritten destination
+cannot establish absence at the captured endpoint.
+
+Compare the `main` object ID with the reported candidate commit and the tag
+object ID with the reported tag-object ID. `--refs` returns the annotated tag
+object itself, not merely its peeled commit. If the tag object matches, the
+release is published. If the query fails, the result is conflicting, or the
+endpoint or candidate differs, stop and resolve the state before retrying. If
+the endpoint is reachable, the tag is confirmed absent, and `main` still equals
+the reported candidate, remove a retained local tag only after confirming it
+was not published or used by another channel; then rerun `publish`, which
+revalidates the candidate:
+
+```sh
+git tag -d "$release_version"
+"$releasekit_bin" publish "$release_version"
+```
+
+A validation failure after the candidate reached `main` leaves an untagged
+candidate there. Correct it with a new commit rather than rewriting or rolling
+back `main`. A conflicting published tag must never be moved or reused. A
+defect in published source requires a corrected commit, a new semantically
+appropriate version, and a matching release note.
+
+## 6. Optional consumer verification
+
+When useful or requested, read back the remote tag or retrieve the exact
+version into a new module without a local `replace` directive. These checks are
+optional and non-gating. Do not poll CI or wait for module propagation. For
+direct-origin verification, use the exact published version in a temporary
+consumer module:
+
+```sh
+release_version=vMAJOR.MINOR.PATCH
 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.
+The output should show the exact module path and selected version. Report this
+optional check separately from publication success.

docs/releases/v0.4.0.md

@@ -0,0 +1,56 @@
+# ConfigKit v0.4.0
+
+This release renames selected configuration profiles to overlays, intentionally
+breaking the previous naming contract without compatibility aliases or fallback.
+
+## Summary
+
+Selected configuration now uses `Options.Overlay` and `overlays/<name>.yml`.
+Selection, merge rules, input precedence, and lifecycle behavior are unchanged.
+See the [selected-overlay contract](../config.md#selected-overlays).
+
+## Compatibility
+
+This is a breaking minor release before v1.0.0. Four surfaces change:
+
+- Go API: `Options.Profile` becomes `Options.Overlay`, `LayerProfile` becomes
+  `LayerOverlay`, and all seven `CodeProfile*` constants become `CodeOverlay*`.
+- Selected files: `profiles/<name>.yml` becomes `overlays/<name>.yml`; there is
+  no legacy-directory fallback.
+- Provenance: the layer string changes from `profile` to `overlay`.
+- Structured diagnostics: the `profile.*` code prefix becomes `overlay.*`.
+
+The old Go identifiers are removed. Application-owned concepts such as
+`ModelProfile` and `model_profile` are unchanged. Ordinary explicit imports from
+a directory named `profiles` remain valid base inputs.
+
+The minimum Go version remains 1.25. Linux remains release-validated and macOS
+remains best-effort. The YAML dependency remains pinned to
+`go.yaml.in/yaml/v4 v4.0.0-rc.6`; this release does not change parser dependencies.
+
+## Upgrade
+
+Update the dependency:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.4.0
+```
+
+Rename the affected Go identifiers, move selected files into `overlays/`, update
+any handling of provenance-layer and diagnostic-code strings, and rebuild.
+The maintained example helper is now `LoadNarratioOverlay` rather than
+`LoadNarratioProfile`. Application CLI flag names remain application-owned;
+ConfigKit does not require renaming domain-specific profile concepts.
+See the [migration guide](../consumers/README.md#selected-overlay-migration).
+
+## Changes
+
+- Rename the selected feature consistently across the public facade, internal
+  engine, diagnostics, source labels, examples, and current documentation.
+- Add regression coverage for the new directory, absence of legacy fallback,
+  public values, explanations, and preservation of application profile fields.
+- Align contributor policies with the current shared standard.
+- Simplify release validation to one local minimum-toolchain check. CI is not a
+  release gate; successful publication of the intended tag completes the
+  source-only release. See the [release procedure](../release.md).
+- Retire completed overlay and release-workflow planning documents to Git history.

docs/releases/v0.4.1.md

@@ -0,0 +1,42 @@
+# ConfigKit v0.4.1
+
+This release formalizes ConfigKit's licensing and upstream attribution
+documentation without changing library behavior.
+
+## Summary
+
+ConfigKit now includes an explicit LGPL-3.0-or-later grant in COPYRIGHT,
+complete LGPLv3 and incorporated GPLv3 license texts, and centralized upstream
+attribution. README and release guidance provide navigation to those materials.
+
+## Compatibility
+
+No Go code, public API, configuration semantics, diagnostic behavior, or
+dependencies changed from v0.4.0. The minimum Go version remains 1.25, Linux
+remains release-validated, and macOS remains best-effort. The YAML dependency
+remains `go.yaml.in/yaml/v4 v4.0.0-rc.6`.
+
+The project grant is explicitly LGPL-3.0-or-later. Third-party material remains
+subject to its respective terms, recorded in the attribution document below.
+Published earlier releases are unchanged.
+
+## Upgrade
+
+Update the module dependency and rebuild to select this release:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.4.1
+```
+
+No application-code or configuration migration is required from v0.4.0.
+
+## Changes
+
+- Add the project COPYRIGHT statement and complete standard project license texts.
+- Document go-yaml attribution with exact upstream revision links and local
+  Apache-2.0 and MIT license texts.
+- Adopt the house OSS policy and connect licensing ownership and contributor guidance.
+- Add license and attribution navigation to README and future release documents,
+  without adding release gates or changing publication mechanics.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.1/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.1/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.1/docs/oss/attribution.md)

docs/releases/v0.4.2.md

@@ -0,0 +1,45 @@
+# ConfigKit v0.4.2
+
+This patch release simplifies and safeguards source publication and aligns
+maintainer policy and licensing guidance. Library behavior is unchanged.
+
+## Summary
+
+Maintainers can now publish a prepared release with one guarded command. It
+validates the exact committed candidate, checks the authoritative destination,
+and pushes only the candidate on `main` and its annotated version tag. The
+[release procedure](../release.md) explains preparation and partial-publication
+recovery; successful tag publication completes the source-only release without
+waiting for CI or consumer retrieval.
+
+## Compatibility
+
+No Go code, public API, configuration semantics, diagnostic behavior, or
+dependencies changed from v0.4.1. The minimum Go version remains 1.25. Linux
+amd64 and arm64 remain release-validated; macOS remains best-effort. The YAML
+dependency remains `go.yaml.in/yaml/v4 v4.0.0-rc.6`.
+
+## Upgrade
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.4.2
+```
+
+No application-code or configuration migration is required from v0.4.1.
+
+## Changes
+
+- Add guarded single-command publication with exact candidate, version, and
+  remote checks; note-derived annotated tags; and bounded failed-push recovery.
+- Keep one offline race suite, one full vet pass, and Linux amd64/arm64 builds
+  as release gates. Formatting and whitespace remain development checks;
+  dependency tidiness is reviewed when dependencies change.
+- Align adopted policies and contributor guidance with their pinned upstream
+  revision, and include the CC BY-SA 4.0 license text and attribution for
+  adapted policy material. The project grant remains LGPL-3.0-or-later.
+- Retire completed release-workflow and policy-alignment planning records while
+  preserving their history in Git. Deferred scope remains documented.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.2/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.2/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.2/docs/oss/attribution.md)

docs/releases/v0.5.0.md

@@ -0,0 +1,53 @@
+# ConfigKit v0.5.0
+
+This minor release adds source-backed layered configuration and built-in sibling
+inheritance, so applications can load one complete configuration while retaining
+presence, source context, and explainable derivation.
+
+## Summary
+
+- `LoadLayered` combines ordered underlay groups beneath the main input, selected
+  overlay, and explicit CLI overrides. Inputs within a group still compose
+  strictly; precedence tiers combine mappings recursively. The main input
+  remains the owner of the optional schema-version check.
+- `NewFilesystem(...).File(...)` accepts caller-owned `fs.FS` sources, and
+  `Mapping(...)` snapshots supported built-in values as a labeled source.
+  `Result.SourceContext` and `Resolution.SourceContext` expose passive resource
+  contexts for values from file-backed sources.
+- `extends` now resolves a mapping against one named sibling after source
+  precedence and before typed decoding. Inherited values count as explicit
+  input. Origins, history, explanations, and redacted rendering retain the
+  source and the structural path by which a value reached its destination.
+  Review corrections preserve immutable nested-history evidence and bounded
+  expansion work.
+
+See the [configuration contract](../config.md#layered-loading),
+[inheritance rules](../config.md#sibling-inheritance), and
+[consumer examples](../consumers/README.md) for current behavior.
+
+## Compatibility and upgrade
+
+This is an intentional pre-v1 incompatible minor release for configurations
+that use `extends` as ordinary data. That key is now reserved wherever an input
+mapping occurs; it is consumed before decoding. Rename any such YAML key and
+corresponding Go field, then rebuild. No collection registration or opt-in flag
+is available. Configurations without `extends` retain their existing meaning.
+
+Applications adopting the new features may move source-backed defaults into
+explicit `Underlay` groups, use `SourceContext` for relative resources, or
+replace application-owned sibling merging with one `extends` relationship per
+mapping. Adoption is optional. The maintained
+[named-preset example](../consumers/README.md#named-sibling-inheritance)
+shows inherited `false` and destination-local nested resolution.
+
+Select the new module version:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.5.0
+```
+
+The minimum Go line remains 1.25. Linux amd64 and arm64 remain the
+release-validated targets; macOS remains best-effort. The YAML dependency is
+unchanged at `go.yaml.in/yaml/v4 v4.0.0-rc.6`.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/docs/oss/attribution.md)

docs/releases/v0.6.0.md

@@ -0,0 +1,63 @@
+# ConfigKit v0.6.0
+
+This minor release reduces avoidable configuration-loading and inspection work,
+adopts ReleaseKit for source publication, and raises the consumer Go minimum to
+1.26.0. The public configuration model remains unchanged from v0.5.0.
+
+## Changes
+
+- Reduce temporary path allocations by sizing pointer-formatting buffers,
+  combining final metadata traversal, reusing parsed sensitivity paths, and
+  avoiding path construction when detecting inheritance. Effective and
+  historical metadata indexes retain independently owned paths.
+- Replace repeated resolution-coverage, metadata-update, CLI-overlap, and
+  sensitivity-propagation scans with private indexes and incremental processing.
+  Preserve diagnostic ordering, history, contextual path shapes, and redaction.
+- Make presence-only metadata queries avoid materializing provenance. Prepare
+  resolution replay descriptors and immutable mapping and inheritance-history
+  lookups for inspection, and use ordered membership indexes for provenance
+  collections. Returned records remain detached; explanations still replay
+  complete trees and render on demand.
+- Add a maintained public-API startup construction benchmark. The initial path
+  formatting and metadata traversal experiments reproduced allocation reductions
+  across local and downstream workloads. Later algorithmic improvements were
+  validated behaviorally without a new benchmark campaign. Busy-host timings
+  were inconclusive; no fixed speedup or allocation threshold is promised.
+- Replace ConfigKit-owned release scripts with a small profile and the
+  module-selected ReleaseKit v0.3.0 tool. The
+  [release procedure](../release.md) documents candidate preparation, tag-only
+  publication, and failed-push recovery. Existing release checks and the
+  successful-tag-push completion point remain unchanged; CI is not a gate.
+- Adopt release policy 1.6, distinguishing the consumer Go minimum from the
+  validation toolchain. Retire completed planning and measurement records into
+  Git history while preserving [deferred work](../roadmap/deferred.md).
+
+## Compatibility and upgrade
+
+The consumer minimum increases from Go 1.25.0 to **Go 1.26.0**. Install Go 1.26.0
+or newer before upgrading, and update application build and CI environments that
+still use Go 1.25. The `go.mod` directive records the consumer minimum; using a
+newer validation patch does not itself raise that minimum. See the
+[consumer minimum policy](../policy/release.md#consumer-minimum-and-release-toolchain).
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.6.0
+```
+
+No application-code or configuration migration is required from v0.5.0. Public
+Go signatures, strict within-tier composition, recursive tier precedence,
+sibling inheritance, presence/null distinctions, provenance, secret handling,
+and synchronous completion retain their existing contracts. See the
+[configuration contract](../config.md) and
+[consumer guide](../consumers/README.md).
+
+The YAML dependency remains `go.yaml.in/yaml/v4 v4.0.0-rc.6`. ReleaseKit is a
+declared Go tool dependency for maintainer commands, not a library runtime
+import; its requirements participate in ConfigKit's module graph. Linux amd64
+and arm64 remain release-validated targets, and macOS remains best-effort.
+Maintainers should use the current release procedure instead of the retired
+`scripts/release.sh` and `scripts/release-check.sh` commands.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/docs/oss/attribution.md)

docs/releases/v0.7.0.md

@@ -0,0 +1,58 @@
+# ConfigKit v0.7.0
+
+This minor release adds structured comparison of completed configurations.
+Applications can inspect changes between two configuration files or the
+effective impact of a selected overlay, with ConfigKit handling comparison and
+safe value disclosure while applications own presentation.
+
+## Changes
+
+- Add `Diff[T](left, right *Result[T]) ([]Difference, error)` through the public
+  facade. It compares frozen effective inspection values from successful loads,
+  including defaults and recorded application resolution, rather than source
+  text or later mutations of `Result.Config`.
+- Return detached changed-region records containing a semantic path, safe left
+  and right `ValueSnapshot` values, and `HiddenChanged`. Mappings compare
+  recursively, sequences report atomic changes, and records follow canonical
+  pointer order. Presence distinguishes additions and removals from explicit
+  null; equal effective values produce no records regardless of provenance or
+  loading history.
+- Protect both snapshots with sensitivity from both loads, including historical
+  and dependency-derived classification. Compatible regions retain public
+  siblings; incompatible structural replacements use complete-region masking
+  where required. `HiddenChanged` identifies actual changes withheld by masking,
+  rather than merely indicating that a snapshot contains redacted content.
+- Preserve visible public scalar changes when only historical sensitive
+  descendants remain, including additions and removals. Historical explanation
+  redaction and masking of incompatible present shapes remain intact.
+- Add maintained file-comparison and base-versus-overlay examples with
+  consumer-owned formatting, plus an executable public API example. See the
+  [consumer guide](../consumers/README.md#effective-configuration-diffs) and
+  [configuration contract](../config.md#effective-configuration-diffs).
+
+## Compatibility and upgrade
+
+No application-code or configuration migration is required from v0.6.0.
+Adopting the new API is optional. Loading, composition, inheritance, provenance,
+existing inspection, and secret-handling contracts remain unchanged.
+
+Load both configurations successfully before calling `Diff`; nil or
+uninitialized results return an error with no records. Comparison does not
+reopen sources, rerun callbacks, or reread environment variables. Typed secret
+references compare by configured reference name, not resolved credential bytes,
+so credential rotation behind an unchanged name is not reported. Returned
+snapshots describe changed regions, not cross-protected full-document context.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.7.0
+```
+
+The consumer minimum remains Go 1.26.0. The YAML dependency remains
+`go.yaml.in/yaml/v4 v4.0.0-rc.6`, and the maintainer tool dependency remains
+ReleaseKit v0.3.0. Linux amd64 and arm64 remain release-validated targets;
+macOS remains best-effort. Publication follows the existing
+[release procedure](../release.md).
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/docs/oss/attribution.md)

docs/releases/v0.7.1.md

@@ -0,0 +1,22 @@
+# ConfigKit v0.7.1
+
+This patch updates the maintainer release tool to ReleaseKit v0.4.1. ConfigKit's
+runtime behavior and public API are unchanged.
+
+## Compatibility and upgrade
+
+No application-code or configuration migration is required from v0.7.0. The
+consumer minimum remains Go 1.26.0, and the YAML dependency is unchanged.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.7.1
+```
+
+ReleaseKit v0.4.1 is a maintainer tool dependency and does not change consumer
+configuration or runtime behavior. Linux amd64 and arm64 remain
+release-validated targets; macOS remains best-effort. Publication follows the
+existing [release procedure](../release.md).
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.1/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.1/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.1/docs/oss/attribution.md)

docs/roadmap/deferred.md

@@ -11,18 +11,47 @@ General environment-variable configuration overlays are not supported. Typed sec
 references and explicit directory bootstrap are the environment convention. Literal
 credential configuration and implicit secret-directory discovery are unsupported.
 
-The initial release deliberately excludes:
+The following remain deliberately excluded:
 
 - automatic `conf.d` discovery;
 - remote imports, arbitrary providers, and additional configuration formats;
-- custom merge strategies, sequence merging, reset operators, and profile inheritance;
+- custom merge strategies, sequence merging, reset operators, multiple-parent
+  inheritance, and cross-scope references;
+- named overlay 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.
+- configuration 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.
+
+Effective configuration diffs are implemented over frozen successful-load
+inspections; their current comparison and disclosure contract is in the
+[configuration contract](../config.md#effective-configuration-diffs), with
+consumer workflows in the [adoption guide](../consumers/README.md#effective-configuration-diffs).
+Fingerprints remain deferred.
+
+Built-in sibling inheritance using `extends` is implemented as described in the
+[configuration contract](../config.md#sibling-inheritance). It is distinct from
+inheritance between selected overlay files, which remains excluded above.
+
+## Deferred performance experiments
+
+The completed performance audit left these internal optimization candidates for
+separate investigation:
+
+- branch-local explanation replay instead of the current whole-tree replay;
+- redesigning the private metadata-copy handoff while preserving detached public
+  records;
+- caching schema descriptors across loads; and
+- indexing complex contributor-trace membership while preserving its equality
+  rules and compact linked traces.
+
+These are not committed implementation work. Reconsider them with evidence that
+the benefit justifies the ownership, retained-memory, or equivalence risks. The
+completed audit and implementation evidence are preserved in Git history at
+`8e91a2b` under `docs/roadmap/`.

docs/roadmap/implementation.md

@@ -0,0 +1,580 @@
+# Implement configuration input destinations
+
+Status: complete. Stages 1–12 are implemented and audited. The
+[feature roadmap](input-destinations.md) owns the approved behavior and rationale; the
+[consumer request](promptkit-feature-request.md#public-acceptance-scenarios)
+supplies the public acceptance scenarios. This document owns execution and
+completion evidence. Planning baseline: `6353598`.
+
+## Working instructions
+
+- Start each stage with `AGENTS.md`, [development.md](../development.md), the
+  applicable policies, and working-tree inspection. Use graph-first code
+  discovery. Preserve unrelated changes and previously completed stages.
+- Implement only that stage and its supporting tests/documentation. Mark it
+  complete here with actual commands and evidence, then commit the nonempty
+  validated unit with a short plain-English message. Subsequent stages remain
+  planned. A stage is not complete solely because its narrow tests pass.
+- Keep early mechanics private until the complete engine behavior is ready.
+  Root destination fields land in Stage 10. Ordinary root loading must remain
+  working throughout. Do not publish a partially supported public placement API.
+- Follow the existing engine/document/YAML boundaries. Reuse strict composition,
+  recursive overlay, sibling inheritance, paths, history, and sensitivity.
+  No dependencies, toolchain changes, alternate mergers, mount registry,
+  per-fragment lifecycle, import-destination syntax, resource discovery, or new
+  release gates are needed. These are scope boundaries, not new prohibitions
+  on separately approved future work.
+- Update current-behavior documentation when public behavior lands; keep future
+  claims here or in the roadmap until then. Add the ADR in Stage 1 under the
+  documentation policy's accepted-decision exception.
+- Use focused offline tests while iterating; run `./scripts/check.sh` before
+  committing production changes. Documentation-only stages need whitespace and
+  local-link checks. Stage 12 adds full ownership/race validation. Tests protect
+  observable behavior, not wrapper allocation or helper-call counts. Reuse
+  coverage rather than repeat an entire loading matrix at every layer.
+- No benchmark campaign or public complexity guarantee is required. Prefer
+  straightforward bounded walks and wrapping each acquired contribution once;
+  avoid rescanning all preceding contributions for every new input.
+- Do not push, tag, publish, update downstream dependencies, or retire unfinished
+  records. Release compatibility review is separate from publication.
+
+## Resolved implementation decisions
+
+1. Add root-owned `Input.Destination Path` and `SourceInfo.Destination Path`,
+   mirrored privately in engine input/source records. Extend `loadInput` and
+   explicit root/engine conversion and cloning. No new loading entry point,
+   public wrapper type, option, or origin field is needed. Ordinary `Load`
+   continues to supply root placement.
+2. The zero path means root. Validate `Path.valid()` and reject every explicit
+   index segment, including a negative index. Normalize parsed tokens to literal
+   mapping-key segments: `/0` means the key `0`, not sequence element zero.
+   Empty, dot, dot-dot, slash-containing, tilde-containing, and numeric keys keep
+   existing semantic-path meaning. Do not split dots, interpret filesystem
+   traversal, enforce identifier grammar, or add namespace uniqueness.
+3. Reject destination segments equal to `configkit` or `extends`: placement
+   would introduce a reserved mapping key with a mapping value. Substrings such
+   as `myextends` are ordinary keys. This preserves the approved structural
+   rules rather than adding an application naming policy. Source-root import
+   directives are still processed before placement.
+4. Validate destination shape/reservations for all declared inputs during
+   grouped option preflight, before reading any source. Reuse
+   `CodeOptionsInvalid` for invalid destination declarations; include the
+   offending semantic path when representable, with no fabricated physical
+   location. An invalid source still follows existing source-option validation.
+   A valid destination on `NoSource` contributes no nodes, source record, or
+   budget charges. No extra error type or exact message contract is introduced.
+5. Separate acquired source syntax from application placement. Retain original
+   parsed source metadata for filesystem/directive handling. Strip source-root
+   directives, then wrap each application body at the normalized destination
+   before fragment version checks and strict composition. Imports inherit the
+   same destination transitively. Do not wrap both a source application and its
+   recomposed result; every retained application must already use effective
+   coordinates. Content roots remain mappings under existing source contracts.
+6. Wrapping a body at `Keys("a", "b")` creates `{a: {b: body}}` as private
+   document mappings. Actual body nodes and entries keep their physical source
+   coordinates. Introduced mappings/entries use the contributing document's root
+   location as a documented source fallback, not purported authored coordinates.
+   They have the same contributing source ID and create no synthetic source.
+   Retained history trees remain immutable and separate from assembly mutation.
+7. A present empty body creates explicit destination mappings, including an
+   import-only source whose application body is empty. It still cannot clear
+   lower mappings. A missing optional import and `NoSource` remain absent. Empty
+   source text remains the existing invalid document, not an empty mapping.
+8. `VersionRule.Path` stays an effective application path. Check main once after
+   its own imports compose and before underlays/overlay/CLI, at the existing
+   phase. Imported and underlay claims are checked against placed application
+   content, including scalar/null/sequence ancestors replacing a protected path.
+   A mapping ancestor with no protected descendant is not a version claim.
+   Mounted domain `version` fields are ordinary data. Existing inheritance,
+   selected-overlay, CLI protection and exact-path numeric conversion remain;
+   no nested blanket conversion or additional fragment checker is introduced.
+9. Placement does not alter physical source identity, filesystem namespace,
+   source IDs, acquisition order, imported-from locations, or resource contexts.
+   Repeated physical sources in one strict group still fail even at different
+   destinations. The same physical source in separate tiers retains the existing
+   independent source applications/records and each record's own destination.
+   Overlay and CLI source records have root `Destination`; they are not mounted
+   merely because main is mounted. Active layered overlays retain their explicit
+   independent filesystem selection and application-root meaning.
+10. Value-level paths use effective coordinates. Keep genuine source-local
+    import-directive diagnostics and parse/read failures at their original
+    locations; do not invent a logical child path when parsing cannot identify
+    one. Prefix existing application-relative diagnostic paths exactly once
+    where available. Preserve structured codes, related physical locations,
+    deterministic ordering, and value-free messages.
+11. Use current depth/node limits, not a new destination cap. With root depth
+    zero, each destination segment adds one level and, under `countInputNodes`,
+    one mapping node plus one key. Retain current raw input accounting (including
+    directives); charge only the additional `2 * segmentCount` synthetic nodes
+    per acquired body, once, to the shared input budget. Placement adds no bytes,
+    acquired sources, import nesting, or glob visits. Validate placed application
+    depth and per-document node bounds before composition; share accounting
+    across inputs and imports. Precheck wrapper depth/count before allocating
+    wrappers. Use existing source/import limit diagnostics in their acquisition
+    contexts; retain projection, expansion, and history bounds independently.
+    Root placement adds no charge or new rejection. NoSource adds none either.
+12. Cloned source records include independently owned destination paths at every
+    existing publication boundary: results, load errors, metadata/inspection,
+    callback input information, and root conversion. Passive resource contexts
+    remain keyed by the real source identity; inspection must work without an
+    open configuration handle after load. This does not extend caller-owned
+    filesystem or callback resource lifetimes.
+
+## Stage 1 — Record the decision and capture the baseline
+
+Status: complete. Scope: decision record and evidence, no feature code.
+
+1. Read the roadmap, request, relevant policies, and ADRs 0001, 0003, 0006,
+   0008, and 0009. Inspect the owners identified in the roadmap and the function
+   map below. Confirm the baseline and worktree before starting.
+2. Run `./scripts/check.sh`; record the commit, Go version, and results here.
+   Locate existing coverage for root loading, strict underlays, version ownership,
+   repeated sources, history, resource contexts, and detached records.
+3. Write `docs/adr/0011-input-destinations.md` after confirming the number is
+   unused. Record the approved input-placement decision as Accepted, with the
+   roadmap owning implementation status. Follow the repository ADR format.
+   Focus on ConfigKit, put rejected options only in Alternatives Considered,
+   and avoid negative parallelism elsewhere. No PromptKit-specific semantics.
+4. Record the compatibility assessment under the release policy: adding fields
+   preserves zero-value/keyed-literal use, but callers using unkeyed literals
+   for `Input` or `SourceInfo` must migrate to keyed literals. Treat the feature
+   as minor-release work; select a version and write its note only during
+   separately authorized release preparation.
+
+Validated: `./scripts/check.sh` (go1.26.8 linux/amd64) passed on candidate
+`d3d468a8b47d780787fca329ff65b107d98ca59f`; this includes formatting,
+whitespace, local documentation links, tests, vet, and build. The planning
+baseline `6353598bd6cce2eb78a5fe990e144cc0cadf1280` is present in history. The
+worktree was clean before this stage.
+
+Coverage located for later destination work:
+
+- Root and equivalent source loading: `TestEquivalentSourceFormsPreserveInputSemantics`
+  (`facade_equivalence_test.go`) and
+  `TestFacadeLayeredZeroUnderlaysMatchesOrdinaryLoad` (`facade_layered_test.go`).
+- Strict underlays and group budgets: `TestGroupedLoadStrictConflictsRepetitionAndBudgets`
+  and `TestGroupedLoadPrecedenceSourceOrderAndSingleLifecycle`
+  (`internal/engine/load_request_test.go`).
+- Version ownership and imported claims: `TestLoadRejectsImportConflictsAndFragmentVersions`
+  (`internal/engine/imports_test.go`) and version-path coverage in
+  `internal/engine/load_test.go`.
+- Repeated physical sources: `TestRepeatedImportsAcrossTiersAndReaderConsumption`
+  (`facade_equivalence_test.go`).
+- History and source contexts: `TestUnderlayHistoryRetainsLaterMappingContributors`
+  (`facade_underlay_origins_test.go`) and
+  `TestFacadeSourceContextsFollowOriginsWithoutQueryIO`
+  (`facade_resource_context_test.go`).
+- Detached records: `TestLoadMetadataReflectsDefaultsInputAndDetachedRecords`
+  (`internal/engine/load_test.go`) and
+  `TestMetadataQueriesReturnDetachedRecords` (`internal/engine/metadata_test.go`).
+
+The decision is recorded in [ADR 0011](../adr/0011-input-destinations.md).
+Compatibility assessment: adding the two public fields breaks callers using
+unkeyed literals for `Input` or `SourceInfo`; keyed and zero-value use remains
+compatible. The release policy permits the intentional pre-1.0 contract change
+in a minor release when the release note states impact and migration. Version
+selection and release-note preparation remain out of scope until separately
+authorized. No feature code or current behavior contract changed.
+
+## Stage 2 — Add private destination normalization and wrapping
+
+Status: complete. Scope: pure private mechanics with bounded unit tests.
+
+1. Add a small engine helper file for normalized mapping-only destinations and
+   constructing placed document bodies. Follow decisions 2–7; keep helpers
+   private and avoid introducing a second path abstraction.
+2. Validate invalid paths, explicit indices, and exact reserved tokens. Normalize
+   parsed numeric tokens into keys, preserving literal text and pointer escaping.
+3. Build wrappers without mutating the supplied tree. Preserve body locations;
+   give introduced entries/mappings the source-root fallback. Implement depth and
+   node measurement/prechecks usable with existing limits, including the extra
+   synthetic-node count; aggregate charging is wired in Stage 7.
+4. Cover root identity/equivalence, nested empty mapping, literal/escaped/numeric
+   keys, invalid index/path, exact versus substring reserved keys, owned paths,
+   unchanged body coordinates, and bounded wrapper checks. Do not count helper
+   invocations or fix an allocation strategy in tests.
+
+Validated with `go test ./internal/engine -run 'Destination|PlaceDocument'`
+and `./scripts/check.sh` (go1.26.8 linux/amd64); both passed. The private
+helpers normalize mapping-only destinations, preserve authored nodes and source
+coordinates, and check per-document depth/node bounds before allocating wrappers.
+No loader accepts a new public field yet.
+
+## Stage 3 — Integrate private non-import input placement
+
+Status: complete. Scope: engine request plumbing and ordinary acquired bodies.
+
+1. Add destination fields to engine `Input`, `loadInput`, and `SourceInfo`.
+   Preserve ownership through engine `LoadLayered` request construction,
+   `cloneSourceInfo`, and source-record copies. Keep the root facade unchanged.
+2. Extend grouped preflight to validate/normalize destinations before source
+   reads. Use the existing diagnostic accumulator and options-invalid category.
+   Ordinary `Load` supplies root placement through its existing request adapter.
+3. Thread placement into `assembleBaseInputTracked` without confusing raw
+   `parsedSource` with effective `assembledInput.document`. Preserve the root
+   path through existing wrappers/callers. For imports-disabled acquisition,
+   validate disabled directives on raw content, place the body, and retain the
+   destination on the acquired source record. NoSource still short-circuits.
+4. Use engine loads to cover mounted main and underlay bodies, omitted versus
+   explicit root destinations, empty mapping versus NoSource, and preflight
+   rejection without consuming a test-owned reader. Reuse current source-kind
+   acquisition tests; full source equivalence is verified publicly in Stage 10.
+
+Validated with `go test ./internal/engine -run 'Destination|PlaceDocument|Grouped'`
+and `./scripts/check.sh` (go1.26.8 linux/amd64); both passed. Private non-import
+placement reaches assembly, with root behavior unchanged. Imports are the next
+private integration step.
+
+## Stage 4 — Carry placement through import acquisition
+
+Status: complete. Scope: source-local directives and placed imported applications.
+
+1. Thread a normalized destination through `loadImportedDocumentsTracked`,
+   `loadImportedWithAccessTracked`, and `importTraversal`, retaining root defaults
+   for current private callers. Place the selected body's application content
+   after `splitImportDirectives`; do the same for every imported body.
+2. Set placement on selected/imported source records, including error records
+   once a source has been acquired. Keep `ImportedFrom` at the physical directive.
+   Run imported `fragmentVersionClaim` against placed bodies and retain placed
+   bodies in `state.documents` before `composeDocuments` builds applications.
+3. Leave filesystem joins, containment, glob traversal, optional missing inputs,
+   active cycles, visited identities, and acquisition IDs/order unchanged.
+   Destination must not become part of the repeated-source identity key.
+4. Test native and host transitive imports, nested source-root directives,
+   import-only bodies, and mounted imported protected versus ordinary version
+   paths. Verify a repeated file at two destinations in one group still fails,
+   including an import/direct-input alias. Reuse cycle/containment regressions.
+
+Validated with `go test ./internal/engine -run 'Destination|PlaceDocument|Import|Grouped'`
+and `./scripts/check.sh` (go1.26.8 linux/amd64); both passed. Retained imported
+contributions have effective placement and physical source identity, with no
+double wrapping or application-level directive leakage.
+
+## Stage 5 — Complete strict tiers and main version ownership
+
+Status: complete. Scope: assembly and version interactions, not a new merger.
+
+1. Review `acquireUnderlayGroup`: version checks must use placed content, and
+   documents taken from `acquired.composition.applications` must already be
+   placed. Recompose strict peers with ordinary `composeDocuments`; preserve
+   per-source applications, origins, UnderlayIndex, and input accounting.
+2. In `executeLoadRequest`, main's version check uses its placed main/import
+   composition at the existing point. `assembleLayeredInput` recursively overlays
+   completed underlay groups beneath main, selected overlay, and CLI as before.
+   Do not mount the final assembly or prepend main's destination to upper tiers.
+3. Test disjoint same-destination peers, different destinations, equal and unequal
+   conflicts, ancestor/descendant destinations, and a strict conflict despite an
+   upper override. Verify recursive mapping overlays, atomic sequences, null,
+   explicit false/zero, and version-only domain overrides retaining lower fields.
+4. Exercise mounted main with a nested VersionRule, absent root declarations,
+   imported/underlay direct and ancestor claims, and nonclaiming mapping ancestors.
+   Check the hook runs once and does not inspect each fragment independently.
+   Preserve exact configured-path integer conversion and ordinary nested typing.
+
+Validate: destination/composition/overlay/version/grouped tests and everyday
+check. Validated with `go test ./internal/engine -run 'LayeredDestination'`,
+`go test ./internal/engine -run 'Destination|Composition|Overlay|Version|Grouped'`,
+and `./scripts/check.sh` (go1.26.8 linux/amd64); all passed. The new mounted
+cases verify strict same-tier conflicts and disjoint composition, independent
+destinations and nested paths, recursive/atomic overlay behavior, explicit
+null/false/zero, and main-only nested version checking after imports compose.
+Done: placed and authored equivalents agree on values and conflicts; main
+ownership and lifecycle timing are preserved.
+
+## Stage 6 — Reconcile diagnostic coordinate systems
+
+Status: complete. Scope: useful paths without fictional source locations.
+
+1. Audit early acquisition, import, fragment-version, and composition failures.
+   Use one narrow path-prefix translation where diagnostics genuinely identify
+   application content; do not blanket-prefix every diagnostic or double-prefix
+   diagnostics already produced by placed trees.
+2. Preserve syntax/read/directive locations and related import sites. Value
+   conflicts and later typed/resolver/validation failures use effective paths
+   but original content coordinates. Invalid introduced structure identifies
+   the destination declaration rather than a nonexistent source wrapper line.
+3. Add focused assertions for a mounted conflict with two real sources, a mounted
+   decode or validation error, an imported version error, invalid destination,
+   and malformed source-local directives. Test stable codes/paths/locations and
+   nil results, not whole messages or callback choreography.
+
+Validate: destination/diagnostic/import/version/decode tests and everyday check.
+Validated with `go test ./internal/engine -run 'LayeredDestinationDiagnostics'`,
+`go test ./internal/engine -run 'Destination|Diagnostic|Import|Version|Decode'`,
+and `./scripts/check.sh` (go1.26.8 linux/amd64); all passed. The mounted
+conflict, decode, and imported-version assertions use effective application
+paths with authored source locations; invalid destinations have no physical
+location; malformed source-root directives retain their authored path and
+location. Later value diagnostics are produced from placed trees, while directive
+diagnostics retain their raw source coordinates. Post-review remediation also
+rebases known Mapping value-builder failure paths, which are produced before
+placement. `TestFacadeDestinationMappingFailurePaths` covers mounted main and
+underlay failures, escaped keys and sequence-index intent, cycles, unchanged
+root paths, and unchanged source-constructor failures. Its mounted value cases
+failed before the fix and pass afterward.
+Done: diagnostics consistently distinguish application paths from source syntax.
+
+## Stage 7 — Complete shared placement accounting
+
+Status: complete. Scope: resource bounds with no new public limit system.
+
+1. Wire Stage 2's prechecks and synthetic-node charges into both tracked ordinary
+   acquisition and import traversal, using `importAccounting` and current limit
+   structures. Charge each source body once, including empty import bodies.
+2. Preserve existing raw byte/source/node accounting and imported directive
+   inspection. Additional wrappers count in per-body depth/node checks and
+   shared aggregate nodes; recomposition must not charge them again. Ensure
+   selected overlays and CLI start from totals including placement charges.
+3. Test below/at/above relevant bounds using small private limit fixtures where
+   existing seams permit; add a narrow private seam if necessary. Cover deep
+   body plus destination, many shallow mounted inputs, imported wrappers, and
+   empty mappings. Verify root placement and NoSource do not add charges.
+   Preserve history/expansion limits; do not turn fixture values into defaults.
+
+Validated with `go test ./internal/engine -run 'Destination|Import|Grouped|Overlay|Override'`
+and `./scripts/check.sh` (go1.26.8 linux/amd64); both passed. New coverage in
+`TestDestinationPlacementCountsAgainstSharedNodeBudget`,
+`TestMultipleMountedInputsSharePlacementCharges`,
+`TestImportedEmptyBodiesChargeEachDestinationWrapper`,
+`TestSelectedOverlayAndCLIUsePlacementNodeCharges`, and
+`TestNoSourceAndRootPlacementAddNoWrapperCharges` checks totals below, at, and
+above the boundary, multiple shallow inputs, imported empty bodies, and charging
+of upper tiers from mounted inputs. `TestPlaceDocumentRootAndBounds` continues
+to cover per-document depth/node boundaries; existing grouped accounting tests
+cover unchanged raw source/byte charging. Done: placement cannot bypass current
+budgets and introduces no unrelated caps.
+
+## Stage 8 — Preserve provenance, history, and source resource contexts
+
+Status: complete. Scope: engine inspection and retained evidence.
+
+1. Audit `compositionOrigins`, `completeCompositionOrigins`, application replay,
+   metadata construction/finalization, and source-record cloning against placed
+   applications. Prefer feeding placed trees into existing processing rather
+   than adding another path-offset index or rewriting final metadata afterward.
+2. Verify explicit destination containers, actual leaves, composite origins,
+   superseded/removed descendants, and root/source fallbacks. Historical and
+   effective indexes keep independently owned paths and existing shape semantics.
+3. Preserve source-ID resource contexts through imports, surviving lower-tier
+   values, and inheritance. Test completed-result and resolution-time context
+   queries against real relative files, without making ConfigKit open the
+   referenced application resource or retain loading handles.
+4. Exercise the same physical source in separate tiers with different placement,
+   inspect each source record, and verify independent destination paths. Repeat
+   inspection after comparison and independent loads to detect retained mutation.
+
+Validated with `go test ./internal/engine -run 'Destination|Metadata|Provenance|History|Resource|SourceContext'`,
+`go test -race ./internal/engine`, and `./scripts/check.sh` (go1.26.8
+linux/amd64); all passed. `TestLayeredDestinationsRetainOriginsHistoryAndDetachedRecords`
+checks synthetic-container fallback locations, authored leaf coordinates,
+composite origins, removed-descendant history, and the same host file retained
+at distinct destinations in separate tiers before and after comparison and an
+independent load. `TestLayeredImportedInheritedValuesRetainResourceContexts`
+uses real temporary files to verify resolution-time and completed-result
+relative-resource lookup through an imported surviving parent and its inherited
+child. Existing metadata and resource-context tests continue to cover detached
+records and loading-handle closure. Done: inspection explains effective
+placement and physical origin without consumer reconstruction.
+
+## Stage 9 — Verify final inheritance, sensitivity, and lifecycle integration
+
+Status: complete. Scope: complete private engine behavior before facade exposure.
+
+1. Resolve sibling inheritance only after full source precedence using the
+   existing expansion path. Cover a child and parent supplied by separate peers,
+   nested destination-local siblings, cancellation, and an inherited relative
+   resource retaining its declaring context. Cross-collection search is unchanged.
+2. Reuse existing inheritance-version, overlay, and CLI guards; add targeted
+   mounted cases for actual protected paths and their ancestors. Do not reject
+   unrelated inheritance merely because it shares a mapping ancestor.
+3. Test effective-path secret predicates, field tags and recorded dependencies,
+   superseded classified values, safe effective output/history/explanation, and
+   bilateral diff redaction using disposable canaries. Compare mounted versus
+   explicitly wrapped loads for equal effective values and no differences.
+4. Verify defaults, decoding, resolution, secret lookup, and validation execute
+   once on the final shape; failures return no usable result. Reuse existing
+   ordinary-load lifetime/ownership coverage and add mounted protection where
+   placement creates a distinct risk.
+
+Validate: destination/inheritance/secret/sensitivity/explain/diff/lifecycle tests
+and everyday check. Validated with
+`go test ./internal/engine -run 'Destination|Inheritance|Secret|Sensitivity|Explain|Diff|Load|Version|Grouped' -count=1`,
+`go test -race ./internal/engine`, and `./scripts/check.sh` (go1.26.8
+linux/amd64); all passed.
+
+`TestLayeredDestinationInheritanceUsesFinalSiblingScopes` verifies parent and
+child contributions from separate peers, a higher-tier parent update, nested
+destination-local siblings, and cancellation. `TestLayeredDestinationInheritanceDoesNotSearchOtherCollections`
+keeps sibling lookup local. `TestLayeredMountedVersionProtectionCoversInheritanceAndUpperTiers`
+protects inherited values, scalar/null ancestors, consumed directives, selected
+overlays, and CLI overrides at mounted paths while allowing nonclaiming mapping
+ancestors and unrelated inheritance. The coverage exposed an ancestor gap in
+the existing inheritance-version check; it now applies the shared fragment claim
+logic to the transferred parent subtree.
+
+`TestLayeredDestinationLifecycleAndSensitiveInspectionUseFinalShape` checks one
+defaults/resolution/secret-lookup/validation lifecycle, final mounted decoding,
+field tags and effective-path predicates, superseded history, and safe effective
+output/explanation against canaries; it compares mounted and explicitly wrapped
+loads for equal configuration and no diff. `TestLayeredDestinationDiffUsesBilateralSensitivity`
+checks redaction from either side and recorded dependencies, and
+`TestLayeredDestinationDecodeFailureReturnsNoResultOrLaterCallbacks` checks a
+mounted decode failure returns no result. Stage 8's
+`TestLayeredImportedInheritedValuesRetainResourceContexts` continues to verify
+the declaring context for inherited relative resources. Done: approved engine
+semantics integrate without per-fragment loading or a post-return processing
+pass.
+
+## Stage 10 — Expose the root-owned facade and public acceptance tests
+
+Status: complete. Scope: the two public fields and complete consumer behavior.
+
+1. Add `Input.Destination Path` and `SourceInfo.Destination Path` to root types.
+   Extend `toEngineInput`, `fromEngineSourceInfo`, `toEngineSourceInfo`, and all
+   source publication paths with owned conversion. Run existing public/import
+   boundary tests; expose no engine nodes, aliases, or bridge methods.
+2. Document zero/root behavior, mapping-only paths, main/underlay parity,
+   inherited import placement, source-coordinate fallback, and source ownership
+   beside declarations. Update the configuration contract for implemented input
+   destinations, preserving canonical ownership of shared semantics.
+3. Add external-package public-load coverage for equivalent host file, native
+   fs.FS file, byte, reader, and Mapping sources. Cover default/root destinations,
+   literal escaped/numeric keys, empty mapping, NoSource, and invalid declarations.
+   Include one public mounted main case, not just underlays.
+4. Add representative public tests tying final configuration to metadata,
+   source placement, history, resources, callbacks, and safe diffs. Mutate returned
+   record copies and repeat queries/loads to confirm detachment. Reuse engine
+   matrices; public tests demonstrate supported integration rather than every
+   private branch. Confirm failed loads expose detached source destinations.
+
+Validated with `go test . -run 'Destination|FacadeLayered|PublicFacadeBoundary|DependencyImportBoundary' -count=1`,
+`go test -race ./...`, and `./scripts/check.sh` (go1.26.8 linux/amd64); all
+passed.
+
+`TestFacadeInputDestinationsAcrossSourceKinds` exercises host, native
+filesystem, bytes, reader, and mapping inputs with literal escaped and
+numeric-looking mapping keys. `TestFacadeDestinationRootEmptyMappingNoSourceAndInvalidPaths`
+covers omitted/empty root placement, empty mapping presence, absent `NoSource`,
+invalid index/reserved destinations, and reader preflight. `TestFacadeImportedSourcesRetainInputDestination`
+checks public transitive import placement. `TestFacadeMountedMainAndUnderlayInspectionResourcesAndDiff`
+ties mounted main/underlay values to metadata, both source destinations, callback
+source context, relative-resource access during and after loading, history, safe
+effective/explanation rendering, and bilateral diff redaction; it also mutates
+returned records and confirms later queries and comparisons remain unchanged.
+`TestFacadeFailedLoadExposesDetachedSourceDestination` verifies failure records,
+and `TestSourceInfoDestinationConversionsAndCopies` covers root/engine source
+record conversion. Existing public-boundary tests pass with the new root-owned
+fields. Done: destination selection and inspection are available through the
+supported ConfigKit import path.
+
+## Stage 11 — Add maintained simple and collection-style consumer examples
+
+Status: complete. Scope: executable adoption examples with consumer-owned types.
+
+1. Add a minimal mounted-fragment example using `Input.Destination` in an
+   ordinary layered load. Show literal keys and root-default compatibility
+   without introducing consumer parsing or generic merge helpers.
+2. Add one representative collection assembly under `examples/consumers`: peer
+   prompt/profile fragments, collection metadata with ordinary `version`, a
+   source-root import, local sibling inheritance, and an intentional higher-tier
+   version-only override retaining the body. Keep the application VersionRule
+   main-owned. Use a controlled native filesystem or temporary files.
+3. Demonstrate effective-path inspection plus source-relative resource lookup
+   during resolution and from the result, including an inherited/surviving field.
+   Application code owns opening/interpreting the resource and any qualified IDs.
+4. Add tests for final values, presence, actual source placement/coordinates,
+   resource context, and expected conflict behavior. Link maintained examples
+   from the consumer guide; add a small executable root example if useful for
+   discovery without duplicating the complete scenario.
+
+Validated with `go test ./examples/consumers -count=1`,
+`go test . -run '^Example' -count=1`, and `./scripts/check.sh` (go1.26.8
+linux/amd64); all passed, including local documentation links.
+
+`Example_inputDestination` demonstrates a literal mounted mapping path and an
+ordinary root input without consumer-side parsing or merging.
+`LoadCollectionBundle` and `TestCollectionBundleAssemblesMountedPeersAndResources`
+exercise prompt/profile peer sources, imported collection metadata with an
+ordinary version label, main-owned schema-version checking, local sibling
+inheritance, and a higher-tier prompt version-only change that retains the rest
+of its body. The resolver and `ReadCollectionPromptContent` follow direct and
+inherited `content_file` provenance to the declaring native-filesystem context.
+The test checks final values, explicit presence, source destinations and authored
+coordinates, relative resources, and the inherited field's origin.
+`TestCollectionPeerPromptConflictsRemainStrict` preserves expected conflict
+behavior. The examples are linked from the consumer adoption guide. Done: simple
+and representative consumers substantiate the same model without a compensating
+structural pass.
+
+## Stage 12 — Reconcile documentation and audit the complete implementation
+
+Status: complete. Scope: final integration audit and evidence, not release work.
+
+1. Update README navigation, consumer guidance, the component overview, and
+   development reading/test maps. Add ADR 0011 to navigation. Update architecture
+   policy only as needed to recognize placement within existing assembly and
+   source ownership, linking precise behavior to the configuration contract.
+   Do not change unrelated policies, deferred boundaries, or published notes.
+2. Audit every resolved decision, roadmap behavior, and request acceptance row
+   against implementation and tests. Record the corresponding test names here;
+   for reused coverage explain the behavior it proves. Do not mark complete
+   based on green tests that do not exercise mounting.
+3. Review the complete baseline-to-candidate diff for accidental strengthening:
+   extra naming rules, namespace uniqueness, main-only restrictions, changed
+   merge/version semantics, invented source-read ordering guarantees, new limits,
+   or expanded secret policies. Remove temporary helpers/unsupported branches
+   left by staged integration. Confirm original source coordinates remain real.
+4. Run `./scripts/check.sh` and
+   `GOTOOLCHAIN=local GOPROXY=off go test -race ./...`. Review source-kind and
+   placement ownership coverage, error disclosure, complete-on-return handling,
+   and root/private boundaries. Record commit, Go version, results, and any
+   limitations. No release command, CI waiting, benchmark, or downstream upgrade.
+5. Mark this plan and the roadmap implemented only when code, public examples,
+   and owning contracts agree. Retain completed planning evidence until separately
+   authorized release preparation.
+
+Audit evidence for the approved decisions and request acceptance surface:
+
+| Behavior reviewed | Evidence and what it proves |
+| --- | --- |
+| Root-owned `Input.Destination` and `SourceInfo.Destination`; detached conversions and publication | `TestSourceInfoDestinationConversionsAndCopies`, `TestFacadeFailedLoadExposesDetachedSourceDestination`, and `TestFacadeMountedMainAndUnderlayInspectionResourcesAndDiff` verify root/engine conversion, error records, and returned record detachment. `TestPublicFacadeBoundary` and `TestDependencyImportBoundary` verify no engine types cross the facade. |
+| Literal mapping keys, parsed numeric keys, reserved structural tokens, and invalid-path preflight | `TestNormalizeDestinationUsesLiteralMappingKeys`, `TestNormalizeDestinationRejectsIndicesAndReservedKeys`, and `TestLayeredLoadPreflightsDestinationsBeforeReadingSources` cover mapping-only normalization and rejection before reader consumption; `TestFacadeDestinationRootEmptyMappingNoSourceAndInvalidPaths` verifies the public error boundary. |
+| Placement of acquired bodies, empty mappings versus absence, imports, all source kinds, physical identity, and authored coordinates | `TestFacadeInputDestinationsAcrossSourceKinds` covers host/native/bytes/reader/mapping sources; `TestFacadeSameFragmentMountsAtSeparateCollectionDestinations` proves independent public collections can retain the same local key. `TestLayeredImportOnlyInputMaterializesDestination`, `TestLayeredHostImportsInheritInputDestinationTransitively`, `TestLayeredNativeFilesystemImportsInheritInputDestination`, and `TestLayeredDestinationsDoNotChangeRepeatedSourceIdentity` cover inherited import placement and unchanged identity. `TestPlaceDocumentWrapsEmptyMappingAndPreservesSourceCoordinates`, `TestPlaceDocumentPreservesAuthoredEntryCoordinates`, and `TestLayeredDestinationDiagnosticsUseEffectivePathsAndAuthoredLocations` distinguish fallback wrapper locations from authored coordinates. |
+| Strict composition, explicit-wrapper equivalence, precedence, empty values, and domain `version` overrides | `TestLayeredDestinationStrictCompositionBoundaries`, `TestLayeredDestinationMatchesExplicitlyWrappedInput`, `TestEmptyMountedMappingRetainsLowerChildren`, and `TestLayeredDestinationOverlayRetainsOrdinaryValueSemantics` prove existing group and overlay semantics are retained; the latter includes ordinary field-only updates rather than whole-record replacement. |
+| Main-owned version checks and imported, underlay, inheritance, overlay, CLI, and ancestor protection | `TestLayeredDestinationChecksVersionOnceOnComposedMain`, `TestLayeredDestinationVersionClaimsUsePlacedApplicationPaths`, `TestLayeredImportedVersionClaimsUseEffectiveDestination`, and `TestLayeredMountedVersionProtectionCoversInheritanceAndUpperTiers` cover effective paths, one main check, protected-path claims/ancestors, and nonclaiming mapping ancestors. `TestLoadVersionDestinationTypesKeepOrdinaryDecoding` preserves exact configured-path conversion and ordinary nested decoding. |
+| Final-shape sibling inheritance, no cross-collection lookup, one lifecycle, and failure without a usable partial result | `TestLayeredDestinationInheritanceUsesFinalSiblingScopes`, `TestLayeredDestinationInheritanceDoesNotSearchOtherCollections`, `TestLayeredDestinationLifecycleAndSensitiveInspectionUseFinalShape`, and `TestLayeredDestinationDecodeFailureReturnsNoResultOrLaterCallbacks` exercise these behaviors. Reused inheritance, lifecycle, and callback tests cover generic behavior that destination placement does not alter. |
+| Effective paths, presence, provenance/history, safe rendering, sensitivity, bilateral diff, and source-relative resources | `TestLayeredDestinationsRetainOriginsHistoryAndDetachedRecords`, `TestLayeredDestinationLifecycleAndSensitiveInspectionUseFinalShape`, `TestLayeredDestinationDiffUsesBilateralSensitivity`, `TestLayeredImportedInheritedValuesRetainResourceContexts`, and `TestFacadeMountedMainAndUnderlayInspectionResourcesAndDiff` connect mounted values to inspection. The imported-inheritance test specifically proves the inherited value keeps the declaring source's passive resource context. |
+| Shared depth/node limits and unchanged root/absence accounting | `TestPlaceDocumentRootAndBounds`, `TestDestinationPlacementCountsAgainstSharedNodeBudget`, `TestMultipleMountedInputsSharePlacementCharges`, `TestImportedEmptyBodiesChargeEachDestinationWrapper`, `TestSelectedOverlayAndCLIUsePlacementNodeCharges`, and `TestNoSourceAndRootPlacementAddNoWrapperCharges` cover per-document and aggregate limits without adding a new limit class. |
+| Maintained consumer usage and strict peer behavior | `Example_inputDestination`, `TestCollectionBundleAssemblesMountedPeersAndResources`, and `TestCollectionPeerPromptConflictsRemainStrict` verify the simple and representative collection examples against actual values, resources, source records, and conflict behavior. |
+
+The wider baseline-to-candidate review covered all 38 changed files (`6353598..2d8df22`): production changes are limited to the root facade fields/conversions, private input/import assembly and accounting, source record ownership, and the inheritance version-claim correction described above. The contract and tests do not impose identifier grammar, namespace uniqueness, main-only placement, source-read ordering, new budgets, nested version conversion, or expanded secret policy. The implementation wraps only acquired application bodies after processing source-root directives; placement does not change physical source identity, authored node locations, filesystem resolution, or later-tier semantics. Stage evidence above reuses existing general composition, resource, inheritance, version, lifecycle, sensitivity, and boundary tests where those behaviors are unchanged; mounted-specific cases are named separately.
+
+Completion record: audited source candidate `2d8df22dd1cb9fde8e60853da3e2260731acf2bb`. On `go1.26.8 linux/amd64`, `./scripts/check.sh` passed, including formatting, whitespace, documentation links, tests, vet, and build. `GOTOOLCHAIN=local GOPROXY=off go test -race ./...` also passed with dependency downloads disabled. No remaining implementation limitations were found; release preparation, CI, benchmark campaigns, and downstream upgrades remain separately scoped.
+
+Done: one complete layered load supports input destinations with coherent values,
+diagnostics, inspection, limits, and source ownership across the approved surface.
+
+## Implementation owner map
+
+Use graph tools to locate these symbols rather than assume line numbers:
+
+- Public input forwarding: `layered.go`, `records.go`, `conversion.go`
+  (source conversions), and existing facade tests.
+- Private request and lifecycle: `internal/engine/layered.go`, `load_request.go`
+  (`loadInput`, `acquireUnderlayGroup`, `assembleLayeredInput`), `load.go`
+  (`executeLoadRequest`), and `load_input.go` (`assembleBaseInputTracked`).
+- Directives and imports: `directives.go` (`splitImportDirectives`,
+  `fragmentVersionClaim`), and `imports.go` (`visitPath`).
+- Shared document semantics: `compose.go`, `document_walk.go`
+  (`countInputNodes`), `path.go`, `parse.go`, and `value_operation.go` limits.
+- Inspection: `metadata.go`, `history.go`, `explain.go`, `resource_context.go`,
+  `inheritance_origin.go`, `render.go`, and `diff.go`; source copies live in
+  engine `records.go`. Confirm current filenames through the component overview.
+
+## Open Questions
+
+None required for implementation. The four policy choices are approved; this
+plan selects API spelling, validation timing/error category, coordinate handling,
+and accounting within them. Surface a newly discovered incompatibility or scope
+change with evidence rather than resolving it by silently narrowing the feature
+or strengthening an unrelated contract.

docs/roadmap/input-destinations.md

@@ -0,0 +1,171 @@
+# Configuration Input Destinations Roadmap
+
+Status: implemented. This roadmap records the approved behavior and rationale
+for the [PromptKit feature request](promptkit-feature-request.md). The
+[implementation plan](implementation.md) records the completed sequence and
+audit evidence; the [configuration contract](../config.md#input-destinations)
+owns current behavior.
+
+## Targeted end state
+
+A consumer can assign a mapping destination to a configuration input and perform
+one ordinary `LoadLayered` call. The input's application content contributes
+beneath that destination while retaining normal composition, precedence,
+inheritance, typed loading, diagnostics, and inspection. PromptKit can assemble
+collections without parsing and rewriting sources, merging values, or rebuilding
+provenance itself.
+
+For example, a fragment defining `summarize.alerts` can contribute beneath
+`collections`, `weatherreporter`, `prompts`. An independent source with the same
+local key can contribute beneath another collection without conflict. Collection
+and prompt names remain ordinary application keys, not ConfigKit namespaces.
+
+The public surface adds an optional `Destination Path` field on `Input`, not a
+new loading entry point or an operation on completed results. Content-bearing
+main inputs and underlay inputs support
+the same destination model; root placement remains the default.
+
+The following behavior defines the target:
+
+- Omitted and empty destinations retain root placement. Destinations consist of
+  literal mapping keys, including dots, slashes, and numeric-looking text.
+  Explicit sequence-index destinations are outside the requested capability.
+- A valid destination on `NoSource` contributes nothing. An actual empty
+  application mapping materializes its destination mappings as explicit input,
+  matching authored wrappers, without clearing lower-tier children.
+- Existing structural reservations apply to introduced mappings. Destinations
+  that materialize `configkit` or a mapping-valued `extends` contrary to existing
+  contracts fail through the structured loading-error boundary. Keys merely
+  containing those words remain ordinary keys.
+- Host files, native filesystem files, bytes, readers, and mapping sources all
+  support placement under their existing acquisition and ownership contracts.
+  Filesystem context, import opt-in, and precedence remain independent choices.
+- Mounted contributions have the same application values and conflict behavior
+  as explicitly wrapped contributions, with accurate original source locations.
+  Disjoint mappings combine; overlapping non-mapping values within a group
+  conflict even when equal. A later tier cannot repair a strict-group conflict.
+- Different destinations can coexist in one group, and the same destination can
+  occur in different tiers. Ancestor/descendant destinations follow ordinary
+  composition and overlay rules without namespace uniqueness restrictions.
+  Later tiers recursively overlay mappings and replace other values. Empty
+  mappings do not clear lower values; sequences remain atomic.
+- A prompt's ordinary `version` override changes that field and retains omitted
+  lower fields. It neither identifies a merge record nor replaces a whole prompt.
+- Source-root `configkit.imports` remains structural. Imported application data
+  inherits the input destination transitively. Import paths still resolve from
+  their declaring source; containment, optionality, cycles, and repeated-source
+  rules remain applicable. Placement does not change physical source identity.
+- Sibling inheritance runs after assembly in the effective destination scope.
+  Peer fragments can supply a child and its sibling parent. Qualified PromptKit
+  references do not acquire cross-collection inheritance meaning.
+- Version ownership is enforced at effective application paths. A collection's
+  mounted `version` is ordinary data, while underlay/import contributions to the
+  actual protected path, or replacements of its ancestors, remain invalid.
+  There is one main-owned version check and no blanket nested numeric conversion.
+- Defaults, decoding, resolution, secret lookup, and validation run once on the
+  assembled application, not independently on fragments. Callbacks and value
+  diagnostics use effective paths. Failures use structured load errors and yield
+  no usable partial result.
+- Presence, provenance, history, explanations, effective rendering, and diffs
+  agree with the final values. Sensitivity is evaluated at effective application
+  paths under the existing historical and dependency rules.
+- Existing source records expose placement, including inherited import
+  placement. Introduced ancestor mappings retain the contributing source's
+  identity; any root-location fallback identifies that document, not an authored
+  wrapper. Actual content retains its real source coordinates.
+- Original source locations and passive resource contexts survive placement,
+  overrides, and inheritance. `Result.SourceContext` and
+  `Resolution.SourceContext` still identify the declaring filesystem and base
+  directory for a field such as `content_file: ./summary.md`.
+- Destinations and the structure they introduce participate in existing load
+  limits; placement cannot bypass depth or node accounting. Source ownership,
+  complete-on-return loading, and detached result behavior remain intact.
+
+Import-destination syntax, independent collection specification headers, and
+cross-collection inheritance are separate possible follow-ups. This work does
+not add directory discovery, collection installation, remote acquisition,
+namespace ownership enforcement, dependency solving, live reload, filename-based
+identity, arbitrary transformations, or new merge strategies. PromptKit owns its
+collection inventory, qualified IDs, and execution semantics.
+
+## Implementation baseline (v0.7.1)
+
+The request reviewed v0.7.1. At that baseline, the relevant boundaries were:
+
+- [Input](../../layered.go) holds a source, import opt-in, and filesystem context,
+  but no destination. Root types delegate to the engine through the existing
+  [facade boundary](../internal/overview.md).
+- [Layered loading](../config.md#layered-loading) already provides strict groups,
+  ordered precedence, shared acquisition budgets, and one final lifecycle.
+  [Underlay acquisition](../../internal/engine/load_request.go) gathers source
+  applications and recomposes them into their strict group.
+- [Input acquisition](../../internal/engine/load_input.go) and
+  [import traversal](../../internal/engine/imports.go) process source-root
+  directives and source identity before final assembly. Imported version claims
+  are checked during traversal; underlay claims are checked before group
+  composition. These currently operate without a destination.
+- [Version claim checks](../../internal/engine/directives.go) inspect a document
+  using the configured application path. Wrapping only an already acquired
+  result would leave early fragment checks and diagnostic paths incorrect.
+- [Sibling inheritance](../adr/0009-sibling-inheritance.md) already runs after
+  source precedence and preserves source-to-destination derivation. Placement
+  must feed that existing mechanism rather than resolve fragments independently.
+- Existing source records identify acquisition and precedence but do not expose
+  input placement. Existing history retains source applications, so moving only
+  the final tree would be insufficient for coherent inspection.
+
+The architectural fit is an extension of input assembly, not a parser extension
+or a second configuration model. The
+[architecture policy](../policy/architecture.md) supplies the ownership and
+consistency constraints; [deferred scope](deferred.md) continues to govern the
+unrequested features above.
+
+## Selected Design Decisions
+
+### Main-input destinations and absent inputs
+
+Allow the same destination model for content-bearing main inputs and underlays,
+with `VersionRule.Path` remaining an effective application path and any
+specification declaration still owned by main. The checker retains its existing
+handling of an absent declaration and its existing lifecycle phase. `NoSource`
+contributes nothing even with a valid destination. This keeps `Input` coherent
+across its roles and preserves absence without per-fragment version checks.
+
+### Public placement inspection and synthetic ancestors
+
+Expose effective placement as `SourceInfo.Destination Path` on existing source
+records, including inherited import placement.
+Use original source identity for introduced ancestor mappings, documenting that
+any root-location fallback identifies the contributing document rather than a
+physically authored wrapper. Actual content keeps its real coordinates. This
+allows consumers to connect placement to sources without a new query system.
+
+Preserve that relationship through inherited fields, composite origins, and the
+same source selected in separate tiers. Placement is per source application;
+consumers can inspect it without inferring it from final paths.
+
+### Reserved structural keys in destinations
+
+Use existing `Path` conventions for literal keys and parsed
+numeric tokens, reject explicit index segments, and preserve current structural
+reservations in the effective tree. Reject destination segments that would
+materialize `configkit` or a mapping-valued `extends` contrary to those contracts,
+with a clear structured destination error. Ordinary keys containing those words,
+dots, or slashes remain ordinary keys. Source-root directives are processed before
+placement and are not rejected simply because their application body is mounted.
+
+Mounted and authored wrappers therefore retain the same structural meaning.
+Destination diagnostics identify placement as the cause of invalid introduced
+structure, using existing error categories where suitable.
+
+### Empty mappings and presence of introduced containers
+
+Mounting an actual empty application mapping materializes its
+destination mappings as explicit contributions, matching authored wrappers.
+This preserves the request's equivalence principle and lets consumers inspect
+an intentionally empty collection. Such mappings still do not clear lower-tier
+children. An absent source contributes nothing.
+
+This applies equally to empty imported application bodies and inputs containing
+only import directives. Presence follows the contribution's meaning rather than
+whether an implementation happened to allocate wrapper nodes.

docs/roadmap/promptkit-feature-request.md

@@ -0,0 +1,319 @@
+# PromptKit Feature Request: Configuration Input Destinations
+
+Status: consumer feature request; not accepted or implemented.
+
+## Requested Capability
+
+Provide a public API through which a consumer can select a destination mapping
+path for each configuration input. The input's application content contributes
+beneath that path while retaining ConfigKit's normal composition, precedence,
+inheritance, validation, diagnostics, and inspection behavior.
+
+PromptKit needs this capability for its proposal 3, **Collections With Mounted
+Configuration Fragments**, recorded in PromptKit's
+`docs/roadmap/configuration-collections-mounted-fragments.md`. This request
+describes the public API needs and observable acceptance criteria. It does not
+prescribe ConfigKit's internal design or an implementation plan.
+
+The reviewed public baseline is ConfigKit v0.7.1. The current
+[layered-loading contract](../config.md#layered-loading) lets consumers choose
+sources, import policy, filesystem context, and precedence groups, but does not
+provide an input destination. Existing behavior remains authoritative until an
+extension is adopted.
+
+## Consumer Problem
+
+PromptKit applications assemble independently maintained collections of prompts,
+inference profiles, and backends. Collections can contain identical local names,
+such as `summarize.alerts`, without identifying the same resource. PromptKit wants
+an effective configuration organized by collection, while letting authors omit
+repeated collection and resource-container wrappers from individual files.
+
+A file selected from a collection's `prompts` directory would contain:
+
+```yaml
+summarize.alerts:
+  version: "1"
+  default_profile: provider/brief
+  messages:
+    - role: user
+      content: Summarize the weather alerts.
+  output:
+    format: text
+    validation_mode: none
+```
+
+Given destination `[collections, weatherreporter, prompts]`, its contribution
+would appear in the final application configuration as:
+
+```yaml
+collections:
+  weatherreporter:
+    prompts:
+      summarize.alerts:
+        version: "1"
+        default_profile: provider/brief
+        messages:
+          - role: user
+            content: Summarize the weather alerts.
+        output:
+          format: text
+          validation_mode: none
+```
+
+Another collection can contribute `summarize.alerts` beneath
+`[collections, homewatch, prompts]` without conflicting. After final handoff,
+PromptKit interprets these as `weatherreporter/summarize.alerts` and
+`homewatch/summarize.alerts`. The profile reference is illustrative and must be
+supplied separately by the application configuration.
+
+PromptKit will support one installed definition per prompt. Its optional flat
+`version` is ordinary domain metadata, not a merge identity. This request does
+not need identified-record sequences, sequence-root documents, or a custom
+merge policy.
+
+Authors could write the complete wrappers today, but that repeats assembly
+details in every fragment. Consumer-side parsing and wrapping would put generic
+source processing back into PromptKit and complicate source attribution.
+PromptKit instead wants to select sources and destinations, then receive one
+final configuration through ConfigKit's supported loading lifecycle.
+
+## Minimum Public API
+
+### Per-Input Destination
+
+Allow a destination to be attached to each `Input` passed to `LoadLayered`.
+One possible public spelling is an optional `Destination Path` field on `Input`.
+The name and exact signature are suggestions; equivalent public API designs
+are welcome.
+
+Illustrative consumer code using the proposed field, which does not exist in
+v0.7.1:
+
+```go
+promptInput := configkit.Input{
+    Source: configkit.File("/usr/local/share/collections/weatherreporter/prompts/alerts.yml"),
+    Destination: configkit.Keys("collections", "weatherreporter", "prompts"),
+}
+
+peers := configkit.Underlay{
+    Inputs: []configkit.Input{promptInput},
+}
+```
+
+The consumer would include `peers` in its existing `LayeredOptions` and invoke
+`LoadLayered` with its normal root main input and application `Definition`.
+This is a partial API sketch, not an executable example or a proposed new
+loading entry point. A single public layered load is sufficient for PromptKit;
+it does not require a separate mounting operation on completed results.
+
+The API must let the consumer:
+
+- Express a destination relative to the application configuration root using
+  literal mapping-key segments. Dots, slashes, and numeric-looking text inside
+  a key are not implicit path separators or array indexes. PromptKit's namespace
+  grammar remains its own domain rule.
+- Omit the destination to retain existing root placement. An explicitly empty
+  destination should have the same meaning. Mapping destinations are sufficient;
+  sequence-index destinations are not needed by this request.
+- Assign different destinations to inputs in the same strict composition group,
+  and assign the same destination to inputs in separate precedence groups.
+- Use the destination with existing content-bearing source kinds: host files,
+  native filesystem files, bytes, readers, and mapping sources, subject to their
+  existing acquisition and ownership contracts.
+- Retain the existing import opt-in and filesystem context independently of the
+  destination. A configuration destination is not a filesystem root or base
+  directory.
+
+PromptKit can continue to enumerate an explicitly selected directory and attach
+the same destination to its returned inputs. ConfigKit does not need a new
+directory scanner, namespace registry, filename convention, or PromptKit schema.
+The namespace comes from the application's assembly declaration, not from an
+implicit interpretation of a directory basename.
+
+### Representative Assembly
+
+These are application choices, not ConfigKit-specific reserved paths:
+
+| Selected inputs | Destination key segments |
+| --- | --- |
+| Application main document | Root |
+| `weatherreporter/collection.yml` | `collections`, `weatherreporter` |
+| YAML files under `weatherreporter/prompts` | `collections`, `weatherreporter`, `prompts` |
+| YAML files under `homewatch/prompts` | `collections`, `homewatch`, `prompts` |
+| YAML files under `provider/profiles` | `collections`, `provider`, `profiles` |
+| YAML files under `provider/backends` | `collections`, `provider`, `backends` |
+
+Peer collection contributions belong to a single strict underlay group when the
+application wants conflict detection among them. Intentional operator overrides
+use later groups or main. Destination selection must not establish precedence
+or make source traversal order authorize replacement.
+
+## Required Observable Behavior
+
+### Composition And Precedence
+
+Mounted and explicitly wrapped contributions must have equivalent application
+values and conflict behavior at their effective destinations. Actual source
+locations naturally differ and must remain accurate.
+
+Retain the [strict composition](../config.md#strict-document-composition) and
+[layering](../config.md#layered-loading) rules:
+
+- Disjoint mapping entries at the same destination combine.
+- Overlapping non-mapping values within a strict group conflict, including equal
+  scalars, nulls, and sequences. Different destinations do not collide merely
+  because the source documents share a local key.
+- Later precedence layers recursively override mappings and replace other
+  values. A strict conflict cannot be repaired by an upper layer.
+- Empty mappings do not clear lower definitions; ordinary arrays remain atomic.
+  In particular, `messages` does not acquire sequence-merging behavior.
+- Overlapping destination paths and contributions at their ancestors follow
+  those same rules. No namespace-level uniqueness restriction is requested.
+
+Changing only a prompt's `version` in a later layer changes that field while
+retaining omitted lower-layer fields. ConfigKit must not treat a domain version
+label as a request to replace an entire definition.
+
+### Imports And Existing Structural Directives
+
+An input with imports enabled must be able to use its existing source-root
+`configkit.imports` directive while contributing application data at a nested
+destination. The directive remains structural configuration, not an application
+field below that destination.
+
+For the minimum API, imports inherit their importing input's destination,
+including transitive imports. Import paths continue to resolve relative to
+their declaring source's filesystem context. A mounted document must not send
+its imported application fields back to the configuration root accidentally.
+
+Existing opt-in, containment, missing-input, cycle, and repeated-source behavior
+must remain applicable. A different destination is not a request to bypass the
+current rejection of a repeated physical source within a strict group.
+
+Sibling `extends` must resolve against mappings at their effective destinations.
+For example, two independently supplied profile fragments mounted beneath the
+same collection's `profiles` mapping can establish `extends: brief` once they
+compose. Inheritance remains ConfigKit-owned; PromptKit does not perform another
+relationship-resolution pass.
+
+### Protected Specification Paths
+
+The distinction between source-local field names and effective application
+paths is essential. PromptKit's main document will declare `version: 1` at the
+configuration root. A separate collection metadata file can contain:
+
+```yaml
+version: "2.4.0"
+```
+
+Mounted at `[collections, weatherreporter]`, that field is an ordinary collection
+release label. It must not be rejected as an attempt to contribute the protected
+root specification field. The same applies to a prompt's nested optional
+version. Only the main-owned specification declaration uses the application's
+configured [VersionRule](../config.md#application-schema-versions).
+
+Conversely, placement must not permit an underlay or imported fragment to claim
+or replace the actual protected specification path or an ancestor that would
+replace it. The public contract must define protection in terms of effective
+application paths, rather than reserving the spelling `version` in every source.
+Protection must remain valid with imports, inheritance, overlays, and explicit
+CLI overrides.
+
+PromptKit needs one main-owned specification check with the existing lifecycle.
+It does not need main mounted away from the root, integer-to-string conversion
+for every nested `version`, or separate compatibility checks for every fragment.
+
+### Typed Loading, Diagnostics, And Inspection
+
+The destination must be visible consistently through the ordinary public
+loading and inspection surface:
+
+- Final typed configuration and application callbacks use effective destination
+  paths. Defaults, resolution, and validation retain the existing single-load
+  lifecycle; fragments are not independently validated as whole applications.
+- Metadata and presence queries distinguish omission, explicit null, and other
+  explicit values at the mounted paths.
+- Diagnostics identify the effective conflicting or invalid path and the real
+  contributing source locations. Consumers can determine which source and
+  placement produced a value without recreating that relationship themselves.
+- Effective rendering, explanations, history, and configuration comparison agree
+  with the final configuration. Existing sensitivity and disclosure rules apply
+  to mounted fields, including application path-based secret classification.
+- Unsupported destination forms and destination-related loading failures use
+  the normal structured error boundary. Exact new diagnostic codes, if needed,
+  are for ConfigKit to define.
+
+PromptKit does not require a separate mount registry or new inspection facade.
+Existing inspection APIs are sufficient if they expose accurate effective paths,
+source attribution, and placement information. ConfigKit should document how
+source-local locations relate to effective paths.
+
+### Source-Relative Resources And Lifecycle
+
+For a mounted field such as `content_file: ./summary.md`, obtaining its effective
+origin and calling `Result.SourceContext` or `Resolution.SourceContext` must
+still identify the declaring source's filesystem and base directory. The same
+holds when that field survives an override or is inherited into another mapping.
+Its configuration destination must not change the meaning of the relative file
+reference.
+
+Retain existing source ownership, load completion, result lifetime, containment,
+and resource-access behavior. Destinations must participate in documented load
+limits without creating a way to bypass them; this request does not prescribe
+new limit values or their accounting machinery.
+
+## Optional Follow-Ups, Not Prerequisites
+
+**Import destinations in YAML.** A destination on an import directive could let
+publishers describe assembly in configuration rather than Go. Proposal 3
+recommends destinations relative to the importing document's destination, with
+omission retaining that destination. This is useful, but per-input placement
+through the public API is sufficient for initial PromptKit adoption. No new
+directive spelling is selected by this request.
+
+**Independent specification headers.** A collection entry document might later
+declare its own supported specification version. That needs a distinct contract
+for checking the header and determining its role in the composed configuration.
+Mounting collection release metadata does not supply this capability. Keep it a
+separate request if PromptKit decides it is necessary.
+
+**Cross-collection inheritance.** Nested collection profiles are not immediate
+siblings across collections. Current sibling inheritance remains sufficient for
+the initial proposal, with authored definitions flattened where necessary.
+Generic cross-path inheritance could be considered separately if consumer needs
+justify revisiting the [deferred scope](deferred.md). Mounting must not silently
+reinterpret `extends: provider/brief` as a cross-collection reference.
+
+PromptKit also does not request collection installation, automatic directory
+discovery, remote fetching, namespace ownership enforcement, dependency solving,
+live reload, filename-derived identity, or arbitrary document transformations.
+Its qualified resource IDs, optional prompt version semantics, collection
+inventory, and execution APIs remain PromptKit responsibilities.
+
+## Public Acceptance Scenarios
+
+Evaluate the extension through supported public loading and inspection APIs:
+
+| Scenario | Expected consumer-visible result |
+| --- | --- |
+| Destination omitted or empty | Existing root-placement behavior |
+| Same fragment shape mounted under two distinct collection keys from independent sources | Two separate effective resources |
+| Disjoint inputs at the same destination in one group | Mapping contributions combine |
+| Equal or unequal overlapping scalar fields in one group | Structured composition conflict with both source locations |
+| Same destination across deliberate precedence tiers | Existing recursive overlay behavior |
+| Version-only prompt override | Label changes; omitted prompt body fields survive |
+| Nested mounted import without a destination directive | Imported fields contribute at the inherited destination |
+| Local profile inheritance across mounted peer fragments | Parent resolves at the effective sibling scope |
+| Collection metadata containing `version` mounted below root | Ordinary nested field, independent of root VersionRule |
+| Mounted/imported contribution actually targeting the protected version path | Rejected under version ownership rules |
+| Mounted field using source-relative content, including inheritance | Original declaring resource context remains available |
+| Mounted explicit null and omitted field | Distinct effective presence information |
+| Mounted validation error or secret field | Effective paths, physical locations, and existing disclosure rules agree |
+| File, native filesystem, byte, reader, or mapping input with equivalent content | Equivalent application values, subject to existing source contracts |
+| Unsupported destination or failed load | Normal structured failure, no usable partial result |
+
+The minimum usable outcome is a consumer that selects collection sources and
+destinations, performs one ordinary layered load, and receives final typed
+configuration with reliable diagnostics and inspection. It should need no
+consumer-side source rewriting, generic merging, or provenance reconstruction.

docs/roadmap/remediation.md

@@ -0,0 +1,89 @@
+# Input destination review findings
+
+Reviewed `d3d468a..945e111` against the approved
+[input-destinations roadmap](input-destinations.md),
+[implementation plan](implementation.md), and
+[ADR 0011](../adr/0011-input-destinations.md).
+
+The root-owned destination fields, placement before strict composition,
+transitive import placement, effective-path version ownership, final lifecycle,
+source records, resource contexts, retained history, and existing disclosure
+mechanisms broadly match the selected architecture. The required correction
+below concerns an early diagnostic path, not loading or composition semantics.
+No additional unintended strengthening of contracts was identified.
+
+## 1. Rebase known Mapping-source failure paths to the input destination (P2)
+
+Status: complete. Known Mapping value-builder paths are now prefixed with the
+normalized destination at acquisition, preserving semantic key/index segments.
+Public regressions cover mounted main and underlay failures, escaped keys,
+sequence indices, cyclic values, unchanged root paths, and source-constructor
+errors retaining source coordinates. The mounted value cases failed before the
+fix and pass afterward. Existing directive/syntax checks remain unchanged.
+Focused destination/source/diagnostic/import tests, `./scripts/check.sh`, and
+`GOTOOLCHAIN=local GOPROXY=off go test -race ./...` passed on
+`go1.26.8 linux/amd64`.
+
+Affected: `assembleBaseInputTracked` in
+[load_input.go](../../internal/engine/load_input.go), lines 27–32 at the reviewed
+commit; the Mapping failure originates in `readSource` in
+[source.go](../../internal/engine/source.go), lines 206–208.
+
+`Mapping` snapshots its input through the existing value builder. When a nested
+value is unsupported, cyclic, or exceeds a value-builder limit, the retained
+issue already identifies a semantic path inside that application mapping.
+`readSource` turns it into a structured diagnostic. The early return from
+`assembleBaseInputTracked` annotates the source record with its destination but
+returns those diagnostic paths unchanged.
+
+For example, select this input as either main or an underlay:
+
+```go
+configkit.Input{
+    Source: configkit.Mapping("bad mapping", map[string]any{
+        "item": map[string]any{"bad": func() {}},
+    }),
+    Destination: configkit.Keys("catalog", "entries"),
+}
+```
+
+The load correctly fails with `source.invalid`, but `Diagnostic.Path` is
+`/item/bad` rather than `/catalog/entries/item/bad`. The source record correctly
+reports `/catalog/entries`, so the two inspection records use inconsistent
+application coordinates. A consumer displaying or associating the failure with
+the final application shape would point to the wrong configuration field.
+
+This diverges from implementation decision 10 and Stage 6: existing paths that
+genuinely identify application content must be translated to effective paths
+exactly once. A Mapping value-builder issue has that information; it is not a
+source-read or YAML syntax failure for which a logical child path is unavailable.
+
+Required remediation:
+
+- Prefix known application-relative Mapping value-builder diagnostic paths with
+  the normalized input destination at the acquisition boundary. Preserve key
+  escaping and sequence-index intent in the existing semantic Path model.
+- Keep source IDs, original primary/related locations, codes, value-free
+  messages, source destinations, and nil-result behavior unchanged. Root
+  placement must retain its current paths.
+- Do not blanket-prefix parse/read failures, source-constructor errors, or
+  structural import-directive diagnostics. Do not invent logical child paths
+  for failures that do not identify application content.
+- Add bounded regression coverage through the public API for mounted main and
+  underlay Mapping failures, alongside unchanged root placement. Reuse existing
+  syntax/directive tests to protect their different coordinate convention.
+- Run the focused destination/source/diagnostic checks and the everyday check.
+  Reconcile the completed plan's diagnostic evidence with this correction;
+  do not redefine the agreed effective-path behavior to accept the gap.
+
+## Review validation
+
+- `GOTOOLCHAIN=local ./scripts/check.sh` passed at `945e111` on
+  `go1.26.8 linux/amd64` (formatting, whitespace, tests, vet, and build).
+- `GOTOOLCHAIN=local GOPROXY=off go test -race ./...` passed.
+- `GOTOOLCHAIN=local GOPROXY=off go test -count=1 ./...` passed.
+- An additional external-package probe in a disposable checkout outside the
+  repository failed for both main and underlay placement, observing `/item/bad`
+  instead of `/catalog/entries/item/bad` with `source.invalid`.
+- No production fixes or regression tests were added to this repository during
+  the review. No performance measurements or new guarantees were inferred.

example_test.go

@@ -30,6 +30,32 @@ func ExampleNoSource() {
 	// Output: 4 0 <nil>
 }
 
+func ExampleDiff() {
+	type Config struct {
+		Workers int `yaml:"workers"`
+	}
+	before, err := configkit.Load[Config](configkit.Mapping("before", map[string]any{"workers": 2}))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	after, err := configkit.Load[Config](configkit.Mapping("after", map[string]any{"workers": 4}))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	differences, err := configkit.Diff(before, after)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	for _, difference := range differences {
+		fmt.Printf("%s: %s -> %s\n", difference.Path.String(),
+			strings.TrimSpace(difference.Left.YAML), strings.TrimSpace(difference.Right.YAML))
+	}
+	// Output: /workers: 2 -> 4
+}
+
 func ExampleValidation_Error() {
 	type Config struct {
 		First  string `yaml:"first"`

examples/consumers/consumers.go

@@ -29,14 +29,14 @@ type NarratioConfig struct {
 
 // LoadNarratio loads the maintained import fixture with Narratio-shaped rules.
 func LoadNarratio(path string) (*configkit.Result[NarratioConfig], error) {
-	return LoadNarratioProfile(path, "")
+	return LoadNarratioOverlay(path, "")
 }
 
-// LoadNarratioProfile selects a ConfigKit configuration overlay independently
+// LoadNarratioOverlay selects a ConfigKit configuration overlay independently
 // from the application's ModelProfile field.
-func LoadNarratioProfile(path, profile string) (*configkit.Result[NarratioConfig], error) {
+func LoadNarratioOverlay(path, overlay string) (*configkit.Result[NarratioConfig], error) {
 	return configkit.Load(configkit.File(path), configkit.Options[NarratioConfig]{
-		Imports: true, Profile: profile,
+		Imports: true, Overlay: overlay,
 		Definition: configkit.Definition[NarratioConfig]{
 			Defaults: func() NarratioConfig { return NarratioConfig{PollInterval: time.Minute} },
 			Resolve:  resolveNarratio,

examples/consumers/consumers_test.go

@@ -52,6 +52,22 @@ func Example_seriatim() {
 	// Output: visitor true the event occurred <nil> <nil>
 }
 
+func Example_inheritedPresets() {
+	result, err := consumers.LoadPresets(configkit.File(fixture("inheritance", "presets.yml")))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	path := configkit.Keys("presets", "custom", "enabled")
+	info, _ := result.Metadata().Lookup(path)
+	custom := result.Config.Presets["custom"]
+	fmt.Println(custom.Enabled, result.Metadata().IsExplicit(path), custom.Settings["secondary"]["width"])
+	fmt.Println(info.Origin.Locations[0].SourceID, info.Origin.Inheritance[0].From)
+	// Output:
+	// false true 20
+	// source-1 /presets/base/enabled
+}
+
 func Example_weatherStartup() {
 	const blockedName = "CONFIGKIT_WEATHER_EXAMPLE_BLOCKED"
 	const tokenName = "CONFIGKIT_WEATHER_EXAMPLE_TOKEN"
@@ -92,10 +108,13 @@ func TestMaintainedConsumerFixturesPreserveDomainOutcomes(t *testing.T) {
 	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)
+	overlaid, err := consumers.LoadNarratioOverlay(fixture("narratio", "pipeline.yml"), "testing")
+	if err != nil || overlaid.Config.PollInterval.String() != "2m0s" ||
+		overlaid.Config.ModelProfile != "concise" || len(overlaid.Sources()) != 3 {
+		t.Fatalf("configuration overlay and application model profile = %#v, %v", overlaid, err)
+	}
+	if source := overlaid.Sources()[2]; source.Label != "overlays/testing.yml" || source.Layer != configkit.LayerOverlay {
+		t.Fatalf("Narratio overlay source = %#v", source)
 	}
 
 	notarius, err := consumers.LoadNotarius(fixture("notarius", "config.yml"))
@@ -200,7 +219,7 @@ func TestMigrationFixturesFailWithStableCodesAndPaths(t *testing.T) {
 			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") {
+			if strings.Contains(diagnostics[0].Message, "wrong-before-overlay") {
 				t.Fatalf("diagnostic exposed raw parser input: %#v", diagnostics[0])
 			}
 		})

examples/consumers/destinations.go

@@ -0,0 +1,159 @@
+package consumers
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"path"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// CollectionBundleConfig is a representative application-owned collection
+// shape. Collection and prompt version labels remain ordinary domain fields.
+type CollectionBundleConfig struct {
+	Version     string                          `yaml:"version"`
+	Collections map[string]CollectionDefinition `yaml:"collections"`
+}
+
+type CollectionDefinition struct {
+	Name        string                       `yaml:"name"`
+	Version     string                       `yaml:"version"`
+	Description string                       `yaml:"description"`
+	Prompts     map[string]CollectionPrompt  `yaml:"prompts"`
+	Profiles    map[string]CollectionProfile `yaml:"profiles"`
+}
+
+type CollectionPrompt struct {
+	Version        string              `yaml:"version"`
+	DefaultProfile string              `yaml:"default_profile"`
+	ContentFile    string              `yaml:"content_file"`
+	ResourceText   string              `yaml:"resource_text"`
+	Messages       []CollectionMessage `yaml:"messages"`
+}
+
+type CollectionMessage struct {
+	Role    string `yaml:"role"`
+	Content string `yaml:"content"`
+}
+
+type CollectionProfile struct {
+	Format      string  `yaml:"format"`
+	Temperature float64 `yaml:"temperature"`
+}
+
+// LoadCollectionBundle demonstrates one layered collection load. Its main
+// document owns the application schema version; imported collection metadata,
+// prompt/profile peers, and an operator prompt-version override use ordinary
+// destinations and domain fields.
+func LoadCollectionBundle() (*configkit.Result[CollectionBundleConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/collection-bundle")
+	if err != nil {
+		return nil, err
+	}
+	bundle := configkit.NewFilesystem("collection bundle", files)
+	collectionDestination := configkit.Keys("collections", "weatherreporter")
+	promptDestination := collectionDestination.Key("prompts")
+	profileDestination := collectionDestination.Key("profiles")
+	return configkit.LoadLayered(
+		configkit.Input{Source: configkit.Bytes("application main", []byte("version: 1\n"))},
+		configkit.LayeredOptions[CollectionBundleConfig]{
+			Definition: configkit.Definition[CollectionBundleConfig]{
+				Version: &configkit.VersionRule{Path: configkit.Keys("version"), Check: func(version configkit.Version, _ *configkit.Validation) error {
+					if !version.Present || version.Text != "1" {
+						return errors.New("unsupported application schema version")
+					}
+					return nil
+				}},
+				Resolve:  resolveCollectionResources,
+				Validate: validateCollectionBundle,
+			},
+			Underlays: []configkit.Underlay{
+				{Inputs: []configkit.Input{
+					{Source: bundle.File("collections/weather/collection.yml"), Imports: true, Destination: collectionDestination},
+					{Source: bundle.File("collections/weather/prompts/brief.yml"), Destination: promptDestination},
+					{Source: bundle.File("collections/weather/prompts/custom.yml"), Destination: promptDestination},
+					{Source: bundle.File("collections/weather/profiles/compact.yml"), Destination: profileDestination},
+					{Source: bundle.File("collections/weather/profiles/weather.yml"), Destination: profileDestination},
+				}},
+				{Inputs: []configkit.Input{{
+					Source: bundle.File("operators/weather-prompt-version.yml"), Destination: promptDestination,
+				}}},
+			},
+		},
+	)
+}
+
+func resolveCollectionResources(configuration *CollectionBundleConfig, resolution *configkit.Resolution) error {
+	for collectionID, collection := range configuration.Collections {
+		for promptID, prompt := range collection.Prompts {
+			if prompt.ContentFile == "" {
+				continue
+			}
+			contentPath := configkit.Keys("collections", collectionID, "prompts", promptID, "content_file")
+			info, found := resolution.InputInfo(contentPath)
+			if !found || len(info.Origin.Locations) != 1 {
+				return fmt.Errorf("prompt resource has no single declaring source")
+			}
+			resourceSource, found := resolution.Source(info.Origin.Locations[0].SourceID)
+			if !found {
+				return fmt.Errorf("prompt resource source is unavailable")
+			}
+			context, found := resolution.SourceContext(resourceSource.ID)
+			if !found {
+				return fmt.Errorf("prompt resource context is unavailable")
+			}
+			data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, prompt.ContentFile))
+			if err != nil {
+				return fmt.Errorf("read prompt resource: %w", err)
+			}
+			prompt.ResourceText = string(data)
+			collection.Prompts[promptID] = prompt
+			resolution.Record(
+				configkit.Keys("collections", collectionID, "prompts", promptID, "resource_text"),
+				configkit.ChangeNormalization, "read source-relative prompt content", contentPath,
+			)
+		}
+		configuration.Collections[collectionID] = collection
+	}
+	return nil
+}
+
+func validateCollectionBundle(configuration *CollectionBundleConfig, validation *configkit.Validation) {
+	for collectionID, collection := range configuration.Collections {
+		for promptID, prompt := range collection.Prompts {
+			if prompt.DefaultProfile == "" {
+				continue
+			}
+			if _, found := collection.Profiles[prompt.DefaultProfile]; !found {
+				validation.Error(configkit.Keys("collections", collectionID, "prompts", promptID, "default_profile"), "must name a profile in this collection")
+			}
+		}
+	}
+}
+
+// ReadCollectionPromptContent follows the effective content field's physical
+// source, including when it was inherited from a peer prompt.
+func ReadCollectionPromptContent(result *configkit.Result[CollectionBundleConfig], collectionID, promptID string) (string, error) {
+	if result == nil {
+		return "", errors.New("collection result is nil")
+	}
+	contentPath := configkit.Keys("collections", collectionID, "prompts", promptID, "content_file")
+	info, found := result.Metadata().Lookup(contentPath)
+	if !found || len(info.Origin.Locations) != 1 {
+		return "", errors.New("prompt resource has no single declaring source")
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found {
+		return "", errors.New("prompt resource context is unavailable")
+	}
+	prompt, found := result.Config.Collections[collectionID].Prompts[promptID]
+	if !found {
+		return "", errors.New("prompt is unavailable")
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, prompt.ContentFile))
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}

examples/consumers/destinations_test.go

@@ -0,0 +1,116 @@
+package consumers_test
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func Example_inputDestination() {
+	mounted, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.Mapping("weather prompt", map[string]any{
+			"summarize.alerts": map[string]any{"version": "1"},
+		}),
+		Destination: configkit.Keys("collections", "weather.reporter", "prompts"),
+	})
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	root, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.Bytes("standalone", []byte("name: standalone\n")),
+	})
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	_, found := mounted.Config["collections"]
+	fmt.Println(found, root.Config["name"])
+	// Output: true standalone
+}
+
+func TestCollectionBundleAssemblesMountedPeersAndResources(t *testing.T) {
+	result, err := consumers.LoadCollectionBundle()
+	if err != nil {
+		t.Fatalf("LoadCollectionBundle: %v", err)
+	}
+	collection := result.Config.Collections["weatherreporter"]
+	if result.Config.Version != "1" || collection.Name != "Weather Reporter" || collection.Version != "2.4.0" {
+		t.Fatalf("main and collection versions = %q, %#v", result.Config.Version, collection)
+	}
+	brief := collection.Prompts["brief"]
+	custom := collection.Prompts["custom"]
+	if brief.Version != "2" || brief.DefaultProfile != "compact" || brief.ContentFile != "welcome.txt" || len(brief.Messages) != 1 {
+		t.Fatalf("version-only override replaced lower prompt fields: %#v", brief)
+	}
+	if custom.Version != "2" || custom.DefaultProfile != "weather" || custom.ContentFile != brief.ContentFile ||
+		len(custom.Messages) != 1 || custom.ResourceText != "Welcome to the weather collection.\n" {
+		t.Fatalf("inherited prompt = %#v", custom)
+	}
+	weatherProfile := collection.Profiles["weather"]
+	if weatherProfile.Format != "text" || weatherProfile.Temperature != 0.4 {
+		t.Fatalf("inherited profile = %#v", weatherProfile)
+	}
+	resource, err := consumers.ReadCollectionPromptContent(result, "weatherreporter", "custom")
+	if err != nil || resource != custom.ResourceText {
+		t.Fatalf("inherited prompt resource = %q, %v", resource, err)
+	}
+
+	versionInfo, ok := result.Metadata().Lookup(configkit.Keys("version"))
+	if !ok || !versionInfo.Explicit || versionInfo.Origin.Layer != configkit.LayerBase || len(versionInfo.Origin.Locations) != 1 {
+		t.Fatalf("main-owned version metadata = %#v, %v", versionInfo, ok)
+	}
+	collectionVersionPath := configkit.Keys("collections", "weatherreporter", "version")
+	collectionVersion, ok := result.Metadata().Lookup(collectionVersionPath)
+	if !ok || !collectionVersion.Explicit || collectionVersion.Origin.Layer != configkit.LayerUnderlay || len(collectionVersion.Origin.Locations) != 1 {
+		t.Fatalf("ordinary collection version metadata = %#v, %v", collectionVersion, ok)
+	}
+	var metadataSource configkit.SourceInfo
+	var parentPromptSource configkit.SourceInfo
+	var promptVersionSource configkit.SourceInfo
+	for _, source := range result.Sources() {
+		switch source.Path {
+		case "collections/weather/metadata.yml":
+			metadataSource = source
+		case "collections/weather/prompts/brief.yml":
+			parentPromptSource = source
+		case "operators/weather-prompt-version.yml":
+			promptVersionSource = source
+		}
+	}
+	collectionVersionLocation := collectionVersion.Origin.Locations[0]
+	if metadataSource.ID == "" || collectionVersionLocation.SourceID != metadataSource.ID ||
+		metadataSource.Destination.String() != "/collections/weatherreporter" || collectionVersionLocation.Line != 1 {
+		t.Fatalf("imported collection version source = %#v, %#v", metadataSource, collectionVersionLocation)
+	}
+	if promptVersionSource.ID == "" || promptVersionSource.Destination.String() != "/collections/weatherreporter/prompts" || promptVersionSource.UnderlayIndex != 2 {
+		t.Fatalf("prompt version source = %#v", promptVersionSource)
+	}
+	contentPath := configkit.Keys("collections", "weatherreporter", "prompts", "custom", "content_file")
+	contentInfo, ok := result.Metadata().Lookup(contentPath)
+	if !ok || !contentInfo.Explicit || len(contentInfo.Origin.Inheritance) != 1 || len(contentInfo.Origin.Locations) != 1 {
+		t.Fatalf("inherited content field metadata = %#v, %v", contentInfo, ok)
+	}
+	contentLocation := contentInfo.Origin.Locations[0]
+	if parentPromptSource.ID == "" || contentLocation.SourceID != parentPromptSource.ID || contentLocation.Line != 4 ||
+		parentPromptSource.Destination.String() != "/collections/weatherreporter/prompts" {
+		t.Fatalf("inherited resource source coordinates = %#v, source %#v", contentLocation, parentPromptSource)
+	}
+}
+
+func TestCollectionPeerPromptConflictsRemainStrict(t *testing.T) {
+	destination := configkit.Keys("collections", "weatherreporter", "prompts")
+	_, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{
+			{Source: configkit.Bytes("peer-a", []byte("brief: {version: '1'}\n")), Destination: destination},
+			{Source: configkit.Bytes("peer-b", []byte("brief: {version: '1'}\n")), Destination: destination},
+		}}},
+	})
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeComposeConflict {
+		t.Fatalf("duplicate peer prompt error = %v", err)
+	}
+}

examples/consumers/diff.go

@@ -0,0 +1,136 @@
+package consumers
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// FileComparisonConfig is a small application-owned shape for comparing
+// caller-selected configuration files.
+type FileComparisonConfig struct {
+	Name    string         `yaml:"name"`
+	Entries map[string]any `yaml:"entries"`
+}
+
+// CompareFiles loads two caller-selected files with the same application
+// definition, then formats only the safe records returned by configkit.Diff.
+func CompareFiles(leftPath, rightPath string) (string, error) {
+	options := configkit.Options[FileComparisonConfig]{
+		Definition: configkit.Definition[FileComparisonConfig]{
+			Defaults: func() FileComparisonConfig {
+				return FileComparisonConfig{Entries: map[string]any{}}
+			},
+		},
+	}
+	left, err := configkit.Load(configkit.File(leftPath), options)
+	if err != nil {
+		return "", fmt.Errorf("load left configuration: %w", err)
+	}
+	right, err := configkit.Load(configkit.File(rightPath), options)
+	if err != nil {
+		return "", fmt.Errorf("load right configuration: %w", err)
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil {
+		return "", err
+	}
+	return FormatDifferences(differences), nil
+}
+
+// RolloutConfig models a service endpoint whose URL is derived by the
+// application resolver and whose credential reference is classified.
+type RolloutConfig struct {
+	Service RolloutService `yaml:"service"`
+}
+
+// RolloutService contains endpoint settings and a classified credential.
+type RolloutService struct {
+	Endpoint   RolloutEndpoint   `yaml:"endpoint"`
+	Credential RolloutCredential `yaml:"credential"`
+}
+
+// RolloutEndpoint is the effective destination used by the application.
+type RolloutEndpoint struct {
+	Host string `yaml:"host"`
+	Port int    `yaml:"port"`
+	URL  string `yaml:"url"`
+}
+
+// RolloutCredential holds only a typed secret reference in configuration.
+type RolloutCredential struct {
+	Token configkit.Secret `yaml:"token" configkit:"secret"`
+}
+
+// LoadRollout loads a base file, optionally with one selected configuration
+// overlay. Resolution derives the effective URL after the overlay is applied.
+func LoadRollout(basePath, overlayName string) (*configkit.Result[RolloutConfig], error) {
+	return configkit.Load(configkit.File(basePath), configkit.Options[RolloutConfig]{
+		Overlay: overlayName,
+		Definition: configkit.Definition[RolloutConfig]{
+			Resolve: resolveRollout,
+		},
+	})
+}
+
+// CompareRolloutOverlay compares the effective base configuration with that
+// same file plus one selected overlay. It presents only safe diff records.
+func CompareRolloutOverlay(basePath, overlayName string) (string, error) {
+	base, err := LoadRollout(basePath, "")
+	if err != nil {
+		return "", fmt.Errorf("load base configuration: %w", err)
+	}
+	withOverlay, err := LoadRollout(basePath, overlayName)
+	if err != nil {
+		return "", fmt.Errorf("load overlay configuration: %w", err)
+	}
+	differences, err := configkit.Diff(base, withOverlay)
+	if err != nil {
+		return "", err
+	}
+	return FormatDifferences(differences), nil
+}
+
+func resolveRollout(configuration *RolloutConfig, resolution *configkit.Resolution) error {
+	endpoint := &configuration.Service.Endpoint
+	if endpoint.Host == "" || endpoint.Port <= 0 {
+		return nil
+	}
+	url := fmt.Sprintf("https://%s:%d", endpoint.Host, endpoint.Port)
+	if endpoint.URL != url {
+		endpoint.URL = url
+		resolution.Record(configkit.Keys("service", "endpoint", "url"),
+			configkit.ChangeNormalization, "derived endpoint URL",
+			configkit.Keys("service", "endpoint", "host"),
+			configkit.Keys("service", "endpoint", "port"))
+	}
+	return nil
+}
+
+// FormatDifferences is consumer-owned presentation. It distinguishes absence
+// through Present and reports withheld changes through HiddenChanged without
+// interpreting redaction marker text.
+func FormatDifferences(differences []configkit.Difference) string {
+	if len(differences) == 0 {
+		return "No effective changes.\n"
+	}
+	var output strings.Builder
+	for _, difference := range differences {
+		fmt.Fprintf(&output, "%s: left=%s right=%s", difference.Path.String(),
+			formatSnapshot(difference.Left), formatSnapshot(difference.Right))
+		if difference.HiddenChanged {
+			output.WriteString(" hidden-change")
+		}
+		output.WriteByte('\n')
+	}
+	return output.String()
+}
+
+func formatSnapshot(snapshot configkit.ValueSnapshot) string {
+	if !snapshot.Present {
+		return "<absent>"
+	}
+	return strconv.Quote(snapshot.YAML)
+}

examples/consumers/diff_test.go

@@ -0,0 +1,187 @@
+package consumers_test
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func ExampleCompareFiles() {
+	directory, err := os.MkdirTemp("", "configkit-diff-files-")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer os.RemoveAll(directory)
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	if err := os.WriteFile(leftPath, []byte("name: before\nentries: {}\n"), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	if err := os.WriteFile(rightPath, []byte("name: after\nentries:\n  added: null\n"), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Print(output)
+	// Output:
+	// /entries/added: left=<absent> right="null\n"
+	// /name: left="before\n" right="after\n"
+}
+
+func ExampleCompareRolloutOverlay() {
+	const baseToken = "CONFIGKIT_DIFF_EXAMPLE_BASE_TOKEN"
+	const overlayToken = "CONFIGKIT_DIFF_EXAMPLE_OVERLAY_TOKEN"
+	defer preserveEnvironment(baseToken)()
+	defer preserveEnvironment(overlayToken)()
+	_ = os.Setenv(baseToken, "disposable-example-base-credential")
+	_ = os.Setenv(overlayToken, "disposable-example-overlay-credential")
+
+	directory, err := os.MkdirTemp("", "configkit-diff-overlay-")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer os.RemoveAll(directory)
+	basePath := filepath.Join(directory, "config.yml")
+	overlayPath := filepath.Join(directory, "overlays", "canary.yml")
+	if err := os.MkdirAll(filepath.Dir(overlayPath), 0o700); err != nil {
+		fmt.Println(err)
+		return
+	}
+	base := "service:\n  endpoint:\n    host: base.example\n    port: 8443\n  credential:\n    token: " + baseToken + "\n"
+	overlay := "service:\n  endpoint:\n    host: canary.example\n  credential:\n    token: " + overlayToken + "\n"
+	if err := os.WriteFile(basePath, []byte(base), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	if err := os.WriteFile(overlayPath, []byte(overlay), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	output, err := consumers.CompareRolloutOverlay(basePath, "canary")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Print(output)
+	// Output:
+	// /service/credential/token: left="'[REDACTED]'\n" right="'[REDACTED]'\n" hidden-change
+	// /service/endpoint/host: left="base.example\n" right="canary.example\n"
+	// /service/endpoint/url: left="https://base.example:8443\n" right="https://canary.example:8443\n"
+}
+
+func TestCompareFilesUsesCallerSelectedSourcesAndSafePresence(t *testing.T) {
+	directory := t.TempDir()
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	writeConfiguration(t, leftPath, "name: before\nentries: {}\n")
+	writeConfiguration(t, rightPath, "name: after\nentries:\n  added: null\n")
+
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "/entries/added: left=<absent> right=\"null\\n\"\n" +
+		"/name: left=\"before\\n\" right=\"after\\n\"\n"
+	if output != want {
+		t.Fatalf("comparison output = %q, want %q", output, want)
+	}
+	if again, err := consumers.CompareFiles(leftPath, rightPath); err != nil || again != output {
+		t.Fatalf("repeat output = %q, %v", again, err)
+	}
+}
+
+func TestCompareFilesReturnsEachLoadError(t *testing.T) {
+	directory := t.TempDir()
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	writeConfiguration(t, leftPath, "name: before\n")
+	writeConfiguration(t, rightPath, "unknown: invalid\n")
+
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err == nil || output != "" || !strings.Contains(err.Error(), "load right configuration") {
+		t.Fatalf("right load failure = %q, %v", output, err)
+	}
+	if _, err := consumers.CompareFiles(filepath.Join(directory, "missing.yml"), rightPath); err == nil || !strings.Contains(err.Error(), "load left configuration") {
+		t.Fatalf("left load failure = %v", err)
+	}
+}
+
+func TestCompareRolloutOverlayShowsResolvedSafeChangesAndRetainsSiblings(t *testing.T) {
+	t.Setenv("CONFIGKIT_DIFF_BASE_TOKEN", "disposable-base-credential")
+	t.Setenv("CONFIGKIT_DIFF_OVERLAY_TOKEN", "disposable-overlay-credential")
+	directory := t.TempDir()
+	basePath := filepath.Join(directory, "config.yml")
+	overlayPath := filepath.Join(directory, "overlays", "canary.yml")
+	if err := os.MkdirAll(filepath.Dir(overlayPath), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	writeConfiguration(t, basePath, "service:\n  endpoint:\n    host: base.example\n    port: 8443\n  credential:\n    token: CONFIGKIT_DIFF_BASE_TOKEN\n")
+	writeConfiguration(t, overlayPath, "service:\n  endpoint:\n    host: canary.example\n  credential:\n    token: CONFIGKIT_DIFF_OVERLAY_TOKEN\n")
+
+	base, err := consumers.LoadRollout(basePath, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	withOverlay, err := consumers.LoadRollout(basePath, "canary")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if base.Config.Service.Endpoint.Port != 8443 || withOverlay.Config.Service.Endpoint.Port != 8443 {
+		t.Fatalf("lower-priority port was not retained: base=%#v overlay=%#v", base.Config.Service.Endpoint, withOverlay.Config.Service.Endpoint)
+	}
+	urlInfo, found := withOverlay.Metadata().Lookup(configkit.Keys("service", "endpoint", "url"))
+	if !found || urlInfo.Origin.Layer != configkit.LayerResolution || withOverlay.Config.Service.Endpoint.URL != "https://canary.example:8443" {
+		t.Fatalf("resolved endpoint = %#v, metadata=%#v, %v", withOverlay.Config.Service.Endpoint, urlInfo, found)
+	}
+
+	differences, err := configkit.Diff(base, withOverlay)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 3 {
+		t.Fatalf("differences = %#v", differences)
+	}
+	wantPaths := []string{"/service/credential/token", "/service/endpoint/host", "/service/endpoint/url"}
+	for index, difference := range differences {
+		if got := difference.Path.String(); got != wantPaths[index] {
+			t.Errorf("path[%d] = %q, want %q", index, got, wantPaths[index])
+		}
+	}
+	secret := differences[0]
+	if !secret.Left.Redacted || !secret.Right.Redacted || !secret.HiddenChanged || secret.Left.YAML != "'[REDACTED]'\n" || secret.Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("classified credential difference = %#v", secret)
+	}
+	if strings.Contains(consumers.FormatDifferences(differences), "disposable-") {
+		t.Fatal("formatted differences exposed credential contents")
+	}
+
+	output, err := consumers.CompareRolloutOverlay(basePath, "canary")
+	if err != nil || output != consumers.FormatDifferences(differences) {
+		t.Fatalf("formatted base/overlay comparison = %q, %v", output, err)
+	}
+}
+
+func TestFormatDifferencesUsesRecordMembershipForNoChange(t *testing.T) {
+	if got := consumers.FormatDifferences(nil); got != "No effective changes.\n" {
+		t.Fatalf("no-change output = %q", got)
+	}
+}
+
+func writeConfiguration(t *testing.T, filename, contents string) {
+	t.Helper()
+	if err := os.WriteFile(filename, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+}

examples/consumers/inheritance.go

@@ -0,0 +1,20 @@
+package consumers
+
+import configkit "gitea.maximumdirect.net/eric/ConfigKit"
+
+// PresetCatalog is ordinary application data; no inheritance fields or
+// registration hooks are required in the typed configuration.
+type PresetCatalog struct {
+	Presets map[string]Preset `yaml:"presets"`
+}
+
+type Preset struct {
+	Enabled  bool                      `yaml:"enabled"`
+	Settings map[string]map[string]int `yaml:"settings"`
+}
+
+// LoadPresets performs one load; callers may inspect inherited presence and
+// source origins on the returned result before choosing a preset to execute.
+func LoadPresets(source configkit.Source) (*configkit.Result[PresetCatalog], error) {
+	return configkit.Load[PresetCatalog](source)
+}

examples/consumers/inheritance_test.go

@@ -0,0 +1,35 @@
+package consumers_test
+
+import (
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func TestPresetCatalogOneLoadAndDerivation(t *testing.T) {
+	result, err := consumers.LoadPresets(configkit.File(fixture("inheritance", "presets.yml")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Config.Presets["base"].Settings["secondary"]["width"] != 10 ||
+		result.Config.Presets["custom"].Settings["secondary"]["width"] != 20 ||
+		result.Config.Presets["custom"].Enabled {
+		t.Fatalf("expanded presets = %#v", result.Config.Presets)
+	}
+	enabled := configkit.Keys("presets", "custom", "enabled")
+	if !result.Metadata().IsExplicit(enabled) {
+		t.Fatal("inherited false lost explicit presence")
+	}
+	info, found := result.Metadata().Lookup(enabled)
+	if !found || info.Origin.Locations[0].SourceID != "source-1" ||
+		len(info.Origin.Inheritance) != 1 || info.Origin.Inheritance[0].From.String() != "/presets/base/enabled" {
+		t.Fatalf("origin and derivation = %#v, %v", info, found)
+	}
+	nested := configkit.Keys("presets", "custom", "settings", "secondary", "width")
+	info, found = result.Metadata().Lookup(nested)
+	if !found || len(info.Origin.Inheritance) != 1 ||
+		info.Origin.Inheritance[0].From.String() != "/presets/custom/settings/primary/width" {
+		t.Fatalf("destination-local derivation = %#v, %v", info, found)
+	}
+}

examples/consumers/layered.go

@@ -0,0 +1,95 @@
+package consumers
+
+import (
+	"embed"
+	"errors"
+	"io/fs"
+	"path"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+//go:embed testdata/native-source testdata/layered-bundle testdata/collection-bundle
+var bundledFiles embed.FS
+
+// BundledConfig is a small configuration loaded directly from an embedded file.
+type BundledConfig struct {
+	Name    string `yaml:"name"`
+	Enabled bool   `yaml:"enabled"`
+}
+
+// LoadBundled demonstrates a native filesystem source without layered options.
+func LoadBundled() (*configkit.Result[BundledConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/native-source")
+	if err != nil {
+		return nil, err
+	}
+	return configkit.Load[BundledConfig](configkit.NewFilesystem("bundled", files).File("config.yml"))
+}
+
+// CatalogConfig keeps resource names and domain rules in the application.
+type CatalogConfig struct {
+	Mode      string            `yaml:"mode"`
+	Enabled   bool              `yaml:"enabled"`
+	Workers   int               `yaml:"workers"`
+	Templates map[string]string `yaml:"templates"`
+	Labels    map[string]string `yaml:"labels"`
+}
+
+// LoadCatalog combines two underlay tiers beneath an independently located
+// host main file. ConfigKit assembles and validates the complete result once.
+func LoadCatalog(mainPath string) (*configkit.Result[CatalogConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/layered-bundle")
+	if err != nil {
+		return nil, err
+	}
+	bundle := configkit.NewFilesystem("catalog bundle", files)
+	return configkit.LoadLayered(configkit.Input{Source: configkit.File(mainPath)},
+		configkit.LayeredOptions[CatalogConfig]{
+			Underlays: []configkit.Underlay{
+				{Inputs: []configkit.Input{{Source: bundle.File("defaults/config.yml"), Imports: true}}},
+				{Inputs: []configkit.Input{{Source: configkit.Mapping("local policy", map[string]any{
+					"workers": 2,
+					"labels":  map[string]any{"tenant": "example"},
+				})}}},
+			},
+			Definition: configkit.Definition[CatalogConfig]{
+				Defaults: func() CatalogConfig { return CatalogConfig{Mode: "demo"} },
+				Validate: validateCatalog,
+			},
+		})
+}
+
+func validateCatalog(configuration *CatalogConfig, validation *configkit.Validation) {
+	if configuration.Mode != "demo" && configuration.Mode != "production" {
+		validation.Error(configkit.Keys("mode"), "must be demo or production")
+	}
+	if configuration.Workers < 0 {
+		validation.Error(configkit.Keys("workers"), "must be nonnegative")
+	}
+	if name := configuration.Templates["welcome"]; name == "." || !fs.ValidPath(name) {
+		validation.Error(configkit.Keys("templates", "welcome"), "must be a relative resource name")
+	}
+}
+
+// ReadWelcome follows the effective template field's one declaring source to
+// its retained resource context. Opening and interpreting the resource remain
+// application work; this does not reopen configuration or change metadata.
+func ReadWelcome(result *configkit.Result[CatalogConfig]) (string, error) {
+	if result == nil {
+		return "", errors.New("catalog result is nil")
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("templates", "welcome"))
+	if !found || len(info.Origin.Locations) != 1 {
+		return "", errors.New("welcome template has no single declaring source")
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found {
+		return "", errors.New("welcome template has no resource context")
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, result.Config.Templates["welcome"]))
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}

examples/consumers/layered_test.go

@@ -0,0 +1,71 @@
+package consumers_test
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func Example_nativeFilesystem() {
+	result, err := consumers.LoadBundled()
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(result.Config.Name, result.Config.Enabled)
+	// Output: embedded false
+}
+
+func Example_layeredCatalog() {
+	result, err := consumers.LoadCatalog(fixture("layered-main", "config.yml"))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	welcome, err := consumers.ReadWelcome(result)
+	fmt.Println(result.Config.Mode, result.Config.Enabled, result.Config.Workers,
+		result.Config.Labels["origin"], result.Config.Labels["tenant"], strings.TrimSpace(welcome), err)
+	// Output: production false 0 bundle example Welcome from the bundle. <nil>
+}
+
+func TestCatalogRetainsPresenceAndSourceContext(t *testing.T) {
+	result, err := consumers.LoadCatalog(fixture("layered-main", "config.yml"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, key := range []string{"enabled", "workers"} {
+		info, found := result.Metadata().Lookup(configkit.Keys(key))
+		if !found || !info.Explicit || info.Origin.Layer != configkit.LayerBase {
+			t.Fatalf("%s presence = %#v, %v", key, info, found)
+		}
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("templates", "welcome"))
+	if !found || info.Origin.Layer != configkit.LayerUnderlay || len(info.Origin.Locations) != 1 {
+		t.Fatalf("welcome origin = %#v, %v", info, found)
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found || context.BaseDir != "defaults" {
+		t.Fatalf("welcome context = %#v, %v", context, found)
+	}
+	welcome, err := consumers.ReadWelcome(result)
+	if err != nil || strings.TrimSpace(welcome) != "Welcome from the bundle." {
+		t.Fatalf("welcome = %q, %v", welcome, err)
+	}
+	if result.Config.Labels["tenant"] != "example" || result.Config.Labels["origin"] != "bundle" {
+		t.Fatalf("recursive underlay labels = %#v", result.Config.Labels)
+	}
+}
+
+func TestCatalogValidationRejectsInvalidMode(t *testing.T) {
+	_, err := consumers.LoadCatalog(fixture("layered-main", "invalid.yml"))
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) == 0 ||
+		loadError.Diagnostics()[0].Code != configkit.CodeValidateError ||
+		loadError.Diagnostics()[0].Path.String() != "/mode" {
+		t.Fatalf("invalid mode error = %v", err)
+	}
+}

examples/consumers/testdata/collection-bundle/collections/weather/collection.yml

@@ -0,0 +1,4 @@
+configkit:
+  imports:
+    - path: metadata.yml
+name: Weather Reporter

examples/consumers/testdata/collection-bundle/collections/weather/metadata.yml

@@ -0,0 +1,2 @@
+version: "2.4.0"
+description: Imported collection metadata.

examples/consumers/testdata/collection-bundle/collections/weather/profiles/compact.yml

@@ -0,0 +1,3 @@
+compact:
+  format: text
+  temperature: 0.2

examples/consumers/testdata/collection-bundle/collections/weather/profiles/weather.yml

@@ -0,0 +1,3 @@
+weather:
+  extends: compact
+  temperature: 0.4

examples/consumers/testdata/collection-bundle/collections/weather/prompts/brief.yml

@@ -0,0 +1,7 @@
+brief:
+  version: "1"
+  default_profile: compact
+  content_file: welcome.txt
+  messages:
+    - role: user
+      content: Write a concise weather summary.

examples/consumers/testdata/collection-bundle/collections/weather/prompts/custom.yml

@@ -0,0 +1,3 @@
+custom:
+  extends: brief
+  default_profile: weather

examples/consumers/testdata/collection-bundle/collections/weather/prompts/welcome.txt

@@ -0,0 +1 @@
+Welcome to the weather collection.

examples/consumers/testdata/collection-bundle/operators/weather-prompt-version.yml

@@ -0,0 +1,2 @@
+brief:
+  version: "2"

examples/consumers/testdata/inheritance/presets.yml

@@ -0,0 +1,10 @@
+presets:
+  base:
+    enabled: false
+    settings:
+      primary: {width: 10}
+      secondary: {extends: primary}
+  custom:
+    extends: base
+    settings:
+      primary: {width: 20}

examples/consumers/testdata/layered-bundle/defaults/config.yml

@@ -0,0 +1,7 @@
+configkit:
+  imports:
+    - path: ../shared.yml
+enabled: true
+workers: 4
+templates:
+  welcome: messages/welcome.txt

examples/consumers/testdata/layered-bundle/defaults/messages/welcome.txt

@@ -0,0 +1 @@
+Welcome from the bundle.

examples/consumers/testdata/layered-bundle/shared.yml

@@ -0,0 +1,2 @@
+labels:
+  origin: bundle

examples/consumers/testdata/layered-main/config.yml

@@ -0,0 +1,3 @@
+mode: production
+enabled: false
+workers: 0

examples/consumers/testdata/layered-main/invalid.yml

@@ -0,0 +1 @@
+mode: unsupported

examples/consumers/testdata/native-source/config.yml

@@ -0,0 +1,2 @@
+name: embedded
+enabled: false

facade_candidate_test.go

@@ -31,6 +31,9 @@ func TestFacadeValuesRemainRootOwned(t *testing.T) {
 		reflect.TypeFor[configkit.Resolution](), reflect.TypeFor[configkit.Validation](),
 		reflect.TypeFor[configkit.Version](), reflect.TypeFor[configkit.VersionRule](),
 		reflect.TypeFor[configkit.FilesystemContext](), reflect.TypeFor[configkit.Override](),
+		reflect.TypeFor[configkit.Filesystem](), reflect.TypeFor[configkit.ResourceContext](),
+		reflect.TypeFor[configkit.Input](), reflect.TypeFor[configkit.Underlay](),
+		reflect.TypeFor[configkit.LayeredOptions[struct{}]](),
 		reflect.TypeFor[configkit.Definition[struct{}]](), reflect.TypeFor[configkit.Options[struct{}]](),
 		reflect.TypeFor[configkit.Result[struct{}]](),
 		reflect.TypeFor[configkit.ValueSnapshot](), reflect.TypeFor[configkit.ValueChange](),

facade_context_test.go

@@ -26,6 +26,7 @@ func TestFacadeInactiveContextsPreserveClosedFailure(t *testing.T) {
 				{"IsExplicit", func() { state.resolution.IsExplicit(path) }},
 				{"InputInfo", func() { state.resolution.InputInfo(path) }},
 				{"Source", func() { state.resolution.Source("source") }},
+				{"SourceContext", func() { state.resolution.SourceContext("source") }},
 				{"Record", func() { state.resolution.Record(path, configkit.ChangeNormalization, "reason") }},
 				{"Resolution.Error", func() { state.resolution.Error(path, "message") }},
 				{"Resolution.Warning", func() { state.resolution.Warning(path, "message") }},

facade_destination_test.go

@@ -0,0 +1,443 @@
+package configkit_test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestFacadeInputDestinationsAcrossSourceKinds(t *testing.T) {
+	directory := t.TempDir()
+	hostPath := filepath.Join(directory, "host.yml")
+	if err := os.WriteFile(hostPath, []byte("value: host\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	native := configkit.NewFilesystem("native", fstest.MapFS{
+		"main.yml": &fstest.MapFile{Data: []byte("value: native\n")},
+	})
+	const literalValuePath = "/collections/weather.reporter/profiles/a~1b/~0/0"
+	destination := configkit.Keys("collections", "weather.reporter", "profiles", "a/b", "~", "0")
+	for _, test := range []struct {
+		name   string
+		source configkit.Source
+		want   string
+		kind   configkit.SourceKind
+	}{
+		{name: "host file", source: configkit.File(hostPath), want: "host", kind: configkit.SourceKindFile},
+		{name: "native filesystem", source: native.File("main.yml"), want: "native", kind: configkit.SourceKindFilesystem},
+		{name: "bytes", source: configkit.Bytes("bytes", []byte("value: bytes\n")), want: "bytes", kind: configkit.SourceKindBytes},
+		{name: "reader", source: configkit.Reader("reader", strings.NewReader("value: reader\n")), want: "reader", kind: configkit.SourceKindReader},
+		{name: "mapping", source: configkit.Mapping("mapping", map[string]any{"value": "mapping"}), want: "mapping", kind: configkit.SourceKindMapping},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: test.source, Destination: destination})
+			if err != nil {
+				t.Fatalf("LoadLayered: %v", err)
+			}
+			if got := facadeDestinationValue(t, result.Config, "collections", "weather.reporter", "profiles", "a/b", "~", "0", "value"); got != test.want {
+				t.Fatalf("placed value = %#v, want %q", got, test.want)
+			}
+			sources := result.Sources()
+			if len(sources) != 1 || sources[0].Kind != test.kind || sources[0].Destination.String() != destination.String() || destination.String() != literalValuePath {
+				t.Fatalf("source placement = %#v, destination = %q", sources, destination.String())
+			}
+		})
+	}
+}
+
+func TestFacadeDestinationRootEmptyMappingNoSourceAndInvalidPaths(t *testing.T) {
+	for _, test := range []struct {
+		name        string
+		destination configkit.Path
+	}{
+		{name: "omitted destination"},
+		{name: "explicit empty destination", destination: configkit.Path{}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Bytes("root", []byte("value: root\n")), Destination: test.destination})
+			if err != nil {
+				t.Fatalf("root load: %v", err)
+			}
+			if result.Config["value"] != "root" || len(result.Sources()) != 1 || result.Sources()[0].Destination.String() != "" {
+				t.Fatalf("root placement = %#v, sources %#v", result.Config, result.Sources())
+			}
+		})
+	}
+
+	emptyPath := configkit.Keys("collections", "empty")
+	empty, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.Mapping("empty", map[string]any{}), Destination: emptyPath,
+	})
+	if err != nil {
+		t.Fatalf("empty mapping load: %v", err)
+	}
+	if value := facadeDestinationMap(t, empty.Config, "collections")["empty"]; value == nil {
+		t.Fatal("empty mapping did not materialize its destination")
+	}
+	if info, ok := empty.Metadata().Lookup(emptyPath); !ok || !info.Explicit {
+		t.Fatalf("empty destination metadata = %#v, %v", info, ok)
+	}
+
+	absentPath := configkit.Keys("collections", "absent")
+	absent, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource(), Destination: absentPath})
+	if err != nil {
+		t.Fatalf("NoSource with a destination: %v", err)
+	}
+	if len(absent.Sources()) != 0 {
+		t.Fatalf("NoSource created source records: %#v", absent.Sources())
+	}
+	if _, ok := absent.Metadata().Lookup(absentPath); ok {
+		t.Fatal("NoSource materialized its destination")
+	}
+
+	reader := &destinationCountingReader{}
+	for _, test := range []struct {
+		name        string
+		source      configkit.Source
+		destination configkit.Path
+		wantReads   int
+	}{
+		{name: "index segment", source: configkit.Reader("unread", reader), destination: configkit.Keys("catalog").Index(0)},
+		{name: "reserved configkit key", source: configkit.NoSource(), destination: configkit.Keys("configkit")},
+		{name: "reserved extends key", source: configkit.NoSource(), destination: configkit.Keys("catalog", "extends")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: test.source, Destination: test.destination})
+			loadError, ok := err.(*configkit.LoadError)
+			if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeOptionsInvalid {
+				t.Fatalf("invalid destination error = %#v", err)
+			}
+			if reader.reads != test.wantReads {
+				t.Fatalf("reader calls = %d, want %d", reader.reads, test.wantReads)
+			}
+		})
+	}
+}
+
+func TestFacadeImportedSourcesRetainInputDestination(t *testing.T) {
+	directory := t.TempDir()
+	mainPath := filepath.Join(directory, "main.yml")
+	fragmentPath := filepath.Join(directory, "fragment.yml")
+	if err := os.WriteFile(mainPath, []byte("configkit: {imports: [{path: fragment.yml}]}\nmain: present\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(fragmentPath, []byte("imported: present\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	destination := configkit.Keys("collections", "weatherreporter")
+	result, err := configkit.LoadLayered[map[string]any](configkit.Input{
+		Source: configkit.File(mainPath), Imports: true, Destination: destination,
+	})
+	if err != nil {
+		t.Fatalf("mounted import load: %v", err)
+	}
+	collection := facadeDestinationMap(t, result.Config, "collections", "weatherreporter")
+	if collection["main"] != "present" || collection["imported"] != "present" {
+		t.Fatalf("mounted imported values = %#v", collection)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != destination.String() || sources[1].Destination.String() != destination.String() {
+		t.Fatalf("mounted import source destinations = %#v", sources)
+	}
+	if sources[1].ImportedFrom == nil {
+		t.Fatalf("imported source lacks its authored directive: %#v", sources[1])
+	}
+}
+
+func TestFacadeSameFragmentMountsAtSeparateCollectionDestinations(t *testing.T) {
+	weatherDestination := configkit.Keys("collections", "weather", "prompts")
+	homeDestination := configkit.Keys("collections", "homewatch", "prompts")
+	result, err := configkit.LoadLayered(
+		configkit.Input{Source: configkit.NoSource()},
+		configkit.LayeredOptions[map[string]any]{Underlays: []configkit.Underlay{{Inputs: []configkit.Input{
+			{Source: configkit.Bytes("weather", []byte("summarize.alerts: {title: weather}\n")), Destination: weatherDestination},
+			{Source: configkit.Bytes("homewatch", []byte("summarize.alerts: {title: home}\n")), Destination: homeDestination},
+		}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	for _, test := range []struct {
+		path []string
+		want string
+	}{
+		{path: []string{"collections", "weather", "prompts", "summarize.alerts", "title"}, want: "weather"},
+		{path: []string{"collections", "homewatch", "prompts", "summarize.alerts", "title"}, want: "home"},
+	} {
+		if got := facadeDestinationValue(t, result.Config, test.path...); got != test.want {
+			t.Errorf("value at %v = %#v, want %q", test.path, got, test.want)
+		}
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != weatherDestination.String() || sources[1].Destination.String() != homeDestination.String() {
+		t.Fatalf("independent collection source placements = %#v", sources)
+	}
+}
+
+type destinationCountingReader struct{ reads int }
+
+func (reader *destinationCountingReader) Read([]byte) (int, error) {
+	reader.reads++
+	return 0, io.EOF
+}
+
+func TestFacadeMountedMainAndUnderlayInspectionResourcesAndDiff(t *testing.T) {
+	t.Parallel()
+
+	filesystem := configkit.NewFilesystem("resources", fstest.MapFS{
+		"assets/summary.md": &fstest.MapFile{Data: []byte("source-relative summary")},
+	})
+	context := &configkit.FilesystemContext{Root: ".", BaseDir: "assets", FS: &filesystem}
+	destination := configkit.Keys("catalog", "entries")
+	tokenPath := configkit.Keys("catalog", "entries", "item", "token")
+	titlePath := configkit.Keys("catalog", "entries", "item", "title")
+	var callbacks struct{ resolve, validate int }
+	load := func(token string) *configkit.Result[map[string]any] {
+		t.Helper()
+		main := configkit.Input{
+			Source:      configkit.Bytes("main", []byte("item:\n  title: main\n  resource: summary.md\n  token: "+token+"\n")),
+			Destination: destination, Filesystem: context,
+		}
+		underlay := configkit.Input{
+			Source:      configkit.Bytes("underlay", []byte("item:\n  title: lower\n  retained: lower-value\n  token: LOWER-TOKEN-CANARY\n")),
+			Destination: destination,
+		}
+		result, err := configkit.LoadLayered(main, configkit.LayeredOptions[map[string]any]{
+			Underlays: []configkit.Underlay{{Inputs: []configkit.Input{underlay}}},
+			Definition: configkit.Definition[map[string]any]{
+				Resolve: func(configuration *map[string]any, resolution *configkit.Resolution) error {
+					callbacks.resolve++
+					resourcePath := configkit.Keys("catalog", "entries", "item", "resource")
+					input, ok := resolution.InputInfo(resourcePath)
+					if !ok || len(input.Origin.Locations) != 1 {
+						return fmt.Errorf("mounted resource input has no single source origin")
+					}
+					source, ok := resolution.Source(input.Origin.Locations[0].SourceID)
+					if !ok || source.Destination.String() != destination.String() {
+						return fmt.Errorf("resolution source did not retain its destination")
+					}
+					resourceContext, ok := resolution.SourceContext(source.ID)
+					if !ok {
+						return fmt.Errorf("resolution source context is unavailable")
+					}
+					data, err := fs.ReadFile(resourceContext.FS, path.Join(resourceContext.BaseDir, "summary.md"))
+					if err != nil {
+						return fmt.Errorf("read source-relative resource: %w", err)
+					}
+					entry := facadeDestinationMap(t, facadeDestinationMap(t, facadeDestinationMap(t, *configuration, "catalog"), "entries"), "item")
+					entry["resource_text"] = string(data)
+					resolution.Record(configkit.Keys("catalog", "entries", "item", "resource_text"), configkit.ChangeNormalization, "read source-relative summary", resourcePath)
+					return nil
+				},
+				Validate: func(*map[string]any, *configkit.Validation) { callbacks.validate++ },
+				Secret:   func(candidate configkit.Path) bool { return candidate.String() == tokenPath.String() },
+			},
+		})
+		if err != nil {
+			t.Fatalf("mounted load: %v", err)
+		}
+		return result
+	}
+
+	first := load("FIRST-TOKEN-CANARY")
+	second := load("SECOND-TOKEN-CANARY")
+	if callbacks.resolve != 2 || callbacks.validate != 2 {
+		t.Fatalf("lifecycle callbacks = %#v", callbacks)
+	}
+	for _, result := range []*configkit.Result[map[string]any]{first, second} {
+		entry := facadeDestinationMap(t, facadeDestinationMap(t, facadeDestinationMap(t, result.Config, "catalog"), "entries"), "item")
+		if entry["title"] != "main" || entry["retained"] != "lower-value" || entry["resource_text"] != "source-relative summary" {
+			t.Fatalf("mounted main/underlay configuration = %#v", entry)
+		}
+		sources := result.Sources()
+		if len(sources) != 2 || sources[0].Destination.String() != destination.String() || sources[1].Destination.String() != destination.String() {
+			t.Fatalf("public source destinations = %#v", sources)
+		}
+		resource, ok := result.SourceContext(sources[0].ID)
+		if !ok || resource.BaseDir != "assets" {
+			t.Fatalf("completed source context = %#v, %v", resource, ok)
+		}
+		data, err := fs.ReadFile(resource.FS, path.Join(resource.BaseDir, "summary.md"))
+		if err != nil || string(data) != "source-relative summary" {
+			t.Fatalf("completed relative resource = %q, %v", data, err)
+		}
+		if info, ok := result.Metadata().Lookup(titlePath); !ok || info.Origin.Layer != configkit.LayerBase {
+			t.Fatalf("mounted title metadata = %#v, %v", info, ok)
+		}
+		if history := result.Metadata().History(titlePath); len(history) < 2 {
+			t.Fatalf("mounted title history = %#v", history)
+		}
+		if explanation, ok := result.Explain(tokenPath); !ok || !explanation.Effective.Redacted {
+			t.Fatalf("mounted secret explanation = %#v, %v", explanation, ok)
+		}
+		var effective, explanation bytes.Buffer
+		if err := result.WriteEffective(&effective); err != nil {
+			t.Fatal(err)
+		}
+		if err := result.WriteExplanation(&explanation, tokenPath); err != nil {
+			t.Fatal(err)
+		}
+		output := effective.String() + explanation.String()
+		for _, canary := range []string{"FIRST-TOKEN-CANARY", "SECOND-TOKEN-CANARY", "LOWER-TOKEN-CANARY"} {
+			if strings.Contains(output, canary) {
+				t.Fatalf("public inspection leaked %q: %s", canary, output)
+			}
+		}
+	}
+
+	// Mutating published records and query copies must not alter the retained result.
+	returnedSources := first.Sources()
+	returnedSources[0].Destination = configkit.Keys("mutated")
+	if got := first.Sources()[0].Destination.String(); got != destination.String() {
+		t.Fatalf("source destination changed after caller mutation: %q", got)
+	}
+	info, ok := first.Metadata().Lookup(titlePath)
+	if !ok {
+		t.Fatal("mounted metadata lookup failed")
+	}
+	info.Origin.Locations[0].SourceID = "mutated"
+	if fresh, ok := first.Metadata().Lookup(titlePath); !ok || fresh.Origin.Locations[0].SourceID == "mutated" {
+		t.Fatalf("metadata copy mutation escaped: %#v, %v", fresh, ok)
+	}
+	history := first.Metadata().History(titlePath)
+	if len(history) == 0 {
+		t.Fatal("mounted history is empty")
+	}
+	history[0].Path = configkit.Keys("mutated")
+	if fresh := first.Metadata().History(titlePath); len(fresh) == 0 || fresh[0].Path.String() != titlePath.String() {
+		t.Fatalf("history copy mutation escaped: %#v", fresh)
+	}
+
+	differences, err := configkit.Diff(first, second)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 1 || differences[0].Path.String() != tokenPath.String() ||
+		!differences[0].Left.Redacted || !differences[0].Right.Redacted ||
+		strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "TOKEN-CANARY") {
+		t.Fatalf("public mounted secret diff = %#v", differences)
+	}
+	if callbacks.resolve != 2 || callbacks.validate != 2 {
+		t.Fatalf("inspection reran lifecycle callbacks: %#v", callbacks)
+	}
+}
+
+func TestFacadeFailedLoadExposesDetachedSourceDestination(t *testing.T) {
+	type configuration struct {
+		Tenant struct {
+			Count int `yaml:"count"`
+		} `yaml:"tenant"`
+	}
+	destination := configkit.Keys("tenant")
+	_, err := configkit.LoadLayered[configuration](configkit.Input{
+		Source: configkit.Bytes("invalid", []byte("count: not-an-integer\n")), Destination: destination,
+	})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("failed load error = %T %v", err, err)
+	}
+	sources := loadError.Sources()
+	if len(sources) != 1 || sources[0].Destination.String() != destination.String() {
+		t.Fatalf("failed source destinations = %#v", sources)
+	}
+	sources[0].Destination = configkit.Keys("caller-change")
+	if fresh := loadError.Sources(); len(fresh) != 1 || fresh[0].Destination.String() != destination.String() {
+		t.Fatalf("failed source destination was not detached: %#v", fresh)
+	}
+}
+
+func TestFacadeDestinationMappingFailurePaths(t *testing.T) {
+	t.Parallel()
+
+	cycle := map[string]any{}
+	cycle["self"] = cycle
+	for _, fixture := range []struct {
+		name   string
+		values map[string]any
+		path   func(configkit.Path) configkit.Path
+	}{
+		{
+			name:   "unsupported nested sequence value",
+			values: map[string]any{"a/b": []any{map[string]any{"~": func() {}}}},
+			path:   func(base configkit.Path) configkit.Path { return base.Key("a/b").Index(0).Key("~") },
+		},
+		{
+			name:   "cyclic value",
+			values: map[string]any{"item": cycle},
+			path:   func(base configkit.Path) configkit.Path { return base.Key("item").Key("self") },
+		},
+		{
+			name: "constructor failure keeps source coordinates",
+			path: func(configkit.Path) configkit.Path { return configkit.Path{} },
+		},
+	} {
+		t.Run(fixture.name, func(t *testing.T) {
+			for _, role := range []string{"root", "mounted main", "mounted underlay"} {
+				t.Run(role, func(t *testing.T) {
+					var destination configkit.Path
+					if role != "root" {
+						destination = configkit.Keys("catalog", "entries")
+					}
+					wantPath := fixture.path(destination)
+					input := configkit.Input{Source: configkit.Mapping("invalid mapping", fixture.values), Destination: destination}
+					var result *configkit.Result[map[string]any]
+					var err error
+					if role == "mounted underlay" {
+						result, err = configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{
+							Underlays: []configkit.Underlay{{Inputs: []configkit.Input{input}}},
+						})
+					} else {
+						result, err = configkit.LoadLayered[map[string]any](input)
+					}
+					var loadError *configkit.LoadError
+					if result != nil || !errors.As(err, &loadError) {
+						t.Fatalf("failed load = %#v, %v", result, err)
+					}
+					diagnostics := loadError.Diagnostics()
+					if len(diagnostics) != 1 {
+						t.Fatalf("diagnostics = %#v", diagnostics)
+					}
+					diagnostic := diagnostics[0]
+					if diagnostic.Code != configkit.CodeSourceInvalid || !reflect.DeepEqual(diagnostic.Path, wantPath) {
+						t.Fatalf("diagnostic = %#v; want source.invalid at %q", diagnostic, wantPath.String())
+					}
+					if diagnostic.Primary == nil || *diagnostic.Primary != (configkit.Location{SourceID: "source-1"}) || len(diagnostic.Related) != 0 {
+						t.Fatalf("mapping source locations = %#v, %#v", diagnostic.Primary, diagnostic.Related)
+					}
+					sources := loadError.Sources()
+					if len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Destination.String() != destination.String() {
+						t.Fatalf("error sources = %#v", sources)
+					}
+				})
+			}
+		})
+	}
+}
+
+func facadeDestinationMap(t *testing.T, value map[string]any, keys ...string) map[string]any {
+	t.Helper()
+	for _, key := range keys {
+		child, ok := value[key].(map[string]any)
+		if !ok {
+			t.Fatalf("%q = %#v, want mapping", key, value[key])
+		}
+		value = child
+	}
+	return value
+}
+
+func facadeDestinationValue(t *testing.T, value map[string]any, keys ...string) any {
+	t.Helper()
+	return facadeDestinationMap(t, value, keys[:len(keys)-1]...)[keys[len(keys)-1]]
+}

facade_diff_test.go

@@ -0,0 +1,327 @@
+package configkit_test
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type facadeDiffConfig struct {
+	Value   string         `yaml:"value"`
+	Count   int            `yaml:"count"`
+	Items   []string       `yaml:"items"`
+	Dynamic map[string]any `yaml:"dynamic"`
+}
+
+type facadeDiffSecretConfig struct {
+	Secret configkit.Secret `yaml:"secret"`
+}
+
+func TestFacadeDiffKeepsPublicSequenceChangesWithHistoricalDescendants(t *testing.T) {
+	type configuration struct {
+		Items []any `yaml:"items"`
+	}
+	load := func(public string) *configkit.Result[configuration] {
+		t.Helper()
+		result, err := configkit.Load(configkit.Bytes("historical-sequence.yml", []byte("items: [{secret: disposable-canary}, "+public+"]\n")),
+			configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+				Secret: func(path configkit.Path) bool { return path.String() == "/items/0/secret" },
+				Resolve: func(current *configuration, resolution *configkit.Resolution) error {
+					current.Items[0] = "public scalar"
+					resolution.Record(configkit.Keys("items"), configkit.ChangeNormalization, "normalize first item")
+					return nil
+				},
+			}})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+	left, right := load("1"), load("2")
+	history, ok := left.Explain(configkit.Keys("items").Index(0).Key("secret"))
+	if !ok || history.Effective.Present || len(history.History) == 0 {
+		t.Fatalf("removed historical secret = %#v, known=%v", history, ok)
+	}
+	maskedHistory := false
+	for _, change := range history.History {
+		maskedHistory = maskedHistory || change.Before.Redacted || change.After.Redacted
+	}
+	if !maskedHistory {
+		t.Fatal("historical sensitivity was not retained")
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/items" {
+		t.Fatalf("sequence difference = %#v, %v", differences, err)
+	}
+	difference := differences[0]
+	if !difference.Left.Present || !difference.Right.Present || difference.Left.Redacted || difference.Right.Redacted || difference.HiddenChanged {
+		t.Fatalf("public sequence change was concealed: %#v", difference)
+	}
+	if difference.Left.YAML != "- public scalar\n- 1\n" || difference.Right.YAML != "- public scalar\n- 2\n" {
+		t.Fatalf("public sequence snapshots = %#v", difference)
+	}
+}
+
+func TestFacadeDiffKeepsPublicScalarAcrossHistoricalAbsence(t *testing.T) {
+	type configuration struct {
+		Dynamic map[string]any `yaml:"dynamic"`
+	}
+	options := configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+		Secret: func(path configkit.Path) bool { return path.String() == "/dynamic/entry/token" },
+		Resolve: func(current *configuration, resolution *configkit.Resolution) error {
+			delete(current.Dynamic, "entry")
+			resolution.Record(configkit.Keys("dynamic", "entry"), configkit.ChangeNormalization, "remove entry")
+			return nil
+		},
+	}}
+	absent, err := configkit.Load(configkit.Bytes("removed-entry.yml", []byte("dynamic: {entry: {token: disposable-canary}}\n")), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	options.Definition.Resolve = nil
+	present, err := configkit.Load(configkit.Bytes("public-entry.yml", []byte("dynamic: {entry: public}\n")), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, test := range []struct {
+		name        string
+		left, right *configkit.Result[configuration]
+		leftPresent bool
+	}{
+		{name: "addition", left: absent, right: present},
+		{name: "removal", left: present, right: absent, leftPresent: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			differences, err := configkit.Diff(test.left, test.right)
+			if err != nil || len(differences) != 1 || differences[0].Path.String() != "/dynamic/entry" {
+				t.Fatalf("entry difference = %#v, %v", differences, err)
+			}
+			difference := differences[0]
+			if difference.Left.Present != test.leftPresent || difference.Right.Present == test.leftPresent ||
+				difference.Left.Redacted || difference.Right.Redacted || difference.HiddenChanged {
+				t.Fatalf("public presence change was concealed: %#v", difference)
+			}
+			wantLeft, wantRight := "", "public\n"
+			if test.leftPresent {
+				wantLeft, wantRight = wantRight, wantLeft
+			}
+			if difference.Left.YAML != wantLeft || difference.Right.YAML != wantRight {
+				t.Fatalf("entry snapshots = %#v", difference)
+			}
+		})
+	}
+}
+
+func TestFacadeDiffReturnsDetachedCanonicalRecords(t *testing.T) {
+	left, err := configkit.Load[facadeDiffConfig](configkit.Bytes("left.yml", []byte("value: before\ncount: 0\nitems: [one, two]\ndynamic: {}\n")))
+	if err != nil {
+		t.Fatalf("load left: %v", err)
+	}
+	right, err := configkit.Load[facadeDiffConfig](configkit.Bytes("right.yml", []byte("value: after\ncount: 0\nitems: [one, three]\ndynamic:\n  added: null\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	differences, err := configkit.Diff(left, right)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wantPaths := []string{"/dynamic/added", "/items", "/value"}
+	if len(differences) != len(wantPaths) {
+		t.Fatalf("differences = %#v", differences)
+	}
+	for index, difference := range differences {
+		if got := difference.Path.String(); got != wantPaths[index] {
+			t.Errorf("path[%d] = %q, want %q", index, got, wantPaths[index])
+		}
+	}
+	if differences[0].Left.Present || !differences[0].Right.Present || differences[0].Right.YAML != "null\n" {
+		t.Errorf("null/absence snapshots = %#v / %#v", differences[0].Left, differences[0].Right)
+	}
+	if differences[1].Left.YAML != "- one\n- two\n" || differences[1].Right.YAML != "- one\n- three\n" {
+		t.Errorf("sequence snapshots = %#v / %#v", differences[1].Left, differences[1].Right)
+	}
+
+	left.Config.Value = "caller mutation"
+	left.Config.Items[0] = "caller mutation"
+	right.Config.Dynamic["added"] = "caller mutation"
+	differences[2].Left.YAML = "record mutation"
+	differences[2].Path = configkit.Keys("rewritten")
+
+	again, err := configkit.Diff(left, right)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if again[2].Path.String() != "/value" || again[2].Left.YAML != "before\n" || again[1].Left.YAML != "- one\n- two\n" {
+		t.Fatalf("repeat diff changed = %#v", again)
+	}
+	explanation, ok := left.Explain(configkit.Keys("value"))
+	if !ok || explanation.Effective.YAML != "before\n" {
+		t.Fatalf("frozen explanation = %#v, %v", explanation, ok)
+	}
+}
+
+func TestFacadeDiffRejectsUnavailableResults(t *testing.T) {
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	loaded, err := configkit.Load[config](configkit.Bytes("valid.yml", []byte("value: valid\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var nilResult *configkit.Result[config]
+	zeroResult := &configkit.Result[config]{Config: config{Value: "must not be inspected"}}
+	for name, left := range map[string]*configkit.Result[config]{"nil": nilResult, "zero": zeroResult} {
+		t.Run(name, func(t *testing.T) {
+			if records, err := configkit.Diff(left, loaded); err == nil || records != nil {
+				t.Fatalf("Diff = %#v, %v; want nil records and error", records, err)
+			}
+		})
+	}
+	if records, err := configkit.Diff(loaded, nilResult); err == nil || records != nil {
+		t.Fatalf("right nil Diff = %#v, %v", records, err)
+	}
+}
+
+func TestFacadeDiffIgnoresSourceAndResolutionHistory(t *testing.T) {
+	type config struct {
+		Count int `yaml:"count"`
+	}
+	defaults := configkit.Options[config]{Definition: configkit.Definition[config]{
+		Defaults: func() config { return config{Count: 7} },
+	}}
+	defaulted, err := configkit.Load[config](configkit.NoSource(), defaults)
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicit, err := configkit.Load[config](configkit.Bytes("explicit.yml", []byte("count: 7\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(defaulted, explicit); err != nil || len(differences) != 0 {
+		t.Fatalf("default/source difference = %#v, %v", differences, err)
+	}
+
+	zeroDefault, err := configkit.Load[config](configkit.NoSource(), configkit.Options[config]{Definition: configkit.Definition[config]{Defaults: func() config { return config{} }}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicitZero, err := configkit.Load[config](configkit.Bytes("explicit-zero.yml", []byte("count: 0\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(zeroDefault, explicitZero); err != nil || len(differences) != 0 {
+		t.Fatalf("explicit/defaulted zero difference = %#v, %v", differences, err)
+	}
+
+	resolve := func(value string) configkit.Options[config] {
+		return configkit.Options[config]{Definition: configkit.Definition[config]{Resolve: func(current *config, resolution *configkit.Resolution) error {
+			current.Count = len(value) + 8
+			resolution.Record(configkit.Keys("count"), configkit.ChangeNormalization, "set effective count")
+			return nil
+		}}}
+	}
+	resolvedA, err := configkit.Load[config](configkit.Bytes("a.yml", []byte("{}\n")), resolve("a"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	resolvedB, err := configkit.Load[config](configkit.Bytes("b.yml", []byte("{}\n")), resolve("b"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(resolvedA, resolvedB); err != nil || len(differences) != 0 {
+		t.Fatalf("equal resolved values = %#v, %v", differences, err)
+	}
+}
+
+func TestFacadeDiffUsesFrozenFilesAndRunsCallbacksOnlyDuringLoad(t *testing.T) {
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	calls := 0
+	options := configkit.Options[config]{Definition: configkit.Definition[config]{Defaults: func() config {
+		calls++
+		return config{}
+	}}}
+	dir := t.TempDir()
+	paths := []string{filepath.Join(dir, "left.yml"), filepath.Join(dir, "right.yml")}
+	for index, contents := range []string{"value: left\n", "value: right\n"} {
+		if err := os.WriteFile(paths[index], []byte(contents), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	left, err := configkit.Load[config](configkit.File(paths[0]), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	right, err := configkit.Load[config](configkit.File(paths[1]), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if calls != 2 {
+		t.Fatalf("defaults calls after loads = %d", calls)
+	}
+	for _, path := range paths {
+		if err := os.Remove(path); err != nil {
+			t.Fatal(err)
+		}
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/value" {
+		t.Fatalf("Diff after removing files = %#v, %v", differences, err)
+	}
+	if calls != 2 {
+		t.Fatalf("defaults reran during Diff: %d calls", calls)
+	}
+}
+
+func TestFacadeDiffCombinesSecretSensitivityAndLoadErrors(t *testing.T) {
+	t.Setenv("FACADE_DIFF_SECRET", "credential-one")
+	secretOptions := configkit.Options[facadeDiffSecretConfig]{Definition: configkit.Definition[facadeDiffSecretConfig]{Secret: func(path configkit.Path) bool {
+		return path.String() == "/secret"
+	}}}
+	left, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("left.yml", []byte("secret: FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("FACADE_DIFF_SECRET", "credential-two")
+	right, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("right.yml", []byte("secret: FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(left, right); err != nil || len(differences) != 0 {
+		t.Fatalf("credential rotation = %#v, %v", differences, err)
+	}
+	t.Setenv("OTHER_FACADE_DIFF_SECRET", "other-credential")
+	otherReference, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("other.yml", []byte("secret: OTHER_FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	differences, err := configkit.Diff(left, otherReference)
+	if err != nil || len(differences) != 1 || !differences[0].HiddenChanged || !differences[0].Left.Redacted || !differences[0].Right.Redacted {
+		t.Fatalf("reference change = %#v, %v", differences, err)
+	}
+	if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "credential") {
+		t.Fatalf("credential leaked: %#v", differences)
+	}
+
+	invalidLeft, leftErr := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("invalid-left.yml", []byte("unknown: true\n")))
+	if invalidLeft != nil || leftErr == nil {
+		t.Fatalf("invalid left load = %#v, %v", invalidLeft, leftErr)
+	}
+	var loadError *configkit.LoadError
+	if !errors.As(leftErr, &loadError) {
+		t.Fatalf("left load error = %T, want *configkit.LoadError", leftErr)
+	}
+	invalidRight, rightErr := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("invalid-right.yml", []byte("unknown: true\n")))
+	if invalidRight != nil || rightErr == nil {
+		t.Fatalf("invalid right load = %#v, %v", invalidRight, rightErr)
+	}
+	if !errors.As(rightErr, &loadError) {
+		t.Fatalf("right load error = %T, want *configkit.LoadError", rightErr)
+	}
+}

facade_equivalence_test.go

@@ -0,0 +1,164 @@
+package configkit_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type equivalentConfig struct {
+	Version  uint64   `yaml:"version"`
+	Name     string   `yaml:"name"`
+	Enabled  bool     `yaml:"enabled"`
+	Count    int      `yaml:"count"`
+	Items    []string `yaml:"items"`
+	Optional *string  `yaml:"optional"`
+}
+
+func TestEquivalentSourceFormsPreserveInputSemantics(t *testing.T) {
+	const yaml = "version: 1\nname: retained\nenabled: false\ncount: 0\nitems: []\noptional: null\n"
+	hostPath := filepath.Join(t.TempDir(), "main.yml")
+	if err := os.WriteFile(hostPath, []byte(yaml), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{"main.yml": &fstest.MapFile{Data: []byte(yaml)}})
+	context := &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &namespace}
+	for _, test := range []struct {
+		name    string
+		source  configkit.Source
+		context *configkit.FilesystemContext
+		kind    configkit.SourceKind
+	}{
+		{"host", configkit.File(hostPath), nil, configkit.SourceKindFile},
+		{"native", namespace.File("main.yml"), nil, configkit.SourceKindFilesystem},
+		{"bytes", configkit.Bytes("memory", []byte(yaml)), context, configkit.SourceKindBytes},
+		{"reader", configkit.Reader("stream", strings.NewReader(yaml)), context, configkit.SourceKindReader},
+		{"mapping", configkit.Mapping("programmatic", map[string]any{
+			"version": uint64(1), "name": "retained", "enabled": false, "count": 0,
+			"items": []any{}, "optional": nil,
+		}), context, configkit.SourceKindMapping},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			checks := 0
+			result, err := configkit.LoadLayered(configkit.Input{Source: test.source, Filesystem: test.context}, configkit.LayeredOptions[equivalentConfig]{
+				Definition: configkit.Definition[equivalentConfig]{Version: &configkit.VersionRule{
+					Path: configkit.Keys("version"), Check: func(version configkit.Version, validation *configkit.Validation) error {
+						checks++
+						if !version.Present || version.Text != "1" {
+							t.Errorf("version = %#v", version)
+						}
+						return nil
+					},
+				}},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			want := equivalentConfig{Version: 1, Name: "retained", Items: []string{}}
+			if !reflect.DeepEqual(result.Config, want) || checks != 1 {
+				t.Fatalf("config = %#v, checks %d", result.Config, checks)
+			}
+			for _, field := range []string{"version", "name", "enabled", "count", "items", "optional"} {
+				info, ok := result.Metadata().Lookup(configkit.Keys(field))
+				if !ok || !info.Explicit || info.Origin.Layer != configkit.LayerBase || info.Null != (field == "optional") {
+					t.Errorf("%s input info = %#v, %v", field, info, ok)
+				}
+			}
+			sources := result.Sources()
+			if len(sources) != 1 || sources[0].Kind != test.kind || sources[0].ID != "source-1" || sources[0].UnderlayIndex != 0 {
+				t.Fatalf("sources = %#v", sources)
+			}
+			resource, ok := result.SourceContext("source-1")
+			if !ok || resource.BaseDir != "." || resource.FS == nil {
+				t.Fatalf("source context = %#v, %v", resource, ok)
+			}
+		})
+	}
+}
+
+func TestRepeatedImportsAcrossTiersAndReaderConsumption(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"left.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: shared.yml}]}\n")},
+		"right.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: shared.yml}]}\n")},
+		"shared.yml": &fstest.MapFile{Data: []byte("name: retained\n")},
+	})
+	type config struct {
+		Name string `yaml:"name"`
+	}
+	left := configkit.Input{Source: namespace.File("left.yml"), Imports: true}
+	right := configkit.Input{Source: namespace.File("right.yml"), Imports: true}
+	_, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{left, right}}},
+	})
+	if got := layeredErrorCode(t, err); got != configkit.CodeImportRepeated {
+		t.Fatalf("same-tier repeated import = %s", got)
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{left}}, {Inputs: []configkit.Input{right}}},
+	})
+	if err != nil || result.Config.Name != "retained" || len(result.Sources()) != 4 {
+		t.Fatalf("cross-tier repeated import = %#v, %v", result, err)
+	}
+	reader := configkit.Reader("single-use", strings.NewReader("name: retained\n"))
+	_, err = configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: reader}}}, {Inputs: []configkit.Input{{Source: reader}}}},
+	})
+	if got := layeredErrorCode(t, err); got != configkit.CodeSourceReaderUsed {
+		t.Fatalf("reader re-consumption = %s", got)
+	}
+}
+
+func TestReplacedSecretRemainsAbsentFromSafeOutput(t *testing.T) {
+	type config struct {
+		Secret string `yaml:"secret"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("secret: new-secret-canary\n"))}, configkit.LayeredOptions[config]{
+		Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"secret": "old-secret-canary"})}}}},
+		Definition: configkit.Definition[config]{Secret: func(path configkit.Path) bool { return path.String() == "/secret" }},
+	})
+	if err != nil || result.Config.Secret != "new-secret-canary" {
+		t.Fatalf("secret load = %#v, %v", result, err)
+	}
+	var effective, history bytes.Buffer
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if err := result.WriteExplanation(&history, configkit.Keys("secret")); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String()+history.String(), "old-secret-canary") || strings.Contains(effective.String()+history.String(), "new-secret-canary") {
+		t.Fatalf("secret output leaked: %q %q", effective.String(), history.String())
+	}
+	_, err = configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("secret: new-secret-canary\n"))}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"secret": "old-secret-canary"})}}}},
+		Definition: configkit.Definition[config]{Validate: func(config *config, validation *configkit.Validation) {
+			validation.Error(configkit.Keys("secret"), "invalid secret")
+		}},
+	})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("validation error = %#v", err)
+	}
+	var diagnostics bytes.Buffer
+	if err := configkit.WriteDiagnostics(&diagnostics, loadError.Diagnostics()); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(diagnostics.String(), "secret-canary") {
+		t.Fatalf("diagnostic leaked secret: %q", diagnostics.String())
+	}
+}
+
+func layeredErrorCode(t *testing.T, err error) configkit.Code {
+	t.Helper()
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 {
+		t.Fatalf("load error = %#v", err)
+	}
+	return loadError.Diagnostics()[0].Code
+}

facade_filesystem_test.go

@@ -0,0 +1,142 @@
+package configkit_test
+
+import (
+	"embed"
+	"io/fs"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+//go:embed testdata/filesystem
+var filesystemFixtures embed.FS
+
+type filesystemFacadeConfig struct {
+	Items map[string]string `yaml:"items"`
+}
+
+func TestFacadeFilesystemSourceImportsOverlayAndReuse(t *testing.T) {
+	fixture, err := fs.Sub(filesystemFixtures, "testdata/filesystem")
+	if err != nil {
+		t.Fatal(err)
+	}
+	namespace := configkit.NewFilesystem("embedded", fixture)
+	copyOfNamespace := namespace
+	source := namespace.File("config/main.yml")
+	for range 2 {
+		result, err := configkit.Load(source, configkit.Options[filesystemFacadeConfig]{
+			Imports: true, Overlay: "testing", Filesystem: &configkit.FilesystemContext{FS: &copyOfNamespace},
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want := (map[string]string{"main": "selected", "shared": "default"}); !reflect.DeepEqual(result.Config.Items, want) {
+			t.Fatalf("items = %#v, want %#v", result.Config.Items, want)
+		}
+		sources := result.Sources()
+		if len(sources) != 3 {
+			t.Fatalf("sources = %#v", sources)
+		}
+		for index, want := range []string{"config/main.yml", "shared.yml", "overlays/testing.yml"} {
+			if sources[index].ID != "source-"+strconv.Itoa(index+1) || sources[index].Path != want || sources[index].Kind != configkit.SourceKindFilesystem {
+				t.Errorf("source %d = %#v", index, sources[index])
+			}
+			if sources[index].Label != "embedded/"+want {
+				t.Errorf("source label = %q", sources[index].Label)
+			}
+		}
+		if sources[1].ImportedFrom == nil || sources[1].ImportedFrom.SourceID != sources[0].ID || sources[1].ImportedFrom.Line == 0 {
+			t.Fatalf("import location = %#v", sources[1].ImportedFrom)
+		}
+	}
+}
+
+func TestFacadeFilesystemContextsAndIdentity(t *testing.T) {
+	first := configkit.NewFilesystem("same", fstest.MapFS{
+		"root/config/main.yml":      &fstest.MapFile{Data: []byte("configkit: {imports: [{path: ../shared.yml}]}\n")},
+		"root/shared.yml":           &fstest.MapFile{Data: []byte("items: {value: first}\n")},
+		"root/overlays/testing.yml": &fstest.MapFile{Data: []byte("items: {overlay: first}\n")},
+	})
+	second := configkit.NewFilesystem("same", fstest.MapFS{
+		"root/config/main.yml": &fstest.MapFile{Data: []byte("items: {value: second}\n")},
+	})
+	context := &configkit.FilesystemContext{Root: "root", FS: &first}
+	result, err := configkit.Load(first.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Imports: true, Overlay: "testing", Filesystem: context})
+	if err != nil || result.Config.Items["value"] != "first" || result.Config.Items["overlay"] != "first" {
+		t.Fatalf("contained load = %#v, %v", result, err)
+	}
+	result, err = configkit.Load[filesystemFacadeConfig](second.File("root/config/main.yml"))
+	if err != nil || result.Config.Items["value"] != "second" {
+		t.Fatalf("same-label independent namespace = %#v, %v", result, err)
+	}
+	checkCode(t, second.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: context}, configkit.CodeOptionsInvalid)
+	checkCode(t, first.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: &configkit.FilesystemContext{Root: "root/config", FS: &first}, Imports: true}, configkit.CodeImportOutsideRoot)
+	checkCode(t, configkit.File("testdata/filesystem/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: context}, configkit.CodeOptionsInvalid)
+
+	for _, input := range []struct {
+		name   string
+		source configkit.Source
+		kind   configkit.SourceKind
+	}{
+		{"bytes", configkit.Bytes("memory", []byte("configkit: {imports: [{path: shared.yml}]}\n")), configkit.SourceKindBytes},
+		{"reader", configkit.Reader("stream", strings.NewReader("configkit: {imports: [{path: shared.yml}]}\n")), configkit.SourceKindReader},
+	} {
+		t.Run(input.name, func(t *testing.T) {
+			result, err := configkit.Load(input.source, configkit.Options[filesystemFacadeConfig]{Imports: true, Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &first}})
+			if err != nil || result.Config.Items["value"] != "first" {
+				t.Fatalf("synthetic import = %#v, %v", result, err)
+			}
+			if got := result.Sources(); len(got) != 2 || got[0].Kind != input.kind || got[0].Path != "" || got[1].Kind != configkit.SourceKindFilesystem {
+				t.Fatalf("sources = %#v", got)
+			}
+		})
+	}
+	checkCode(t, configkit.Bytes("memory", []byte("items: {}\n")), configkit.Options[filesystemFacadeConfig]{Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: ".", FS: &first}}, configkit.CodeImportOutsideRoot)
+	result, err = configkit.Load(configkit.Bytes("memory", []byte("items: {value: main}\n")), configkit.Options[filesystemFacadeConfig]{
+		Overlay: "testing", Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &first},
+	})
+	if err != nil || result.Config.Items["value"] != "main" || result.Config.Items["overlay"] != "first" {
+		t.Fatalf("synthetic selected overlay = %#v, %v", result, err)
+	}
+}
+
+func TestFacadeSourceKindsIncludeHostAndCLI(t *testing.T) {
+	result, err := configkit.Load(configkit.File("testdata/filesystem/shared.yml"), configkit.Options[filesystemFacadeConfig]{
+		Overrides: []configkit.Override{{Path: configkit.Keys("items", "shared"), Value: "changed", Flag: "--shared"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Kind != configkit.SourceKindFile || sources[0].Path == "" || sources[1].Kind != configkit.SourceKindCLI || sources[1].Path != "" {
+		t.Fatalf("host and CLI sources = %#v", sources)
+	}
+}
+
+func TestFacadeFilesystemConstructorDiagnostics(t *testing.T) {
+	var nilFilesystem fs.FS
+	var zero configkit.Filesystem
+	for _, source := range []configkit.Source{
+		zero.File("main.yml"),
+		configkit.NewFilesystem("", fstest.MapFS{}).File("main.yml"),
+		configkit.NewFilesystem("bad\nlabel", fstest.MapFS{}).File("main.yml"),
+		configkit.NewFilesystem("good", nilFilesystem).File("main.yml"),
+		configkit.NewFilesystem("good", fstest.MapFS{}).File("."),
+		configkit.NewFilesystem("good", fstest.MapFS{}).File("../outside.yml"),
+	} {
+		checkCode(t, source, configkit.Options[filesystemFacadeConfig]{}, configkit.CodeSourceInvalid)
+	}
+}
+
+func checkCode(t *testing.T, source configkit.Source, options configkit.Options[filesystemFacadeConfig], want configkit.Code) {
+	t.Helper()
+	_, err := configkit.Load(source, options)
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != want {
+		t.Fatalf("error = %#v, want %s", err, want)
+	}
+}

facade_inheritance_test.go

@@ -0,0 +1,284 @@
+package configkit_test
+
+import (
+	"bytes"
+	"io/fs"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type inheritedEntry struct {
+	Enabled  bool                         `yaml:"enabled"`
+	Count    int                          `yaml:"count"`
+	Resource string                       `yaml:"resource"`
+	Settings map[string]map[string]string `yaml:"settings"`
+}
+
+type inheritedConfiguration struct {
+	Version string                    `yaml:"version"`
+	Entries map[string]inheritedEntry `yaml:"entries"`
+}
+
+func TestFacadeInheritancePreservesPresenceOriginsAndHistory(t *testing.T) {
+	input := []byte(`entries:
+  parent:
+    enabled: false
+    count: 0
+    settings:
+      primary: {value: original}
+      secondary: {extends: primary}
+  child:
+    extends: parent
+    settings:
+      primary: {value: custom}
+`)
+	defaultRan := false
+	result, err := configkit.Load(configkit.Bytes("config.yml", input), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+		Resolve: func(config *inheritedConfiguration, resolution *configkit.Resolution) error {
+			if !resolution.IsExplicit(configkit.Keys("entries", "child", "enabled")) {
+				t.Error("inherited false is not explicit")
+			}
+			info, found := resolution.InputInfo(configkit.Keys("entries", "child", "enabled"))
+			if !found || len(info.Origin.Inheritance) != 1 {
+				t.Errorf("resolver inherited origin = %#v, %v", info, found)
+			}
+			child := config.Entries["child"]
+			configkit.Default(resolution, configkit.Keys("entries", "child", "enabled"), &child.Enabled, true, "use fallback")
+			defaultRan = child.Enabled
+			config.Entries["child"] = child
+			return nil
+		},
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	child := result.Config.Entries["child"]
+	if defaultRan || child.Enabled || child.Count != 0 || child.Settings["secondary"]["value"] != "custom" {
+		t.Fatalf("inherited values = %#v, default ran %v", child, defaultRan)
+	}
+	path := configkit.Keys("entries", "child", "settings", "secondary", "value")
+	info, found := result.Metadata().Lookup(path)
+	if !found || !info.Explicit || len(info.Origin.Inheritance) != 1 ||
+		info.Origin.Inheritance[0].From.String() != "/entries/child/settings/primary/value" ||
+		info.Origin.Inheritance[0].To.String() != path.String() {
+		t.Fatalf("nested destination origin = %#v, %v", info, found)
+	}
+	info.Origin.Inheritance[0].Locations[0].SourceID = "mutated"
+	info, _ = result.Metadata().Lookup(path)
+	if info.Origin.Inheritance[0].Locations[0].SourceID == "mutated" {
+		t.Fatal("public inheritance record aliases inspection state")
+	}
+	if _, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "extends")); found {
+		t.Fatal("consumed directive remained effective")
+	}
+	control, found := result.Explain(configkit.Keys("entries", "child", "extends"))
+	if !found || control.Effective.Present || len(control.History) < 2 ||
+		control.History[len(control.History)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("consumed directive = %#v, %v", control, found)
+	}
+	explanation, found := result.Explain(path)
+	if !found || explanation.Effective.YAML != "custom\n" {
+		t.Fatalf("nested explanation = %#v, %v", explanation, found)
+	}
+	for _, change := range explanation.History {
+		if change.Origin.Operation == configkit.ChangeDecoding {
+			t.Fatalf("inherited value attributed to decoding: %#v", explanation.History)
+		}
+	}
+}
+
+func TestFacadeInheritanceLayeredPrecedenceAndSourceContext(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"config/main.yml":          &fstest.MapFile{Data: []byte("entries: {child: {extends: parent}}\n")},
+		"shared/parent.yml":        &fstest.MapFile{Data: []byte("entries: {parent: {resource: icon.txt, count: 1}}\n")},
+		"shared/flags.yml":         &fstest.MapFile{Data: []byte("entries: {parent: {enabled: false}}\n")},
+		"shared/icon.txt":          &fstest.MapFile{Data: []byte("parent-resource")},
+		"config/overlays/test.yml": &fstest.MapFile{Data: []byte("entries: {parent: {count: 2}}\n")},
+	})
+	result, err := configkit.LoadLayered(configkit.Input{Source: filesystem.File("config/main.yml")}, configkit.LayeredOptions[inheritedConfiguration]{
+		Underlays: []configkit.Underlay{
+			{Inputs: []configkit.Input{{Source: filesystem.File("shared/parent.yml")}}},
+			{Inputs: []configkit.Input{{Source: filesystem.File("shared/flags.yml")}}},
+		},
+		Overlay: "test", OverlayFilesystem: &configkit.FilesystemContext{Root: "config", BaseDir: "config", FS: &filesystem},
+		Overrides: []configkit.Override{{Path: configkit.Keys("entries", "child", "count"), Value: 3, Flag: "--count"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	child := result.Config.Entries["child"]
+	if child.Count != 3 || child.Resource != "icon.txt" {
+		t.Fatalf("layered inheritance = %#v", child)
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "resource"))
+	if !found || len(info.Origin.Inheritance) != 1 || len(info.Origin.Locations) != 1 {
+		t.Fatalf("resource origin = %#v, %v", info, found)
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found || context.BaseDir != "shared" || context.FS == nil {
+		t.Fatalf("source context = %#v, %v", context, found)
+	}
+	data, err := fs.ReadFile(context.FS, context.BaseDir+"/"+child.Resource)
+	if err != nil || string(data) != "parent-resource" {
+		t.Fatalf("source-relative resource = %q, %v", data, err)
+	}
+	count, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "count"))
+	if !found || count.Origin.Layer != configkit.LayerCLI || len(count.Origin.Inheritance) != 0 {
+		t.Fatalf("CLI replacement origin = %#v, %v", count, found)
+	}
+	var rendered bytes.Buffer
+	if err := result.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), "extends:") {
+		t.Fatalf("effective rendering = %q, %v", rendered.String(), err)
+	}
+}
+
+func TestFacadeInheritanceMappingMatchesBytes(t *testing.T) {
+	values := map[string]any{"entries": map[string]any{
+		"parent": map[string]any{"enabled": false, "count": 0},
+		"child":  map[string]any{"extends": "parent"},
+	}}
+	mapped, err := configkit.Load[inheritedConfiguration](configkit.Mapping("mapping", values))
+	if err != nil {
+		t.Fatal(err)
+	}
+	fromYAML, err := configkit.Load[inheritedConfiguration](configkit.Bytes("config.yml", []byte("entries: {parent: {enabled: false, count: 0}, child: {extends: parent}}\n")))
+	if err != nil || !reflect.DeepEqual(mapped.Config, fromYAML.Config) {
+		t.Fatalf("mapping and bytes differ: %#v, %#v, %v", mapped, fromYAML, err)
+	}
+}
+
+func TestFacadeInheritanceProtectsVersionOwnership(t *testing.T) {
+	for _, test := range []struct {
+		name, input string
+		path        configkit.Path
+		wantFailure bool
+	}{
+		{"inherited version", "entries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("entries", "child", "count"), true},
+		{"overridden version", "entries: {parent: {count: 1}, child: {extends: parent, count: 1}}\n", configkit.Keys("entries", "child", "count"), true},
+		{"direct unchanged version", "entries: {parent: {enabled: false}, child: {extends: parent, count: 1}}\n", configkit.Keys("entries", "child", "count"), false},
+		{"consumed checked directive", "entries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("entries", "child", "extends"), true},
+		{"independent top-level version", "version: v1\nentries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("version"), false},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			checks := struct{ version, defaults, resolution, validation int }{}
+			result, err := configkit.Load(configkit.Bytes("version.yml", []byte(test.input)), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+				Version: &configkit.VersionRule{Path: test.path, Check: func(configkit.Version, *configkit.Validation) error {
+					checks.version++
+					return nil
+				}},
+				Defaults: func() inheritedConfiguration { checks.defaults++; return inheritedConfiguration{} },
+				Resolve:  func(*inheritedConfiguration, *configkit.Resolution) error { checks.resolution++; return nil },
+				Validate: func(*inheritedConfiguration, *configkit.Validation) { checks.validation++ },
+			}})
+			if test.wantFailure {
+				if result != nil || err == nil {
+					t.Fatalf("protected version = %#v, %v", result, err)
+				}
+				loadError, ok := err.(*configkit.LoadError)
+				if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeInheritanceVersion ||
+					checks != (struct{ version, defaults, resolution, validation int }{version: 1}) {
+					t.Fatalf("version failure = %v, callbacks %#v", err, checks)
+				}
+			} else if err != nil || result == nil || checks != (struct{ version, defaults, resolution, validation int }{1, 1, 1, 1}) {
+				t.Fatalf("unchanged version = %#v, %v, callbacks %#v", result, err, checks)
+			}
+		})
+	}
+}
+
+func TestFacadeInheritanceFailureStopsLaterCallbacks(t *testing.T) {
+	checks := struct{ defaults, resolution, validation int }{}
+	result, err := configkit.Load(configkit.Bytes("invalid.yml", []byte("entries: {child: {extends: PRIVATE-REFERENCE-CANARY}}\n")), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+		Defaults: func() inheritedConfiguration { checks.defaults++; return inheritedConfiguration{} },
+		Resolve:  func(*inheritedConfiguration, *configkit.Resolution) error { checks.resolution++; return nil },
+		Validate: func(*inheritedConfiguration, *configkit.Validation) { checks.validation++ },
+	}})
+	if result != nil || err == nil || checks != (struct{ defaults, resolution, validation int }{}) || strings.Contains(err.Error(), "PRIVATE-REFERENCE-CANARY") {
+		t.Fatalf("invalid inheritance = %#v, %v, callbacks %#v", result, err, checks)
+	}
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeInheritanceMissing {
+		t.Fatalf("missing reference diagnostics = %v", err)
+	}
+}
+
+func TestFacadeInheritanceResolvesAfterImportsAndCLIReferenceChange(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: parent.yml}]}\nentries: {child: {extends: first}}\n")},
+		"parent.yml": &fstest.MapFile{Data: []byte("entries: {first: {count: 1}, second: {count: 2}}\n")},
+	})
+	result, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[inheritedConfiguration]{
+		Imports: true, Overrides: []configkit.Override{{Path: configkit.Keys("entries", "child", "extends"), Value: "second", Flag: "--extends"}},
+	})
+	if err != nil || result.Config.Entries["child"].Count != 2 {
+		t.Fatalf("import and CLI reference = %#v, %v", result, err)
+	}
+	history := result.Metadata().History(configkit.Keys("entries", "child", "extends"))
+	if len(history) < 3 || history[len(history)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("superseded reference history = %#v", history)
+	}
+	conflicting := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: parent.yml}]}\nentries: {child: {extends: first}, first: {count: 3}}\n")},
+		"parent.yml": &fstest.MapFile{Data: []byte("entries: {first: {count: 1}}\n")},
+	})
+	_, err = configkit.Load[inheritedConfiguration](conflicting.File("main.yml"), configkit.Options[inheritedConfiguration]{Imports: true})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeComposeConflict {
+		t.Fatalf("strict conflict before expansion = %v", err)
+	}
+}
+
+func TestFacadeInheritanceSelectedOverlayCancelsRelationship(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":          &fstest.MapFile{Data: []byte("entries: {parent: {count: 7}, child: {extends: parent, enabled: false}}\n")},
+		"overlays/test.yml": &fstest.MapFile{Data: []byte("entries: {child: {extends: null}}\n")},
+	})
+	result, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[inheritedConfiguration]{
+		Overlay: "test", Filesystem: &configkit.FilesystemContext{Root: ".", FS: &filesystem},
+	})
+	if err != nil || result.Config.Entries["child"].Count != 0 || result.Config.Entries["child"].Enabled {
+		if loadError, ok := err.(*configkit.LoadError); ok {
+			t.Fatalf("cancelled relationship = %#v, %v, diagnostics %#v", result, err, loadError.Diagnostics())
+		}
+		t.Fatalf("cancelled relationship = %#v, %v", result, err)
+	}
+	history := result.Metadata().History(configkit.Keys("entries", "child", "extends"))
+	if len(history) < 3 || history[len(history)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("cancellation history = %#v", history)
+	}
+}
+
+func TestFacadeInheritedSecretsStayRedactedAfterLookup(t *testing.T) {
+	const canary = "INHERITED-RELEASE-TEST-CREDENTIAL"
+	t.Setenv("CONFIGKIT_INHERITED_TEST_SECRET", canary)
+	type entry struct {
+		Credential configkit.Secret `yaml:"credential"`
+		Plain      string           `yaml:"plain"`
+	}
+	type configuration struct {
+		Entries map[string]entry `yaml:"entries"`
+	}
+	result, err := configkit.Load[configuration](configkit.Bytes("secret.yml", []byte("entries: {parent: {credential: CONFIGKIT_INHERITED_TEST_SECRET, plain: visible}, child: {extends: parent}}\n")), configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+		Secret: func(path configkit.Path) bool { return path.String() == "/entries/parent/credential" },
+	}})
+	if err != nil || result.Config.Entries["child"].Credential.Value() != canary {
+		t.Fatalf("inherited credential = %#v, %v", result, err)
+	}
+	path := configkit.Keys("entries", "child", "credential")
+	explanation, found := result.Explain(path)
+	if !found || !explanation.Effective.Redacted || strings.Contains(explanation.Effective.YAML, canary) {
+		t.Fatalf("credential explanation = %#v, %v", explanation, found)
+	}
+	var rendered bytes.Buffer
+	if err := result.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), canary) || !strings.Contains(rendered.String(), "visible") {
+		t.Fatalf("effective secret rendering = %q, %v", rendered.String(), err)
+	}
+	rendered.Reset()
+	if err := result.WriteExplanation(&rendered, path); err != nil || strings.Contains(rendered.String(), canary) {
+		t.Fatalf("historical secret rendering = %q, %v", rendered.String(), err)
+	}
+}

facade_layered_test.go

@@ -0,0 +1,243 @@
+package configkit_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type layeredFacadeConfig struct {
+	Settings map[string]string `yaml:"settings"`
+	Count    int               `yaml:"count"`
+	Enabled  bool              `yaml:"enabled"`
+	List     []string          `yaml:"list"`
+	Optional *string           `yaml:"optional"`
+	Derived  string            `yaml:"derived"`
+}
+
+func TestFacadeLayeredMixedSourcesAndInspection(t *testing.T) {
+	mainPath := filepath.Join(t.TempDir(), "main.yml")
+	if err := os.WriteFile(mainPath, []byte("settings: {shared: main, secret: new-canary}\ncount: 0\nenabled: false\nlist: []\noptional: null\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	lower := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":             &fstest.MapFile{Data: []byte("configkit: {imports: [{path: defaults.yml}]}\nsettings: {shared: lower, native: retained, secret: old-canary}\nlist: [lower]\n")},
+		"defaults.yml":         &fstest.MapFile{Data: []byte("settings: {imported: retained}\n")},
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("settings: {from_lower_namespace: retained}\n")},
+	})
+	selected := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("settings: {shared: overlay}\n")},
+	})
+	checks := struct{ defaults, resolution, validation int }{}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(mainPath)}, configkit.LayeredOptions[layeredFacadeConfig]{
+		Underlays: []configkit.Underlay{
+			{Inputs: []configkit.Input{
+				{Source: lower.File("main.yml"), Imports: true},
+				{Source: configkit.Mapping("programmatic-low", map[string]any{"settings": map[string]any{"mapping": "retained"}, "count": 7, "enabled": true})},
+				{Source: lower.File("overlays/testing.yml")},
+			}},
+			{Inputs: []configkit.Input{{Source: configkit.Mapping("programmatic-high", map[string]any{
+				"settings": map[string]any{"shared": "middle", "second": "retained"}, "list": []any{"middle"},
+			})}}},
+		},
+		Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &selected},
+		Overrides: []configkit.Override{{Path: configkit.Keys("settings", "shared"), Value: "cli", Flag: "--shared"}},
+		Definition: configkit.Definition[layeredFacadeConfig]{
+			Defaults: func() layeredFacadeConfig { checks.defaults++; return layeredFacadeConfig{} },
+			Resolve: func(config *layeredFacadeConfig, resolution *configkit.Resolution) error {
+				checks.resolution++
+				if source, ok := resolution.Source("source-2"); !ok || source.Layer != configkit.LayerUnderlay || source.UnderlayIndex != 1 || source.Kind != configkit.SourceKindFilesystem {
+					t.Errorf("resolver source = %#v, %v", source, ok)
+				}
+				configkit.Default(resolution, configkit.Keys("derived"), &config.Derived, config.Settings["native"], "derive from native", configkit.Keys("settings", "native"))
+				return nil
+			},
+			Validate: func(config *layeredFacadeConfig, validation *configkit.Validation) { checks.validation++ },
+			Secret:   func(path configkit.Path) bool { return path.String() == "/settings/secret" },
+		},
+	})
+	if err != nil {
+		t.Fatalf("layered load: %v, diagnostics %#v", err, err.(*configkit.LoadError).Diagnostics())
+	}
+	wantSettings := map[string]string{
+		"shared": "cli", "secret": "new-canary", "native": "retained", "imported": "retained",
+		"mapping": "retained", "from_lower_namespace": "retained", "second": "retained",
+	}
+	if !reflect.DeepEqual(result.Config.Settings, wantSettings) || result.Config.Count != 0 || result.Config.Enabled || result.Config.List == nil || len(result.Config.List) != 0 || result.Config.Optional != nil || result.Config.Derived != "retained" {
+		t.Fatalf("configuration = %#v", result.Config)
+	}
+	if checks != (struct{ defaults, resolution, validation int }{1, 1, 1}) {
+		t.Fatalf("lifecycle calls = %#v", checks)
+	}
+	sources := result.Sources()
+	if len(sources) != 8 {
+		t.Fatalf("sources = %#v", sources)
+	}
+	wantLayers := []configkit.Layer{configkit.LayerBase, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerOverlay, configkit.LayerCLI}
+	wantIndexes := []int{0, 1, 1, 1, 1, 2, 0, 0}
+	wantKinds := []configkit.SourceKind{configkit.SourceKindFile, configkit.SourceKindFilesystem, configkit.SourceKindFilesystem, configkit.SourceKindMapping, configkit.SourceKindFilesystem, configkit.SourceKindMapping, configkit.SourceKindFilesystem, configkit.SourceKindCLI}
+	for index, source := range sources {
+		if source.ID != "source-"+strconv.Itoa(index+1) || source.Layer != wantLayers[index] || source.UnderlayIndex != wantIndexes[index] || source.Kind != wantKinds[index] {
+			t.Errorf("source %d = %#v", index, source)
+		}
+	}
+	if sources[4].Path != sources[6].Path || sources[4].Label != sources[6].Label {
+		t.Fatalf("namespace-collision fixture changed: %#v, %#v", sources[4], sources[6])
+	}
+	sources[1].Label = "changed"
+	if result.Sources()[1].Label != "bundle/main.yml" {
+		t.Fatal("source records alias retained metadata")
+	}
+	for _, test := range []struct {
+		path string
+		want configkit.Layer
+	}{
+		{"/settings/native", configkit.LayerUnderlay},
+		{"/settings/shared", configkit.LayerCLI},
+		{"/derived", configkit.LayerResolution},
+	} {
+		path, err := configkit.ParsePath(test.path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		info, ok := result.Metadata().Lookup(path)
+		if !ok || info.Origin.Layer != test.want {
+			t.Errorf("%s metadata = %#v, %v", test.path, info, ok)
+		}
+	}
+	container, ok := result.Metadata().Lookup(configkit.Keys("settings"))
+	if !ok || len(container.Origin.Locations) < 4 {
+		t.Fatalf("composite mapping origin = %#v, %v", container, ok)
+	}
+	explanation, ok := result.Explain(configkit.Keys("settings", "shared"))
+	if !ok || len(explanation.History) < 5 || explanation.Origin.Layer != configkit.LayerCLI {
+		t.Fatalf("shared history = %#v, %v", explanation, ok)
+	}
+	var effective, rendered bytes.Buffer
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if err := result.WriteExplanation(&rendered, configkit.Keys("settings", "secret")); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String()+rendered.String(), "old-canary") || strings.Contains(effective.String()+rendered.String(), "new-canary") || !strings.Contains(rendered.String(), "redacted") {
+		t.Fatalf("secret rendering leaked: effective %q, explanation %q", effective.String(), rendered.String())
+	}
+}
+
+func TestFacadeLayeredDisplacedDescendantRemainsInspectable(t *testing.T) {
+	type config struct {
+		Choice string `yaml:"choice"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.Mapping("main", map[string]any{"choice": "flat"})}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{
+			"choice": map[string]any{"nested": "historical"},
+		})}}}},
+	})
+	if err != nil || result.Config.Choice != "flat" {
+		t.Fatalf("replacement load = %#v, %v", result, err)
+	}
+	path := configkit.Keys("choice", "nested")
+	explanation, ok := result.Explain(path)
+	if !ok || explanation.Effective.Present || len(explanation.History) == 0 || explanation.History[0].Origin.Layer != configkit.LayerUnderlay {
+		t.Fatalf("deleted descendant = %#v, %v", explanation, ok)
+	}
+}
+
+func TestFacadeLayeredZeroUnderlaysMatchesOrdinaryLoad(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("value: selected\n")},
+	})
+	context := &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem}
+	source := configkit.Bytes("main", []byte("value: base\n"))
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	ordinary, err := configkit.Load(source, configkit.Options[config]{Overlay: "testing", Filesystem: context})
+	if err != nil {
+		t.Fatal(err)
+	}
+	layered, err := configkit.LoadLayered(configkit.Input{Source: source}, configkit.LayeredOptions[config]{Overlay: "testing", OverlayFilesystem: context})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ordinary.Config != layered.Config || !reflect.DeepEqual(ordinary.Sources(), layered.Sources()) {
+		t.Fatalf("ordinary = %#v, %#v; layered = %#v, %#v", ordinary.Config, ordinary.Sources(), layered.Config, layered.Sources())
+	}
+	var first, second bytes.Buffer
+	if err := ordinary.WriteEffective(&first); err != nil {
+		t.Fatal(err)
+	}
+	if err := layered.WriteEffective(&second); err != nil {
+		t.Fatal(err)
+	}
+	if first.String() != second.String() {
+		t.Fatalf("effective ordinary %q, layered %q", first.String(), second.String())
+	}
+	if ordinaryExplanation, ok := ordinary.Explain(configkit.Keys("value")); !ok {
+		t.Fatal("ordinary explanation missing")
+	} else if layeredExplanation, ok := layered.Explain(configkit.Keys("value")); !ok || !reflect.DeepEqual(ordinaryExplanation, layeredExplanation) {
+		t.Fatalf("explanations differ: %#v, %#v", ordinaryExplanation, layeredExplanation)
+	}
+}
+
+func TestFacadeLayeredOptionAndVersionFailures(t *testing.T) {
+	type config struct {
+		Version string `yaml:"version"`
+	}
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("{}\n")},
+	})
+	for _, test := range []struct {
+		name    string
+		main    configkit.Input
+		options []configkit.LayeredOptions[config]
+		want    configkit.Code
+	}{
+		{"zero source", configkit.Input{}, nil, configkit.CodeSourceInvalid},
+		{"multiple options", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{}, {}}, configkit.CodeOptionsInvalid},
+		{"missing overlay context", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{Overlay: "testing"}}, configkit.CodeOptionsInvalid},
+		{"selected file already acquired", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{
+			Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &namespace},
+			Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: namespace.File("overlays/testing.yml")}}}},
+		}}, configkit.CodeOverlayRepeated},
+		{"underlay version", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{
+			Definition: configkit.Definition[config]{Version: &configkit.VersionRule{Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil }}},
+			Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"version": "forbidden"})}}}},
+		}}, configkit.CodeUnderlayVersion},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := configkit.LoadLayered(test.main, test.options...)
+			loadError, ok := err.(*configkit.LoadError)
+			if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != test.want {
+				t.Fatalf("error = %#v, want %s", err, test.want)
+			}
+		})
+	}
+}
+
+func TestFacadeLayeredExplicitHostOverlayWithoutMain(t *testing.T) {
+	root := t.TempDir()
+	if err := os.Mkdir(filepath.Join(root, "overlays"), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(root, "overlays", "testing.yml"), []byte("value: selected\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: root, BaseDir: root},
+	})
+	if err != nil || result.Config.Value != "selected" || len(result.Sources()) != 1 || result.Sources()[0].Kind != configkit.SourceKindFile {
+		t.Fatalf("explicit host overlay = %#v, %v", result, err)
+	}
+}

facade_mapping_test.go

@@ -0,0 +1,133 @@
+package configkit_test
+
+import (
+	"math"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type mappingFacadeConfig struct {
+	Enabled  bool     `yaml:"enabled"`
+	Count    uint64   `yaml:"count"`
+	Optional *string  `yaml:"optional"`
+	Items    []string `yaml:"items"`
+	Name     string   `yaml:"name"`
+}
+
+func TestFacadeMappingSnapshotsAndMatchesYAML(t *testing.T) {
+	items := []any{"one", "two"}
+	values := map[string]any{
+		"name": "original", "enabled": false, "count": ^uint64(0),
+		"optional": nil, "items": items,
+	}
+	source := configkit.Mapping("programmatic", values)
+	values["name"] = "mutated"
+	items[0] = "mutated"
+	for range 2 {
+		result, err := configkit.Load[mappingFacadeConfig](source)
+		if err != nil {
+			t.Fatal(err)
+		}
+		want := mappingFacadeConfig{Count: ^uint64(0), Items: []string{"one", "two"}, Name: "original"}
+		if !reflect.DeepEqual(result.Config, want) {
+			t.Fatalf("mapping config = %#v, want %#v", result.Config, want)
+		}
+		if sources := result.Sources(); len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Kind != configkit.SourceKindMapping || sources[0].Layer != configkit.LayerBase || sources[0].Path != "" {
+			t.Fatalf("mapping sources = %#v", sources)
+		}
+		info, ok := result.Metadata().Lookup(configkit.Keys("name"))
+		if !ok || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != "source-1" || info.Origin.Locations[0].Line != 0 {
+			t.Fatalf("mapping name origin = %#v, %v", info, ok)
+		}
+		fromYAML, err := configkit.Load[mappingFacadeConfig](configkit.Bytes("equivalent", []byte("enabled: false\ncount: 18446744073709551615\noptional: null\nitems: [one, two]\nname: original\n")))
+		if err != nil || !reflect.DeepEqual(result.Config, fromYAML.Config) {
+			t.Fatalf("equivalent YAML = %#v, %v", fromYAML, err)
+		}
+	}
+}
+
+func TestFacadeMappingNullEmptyAndVersion(t *testing.T) {
+	result, err := configkit.Load[mappingFacadeConfig](configkit.Mapping("empty", map[string]any{}))
+	if err != nil || len(result.Sources()) != 1 {
+		t.Fatalf("empty mapping = %#v, %v", result, err)
+	}
+	var nilSequence []any
+	var nilMap map[string]any
+	var checked string
+	result, err = configkit.Load(configkit.Mapping("nullable", map[string]any{
+		"items": nilSequence, "optional": nilMap, "enabled": false, "count": uint64(0),
+	}), configkit.Options[mappingFacadeConfig]{Definition: configkit.Definition[mappingFacadeConfig]{
+		Version: &configkit.VersionRule{Path: configkit.Keys("count"), Check: func(version configkit.Version, validation *configkit.Validation) error {
+			checked = version.Text
+			return nil
+		}},
+	}})
+	if err != nil || checked != "0" || result.Config.Items != nil || result.Config.Optional != nil {
+		t.Fatalf("null and zero mapping = %#v, checked %q, %v", result, checked, err)
+	}
+	info, ok := result.Metadata().Lookup(configkit.Keys("items"))
+	if !ok || !info.Null || !info.Explicit {
+		t.Fatalf("null metadata = %#v, %v", info, ok)
+	}
+}
+
+func TestFacadeMappingImportsUseExplicitContext(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"root/shared.yml": &fstest.MapFile{Data: []byte("name: imported\n")},
+	})
+	source := configkit.Mapping("programmatic", map[string]any{
+		"configkit": map[string]any{"imports": []any{map[string]any{"path": "shared.yml"}}},
+		"enabled":   true,
+	})
+	result, err := configkit.Load(source, configkit.Options[mappingFacadeConfig]{
+		Imports: true, Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &namespace},
+	})
+	if err != nil || result.Config.Name != "imported" || !result.Config.Enabled {
+		t.Fatalf("mapping import = %#v, %v", result, err)
+	}
+	if sources := result.Sources(); len(sources) != 2 || sources[0].Kind != configkit.SourceKindMapping || sources[1].Kind != configkit.SourceKindFilesystem || sources[1].Layer != configkit.LayerBase {
+		t.Fatalf("mapping import sources = %#v", sources)
+	}
+	checkMappingCode(t, source, configkit.Options[mappingFacadeConfig]{Imports: true}, configkit.CodeOptionsInvalid)
+	checkMappingCode(t, source, configkit.Options[mappingFacadeConfig]{}, configkit.CodeImportDisabled)
+}
+
+func TestFacadeMappingDeferredValueDiagnostics(t *testing.T) {
+	cycle := map[string]any{}
+	cycle["secret"] = cycle
+	for _, test := range []struct {
+		name   string
+		source configkit.Source
+		want   configkit.Code
+		path   string
+	}{
+		{"nil root", configkit.Mapping("nil", nil), configkit.CodeSourceInvalid, ""},
+		{"bad label", configkit.Mapping("bad\nlabel", map[string]any{}), configkit.CodeSourceInvalid, ""},
+		{"unsupported", configkit.Mapping("bad", map[string]any{"secret": struct{ Value string }{"canary-value"}}), configkit.CodeSourceInvalid, "/secret"},
+		{"nonfinite", configkit.Mapping("bad", map[string]any{"count": math.NaN()}), configkit.CodeSourceInvalid, "/count"},
+		{"cycle", configkit.Mapping("bad", cycle), configkit.CodeSourceInvalid, "/secret"},
+		{"too many bytes", configkit.Mapping("bad", map[string]any{"name": strings.Repeat("x", 8<<20+1)}), configkit.CodeSourceLimitExceeded, "/name"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			loadError := checkMappingCode(t, test.source, configkit.Options[mappingFacadeConfig]{}, test.want)
+			diagnostic := loadError.Diagnostics()[0]
+			if diagnostic.Path.String() != test.path || diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || strings.Contains(diagnostic.Message, "canary-value") {
+				t.Fatalf("diagnostic = %#v", diagnostic)
+			}
+		})
+	}
+}
+
+func checkMappingCode(t *testing.T, source configkit.Source, options configkit.Options[mappingFacadeConfig], want configkit.Code) *configkit.LoadError {
+	t.Helper()
+	_, err := configkit.Load(source, options)
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != want {
+		t.Fatalf("error = %#v, want %s", err, want)
+	}
+	return loadError
+}

facade_resource_context_test.go

@@ -0,0 +1,221 @@
+package configkit_test
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type observedResourceFS struct {
+	files  fstest.MapFS
+	opens  int
+	closed bool
+}
+
+func (f *observedResourceFS) Open(name string) (fs.File, error) {
+	f.opens++
+	if f.closed {
+		return nil, fs.ErrClosed
+	}
+	return f.files.Open(name)
+}
+
+type resourceFacadeConfig struct {
+	Native   string `yaml:"native"`
+	Shared   string `yaml:"shared"`
+	Host     string `yaml:"host"`
+	Selected string `yaml:"selected"`
+}
+
+func TestFacadeSourceContextsFollowOriginsWithoutQueryIO(t *testing.T) {
+	host := t.TempDir()
+	if err := os.WriteFile(filepath.Join(host, "main.yml"), []byte("host: host.txt\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(host, "host.txt"), []byte("host-v1"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	borrowed := &observedResourceFS{files: fstest.MapFS{
+		"bundle/config/defaults.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: ../shared.yml}]}\nnative: icon.txt\n")},
+		"bundle/shared.yml":           &fstest.MapFile{Data: []byte("shared: shared.txt\n")},
+		"bundle/overlays/test.yml":    &fstest.MapFile{Data: []byte("selected: overlay.txt\n")},
+		"bundle/config/icon.txt":      &fstest.MapFile{Data: []byte("icon-v1")},
+		"bundle/shared.txt":           &fstest.MapFile{Data: []byte("shared-v1")},
+		"bundle/overlays/overlay.txt": &fstest.MapFile{Data: []byte("overlay-v1")},
+	}}
+	namespace := configkit.NewFilesystem("bundle", borrowed)
+	var resolverContext configkit.ResourceContext
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(filepath.Join(host, "main.yml"))}, configkit.LayeredOptions[resourceFacadeConfig]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: namespace.File("bundle/config/defaults.yml"), Imports: true,
+			Filesystem: &configkit.FilesystemContext{Root: "bundle", FS: &namespace},
+		}}}},
+		Overlay: "test", OverlayFilesystem: &configkit.FilesystemContext{Root: "bundle", BaseDir: "bundle", FS: &namespace},
+		Definition: configkit.Definition[resourceFacadeConfig]{Resolve: func(config *resourceFacadeConfig, resolution *configkit.Resolution) error {
+			before := borrowed.opens
+			var ok bool
+			resolverContext, ok = resolution.SourceContext("source-2")
+			if !ok || resolverContext.BaseDir != "config" || resolverContext.FS == nil {
+				t.Errorf("resolver context = %#v, %v", resolverContext, ok)
+			}
+			if _, ok := resolution.SourceContext("missing"); ok || borrowed.opens != before {
+				t.Error("resolution context query performed I/O or guessed an unknown source")
+			}
+			return nil
+		}},
+	})
+	if err != nil {
+		t.Fatalf("load: %v, diagnostics %#v", err, err.(*configkit.LoadError).Diagnostics())
+	}
+	queriesBefore := borrowed.opens
+	for _, test := range []struct {
+		field, wantBase, wantData string
+	}{
+		{"native", "config", "icon-v1"},
+		{"shared", ".", "shared-v1"},
+		{"selected", "overlays", "overlay-v1"},
+		{"host", ".", "host-v1"},
+	} {
+		origin, ok := result.Metadata().Lookup(configkit.Keys(test.field))
+		if !ok || len(origin.Origin.Locations) != 1 {
+			t.Fatalf("%s origin = %#v, %v", test.field, origin, ok)
+		}
+		context, ok := result.SourceContext(origin.Origin.Locations[0].SourceID)
+		if !ok || context.FS == nil || context.BaseDir != test.wantBase {
+			t.Fatalf("%s context = %#v, %v", test.field, context, ok)
+		}
+		if borrowed.opens != queriesBefore {
+			t.Fatal("result SourceContext query performed filesystem I/O")
+		}
+		resourceName := path.Join(context.BaseDir, map[string]string{
+			"native": result.Config.Native, "shared": result.Config.Shared,
+			"selected": result.Config.Selected, "host": result.Config.Host,
+		}[test.field])
+		data, err := fs.ReadFile(context.FS, resourceName)
+		if err != nil || string(data) != test.wantData {
+			t.Fatalf("%s resource = %q, %v", test.field, data, err)
+		}
+		queriesBefore = borrowed.opens
+	}
+	if _, ok := result.SourceContext(""); ok {
+		t.Fatal("empty source ID returned a context")
+	}
+	var nilResult *configkit.Result[resourceFacadeConfig]
+	if _, ok := nilResult.SourceContext("source-2"); ok {
+		t.Fatal("nil result returned a context")
+	}
+	if _, err := fs.ReadFile(resolverContext.FS, "../shared.txt"); !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("native root escape = %v", err)
+	}
+	borrowed.files["bundle/config/icon.txt"].Data = []byte("icon-v2")
+	data, err := fs.ReadFile(resolverContext.FS, "config/icon.txt")
+	if err != nil || string(data) != "icon-v2" {
+		t.Fatalf("updated native resource = %q, %v", data, err)
+	}
+	borrowed.closed = true
+	if result.Config.Native != "icon.txt" {
+		t.Fatal("closing caller filesystem altered loaded configuration")
+	}
+	if _, err := fs.ReadFile(resolverContext.FS, "config/icon.txt"); !errors.Is(err, fs.ErrClosed) {
+		t.Fatalf("closed caller filesystem error = %v", err)
+	}
+}
+
+func TestFacadeHostContextSurvivesConfigurationDeletion(t *testing.T) {
+	root := t.TempDir()
+	main := filepath.Join(root, "main.yml")
+	resource := filepath.Join(root, "asset.txt")
+	if err := os.WriteFile(main, []byte("native: asset.txt\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(resource, []byte("first"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	result, err := configkit.Load[resourceFacadeConfig](configkit.File(main))
+	if err != nil {
+		t.Fatal(err)
+	}
+	context, ok := result.SourceContext("source-1")
+	if !ok || context.BaseDir != "." {
+		t.Fatalf("host context = %#v, %v", context, ok)
+	}
+	if err := os.Remove(main); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(resource, []byte("second"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, result.Config.Native))
+	if err != nil || string(data) != "second" || result.Config.Native != "asset.txt" {
+		t.Fatalf("resource after source deletion = %q, %v", data, err)
+	}
+	if _, err := fs.ReadFile(context.FS, "../asset.txt"); !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("invalid host path = %v", err)
+	}
+	if err := os.Remove(resource); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := fs.ReadFile(context.FS, "asset.txt"); !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("missing later resource = %v", err)
+	}
+	outside := filepath.Join(t.TempDir(), "outside.txt")
+	if err := os.WriteFile(outside, []byte("outside"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(outside, filepath.Join(root, "escape")); err == nil {
+		if _, err := fs.ReadFile(context.FS, "escape"); !errors.Is(err, fs.ErrPermission) {
+			t.Fatalf("host symlink escape = %v", err)
+		}
+	}
+}
+
+func TestFacadeSyntheticSourceContextsAndContextlessSources(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"bundle/config/icon.txt": &fstest.MapFile{Data: []byte("icon")},
+	})
+	context := &configkit.FilesystemContext{Root: "bundle", BaseDir: "bundle/config", FS: &namespace}
+	for _, test := range []struct {
+		name   string
+		source configkit.Source
+	}{
+		{"bytes", configkit.Bytes("memory", []byte("native: icon.txt\n"))},
+		{"reader", configkit.Reader("stream", strings.NewReader("native: icon.txt\n"))},
+		{"mapping", configkit.Mapping("values", map[string]any{"native": "icon.txt"})},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.Load(test.source, configkit.Options[resourceFacadeConfig]{Filesystem: context})
+			if err != nil {
+				t.Fatal(err)
+			}
+			resource, ok := result.SourceContext("source-1")
+			if !ok || resource.BaseDir != "config" {
+				t.Fatalf("synthetic context = %#v, %v", resource, ok)
+			}
+			data, err := fs.ReadFile(resource.FS, path.Join(resource.BaseDir, result.Config.Native))
+			if err != nil || string(data) != "icon" {
+				t.Fatalf("synthetic resource = %q, %v", data, err)
+			}
+		})
+	}
+	result, err := configkit.Load[resourceFacadeConfig](configkit.Bytes("plain", []byte("native: none\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := result.SourceContext("source-1"); ok {
+		t.Fatal("contextless byte source returned a context")
+	}
+	result, err = configkit.Load(configkit.NoSource(), configkit.Options[resourceFacadeConfig]{Overrides: []configkit.Override{{Path: configkit.Keys("native"), Value: "from-cli", Flag: "--native"}}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := result.SourceContext("source-1"); ok {
+		t.Fatal("CLI source returned a context")
+	}
+}

facade_underlay_origins_test.go

@@ -0,0 +1,107 @@
+package configkit_test
+
+import (
+	"reflect"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestUnderlaySequenceDescendantOrigins(t *testing.T) {
+	for _, later := range []bool{false, true} {
+		name := "first tier"
+		if later {
+			name = "later tier"
+		}
+		t.Run(name, func(t *testing.T) {
+			bundle := configkit.NewFilesystem("bundle", fstest.MapFS{
+				"main.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: items.yml}]}\n")},
+				"items.yml": &fstest.MapFile{Data: []byte("items: [{name: first, nested: [false]}]\n")},
+			})
+			tiers := []configkit.Underlay{}
+			if later {
+				tiers = append(tiers, configkit.Underlay{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"other": true})}}})
+			}
+			tiers = append(tiers, configkit.Underlay{Inputs: []configkit.Input{{Source: bundle.File("main.yml"), Imports: true}}})
+			paths := []string{"/items", "/items/0", "/items/0/name", "/items/0/nested", "/items/0/nested/0"}
+			entry := map[string]configkit.Origin{}
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Mapping("main", map[string]any{"main": true})}, configkit.LayeredOptions[map[string]any]{
+				Underlays: tiers,
+				Definition: configkit.Definition[map[string]any]{Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
+					for _, pointer := range paths {
+						path, _ := configkit.ParsePath(pointer)
+						info, ok := resolution.InputInfo(path)
+						if !ok {
+							t.Errorf("missing resolver input %s", pointer)
+						}
+						entry[pointer] = info.Origin
+					}
+					return nil
+				}},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			var sourceID string
+			for _, source := range result.Sources() {
+				if source.Path == "items.yml" {
+					sourceID = source.ID
+				}
+			}
+			if sourceID == "" {
+				t.Fatal("missing imported source")
+			}
+			for _, pointer := range paths {
+				path, _ := configkit.ParsePath(pointer)
+				info, ok := result.Metadata().Lookup(path)
+				if !ok || info.Origin.Layer != configkit.LayerUnderlay || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != sourceID {
+					t.Errorf("%s origin = %+v", pointer, info.Origin)
+				}
+				if !reflect.DeepEqual(entry[pointer], info.Origin) {
+					t.Errorf("%s resolver origin differs", pointer)
+				}
+				explanation, ok := result.Explain(path)
+				if !ok || !reflect.DeepEqual(explanation.Origin, info.Origin) {
+					t.Errorf("%s explanation origin differs", pointer)
+				}
+				history := result.Metadata().History(path)
+				if len(history) == 0 || !reflect.DeepEqual(history[len(history)-1].Origin, info.Origin) {
+					t.Errorf("%s history = %+v", pointer, history)
+				}
+			}
+		})
+	}
+}
+
+func TestUnderlayHistoryRetainsLaterMappingContributors(t *testing.T) {
+	result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{Underlays: []configkit.Underlay{
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("one", map[string]any{"x": 1})}}},
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("two", map[string]any{"x": map[string]any{}})}}},
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("three", map[string]any{"x": map[string]any{"y": 2}})}}},
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := configkit.Keys("x")
+	info, _ := result.Metadata().Lookup(path)
+	wantOrigin := configkit.Origin{Layer: configkit.LayerUnderlay, Locations: []configkit.Location{{SourceID: "source-2"}, {SourceID: "source-3"}}}
+	if !reflect.DeepEqual(info.Origin, wantOrigin) {
+		t.Fatalf("effective origin = %+v, want %+v", info.Origin, wantOrigin)
+	}
+	history := result.Metadata().History(path)
+	if len(history) != 3 || !reflect.DeepEqual(history[len(history)-1].Origin, info.Origin) {
+		t.Fatalf("history = %+v, effective origin = %+v", history, info.Origin)
+	}
+	explanation, ok := result.Explain(path)
+	if !ok || !reflect.DeepEqual(explanation.Origin, info.Origin) {
+		t.Fatal("explanation and metadata disagree")
+	}
+	if len(explanation.History) != 3 {
+		t.Fatalf("explanation history = %+v", explanation.History)
+	}
+	last := explanation.History[len(explanation.History)-1].Origin
+	if last.Layer != configkit.LayerUnderlay || len(last.Locations) != 1 || last.Locations[0].SourceID != "source-3" {
+		t.Fatalf("last explanation origin = %+v", last)
+	}
+}

go.mod

@@ -1,5 +1,13 @@
 module gitea.maximumdirect.net/eric/ConfigKit
 
-go 1.25.0
+go 1.26.0
 
 require go.yaml.in/yaml/v4 v4.0.0-rc.6
+
+require (
+	gitea.maximumdirect.net/eric/ReleaseKit v0.4.1 // indirect
+	github.com/yuin/goldmark/v2 v2.1.6 // indirect
+	golang.org/x/mod v0.40.0 // indirect
+)
+
+tool gitea.maximumdirect.net/eric/ReleaseKit/cmd/releasekit

go.sum

@@ -1,2 +1,8 @@
+gitea.maximumdirect.net/eric/ReleaseKit v0.4.1 h1:Qn+mdebYOzCVCEXgCbuGSO/WHmJtItwaQ8e3XwY3qtI=
+gitea.maximumdirect.net/eric/ReleaseKit v0.4.1/go.mod h1:LVodqNQ3OIKv9NBq+fFeyMByd1lHY25YOmmpah/NaNI=
+github.com/yuin/goldmark/v2 v2.1.6 h1:igy0q9JcqovoFXFx/XjtU8sHVhcUHavPj2sA3EoKJUE=
+github.com/yuin/goldmark/v2 v2.1.6/go.mod h1:G6M4/qOFtfn01/o14BU1UR2Lo5N3S9Qo7xCuz/sHjGQ=
 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=
+golang.org/x/mod v0.40.0 h1:hUv+3cXcdRHz08UmSiOob7sadHig73uo5bkXxQ/tvUs=
+golang.org/x/mod v0.40.0/go.mod h1:0/weTWkPWGBikyTWAX3dkjVztMmBA5hM0DH6BElSupE=

inspection.go

@@ -39,7 +39,8 @@ type Explanation struct {
 }
 
 // Explain returns safe effective and chronological values for a known final or
-// historical path. Unknown and malformed paths return false.
+// historical path. Structural inheritance appears between source applications
+// and typed reconciliation. Unknown and malformed paths return false.
 func (r *Result[T]) Explain(path Path) (Explanation, bool) {
 	if r == nil {
 		return Explanation{}, false

internal/engine/assembly_bench_test.go

@@ -0,0 +1,119 @@
+package engine
+
+import (
+	"strconv"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func BenchmarkComposeWideMappings(b *testing.B) {
+	for _, width := range []int{128, 256, 512} {
+		b.Run(strconv.Itoa(width), func(b *testing.B) {
+			left := benchmarkCatalog("source-1", 0, width)
+			right := benchmarkCatalog("source-2", width, width)
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := composeDocuments(left, right); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkOverlayRecursiveMappings(b *testing.B) {
+	for _, width := range []int{128, 256, 512} {
+		b.Run(strconv.Itoa(width), func(b *testing.B) {
+			base, diagnostics := composeDocuments(benchmarkCatalog("source-1", 0, width))
+			if len(diagnostics) != 0 {
+				b.Fatal(diagnostics)
+			}
+			overlay := benchmarkCatalog("source-2", width/2, width)
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := overlayDocuments(base, overlay); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkOverlayManyApplications(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		inputs := make([]*document.Node, count)
+		for index := range inputs {
+			inputs[index] = benchmarkCatalog("source-2", index, 1)
+		}
+		benchmarkOverlayApplications(b, count, inputs)
+	}
+}
+
+func BenchmarkOverlayRepeatedReplacement(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		inputs := make([]*document.Node, count)
+		for index := range inputs {
+			location := document.Location{SourceID: "source-2"}
+			value := &document.Node{Kind: document.String, Text: "replacement", Location: location}
+			if index%2 == 0 {
+				value = &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+					Key: "enabled", Value: &document.Node{Kind: document.Boolean, Text: "false", Location: location},
+				}}}
+			}
+			inputs[index] = &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+				Key: "catalog", Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+					Key: "item-0", Value: value,
+				}}},
+			}}}
+		}
+		benchmarkOverlayApplications(b, count, inputs)
+	}
+}
+
+func benchmarkOverlayApplications(b *testing.B, count int, inputs []*document.Node) {
+	base, diagnostics := composeDocuments(benchmarkCatalog("source-1", 0, 1))
+	if len(diagnostics) != 0 {
+		b.Fatal(diagnostics)
+	}
+	for _, mode := range []string{"owned", "copy-preserving"} {
+		b.Run(mode+"/"+strconv.Itoa(count), func(b *testing.B) {
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if mode == "owned" {
+					assembly := newAssemblyAccumulator(cloneComposedDocument(base))
+					for _, input := range inputs {
+						if diagnostics := assembly.applyOverlay(input, nil, nil, LayerOverlay); len(diagnostics) != 0 {
+							b.Fatal(diagnostics)
+						}
+					}
+					continue
+				}
+				current := base
+				for _, input := range inputs {
+					current, diagnostics = overlayDocuments(current, input)
+					if len(diagnostics) != 0 {
+						b.Fatal(diagnostics)
+					}
+				}
+			}
+		})
+	}
+}
+
+func benchmarkCatalog(source string, first, width int) *document.Node {
+	location := document.Location{SourceID: source}
+	entries := make([]document.Entry, width)
+	for index := range entries {
+		entries[index] = document.Entry{
+			Key:   "item-" + strconv.Itoa(first+index),
+			Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{Key: "enabled", Value: &document.Node{Kind: document.Boolean, Text: "true", Location: location}}}},
+		}
+	}
+	return &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+		Key: "catalog", Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: entries},
+	}}}
+}

internal/engine/compose.go

@@ -15,6 +15,17 @@ type composedDocument struct {
 	inputNodes   int
 }
 
+// assemblyAccumulator owns one mutable working document. Acquired trees held by
+// history applications are separate from its root and remain unchanged.
+type assemblyAccumulator struct {
+	current *composedDocument
+	index   mappingIndex
+}
+
+func newAssemblyAccumulator(owned *composedDocument) *assemblyAccumulator {
+	return &assemblyAccumulator{current: owned}
+}
+
 // cloneComposedDocument detaches mutable composition and provenance state;
 // history application trees are immutable and retain their existing sharing.
 func cloneComposedDocument(base *composedDocument) *composedDocument {
@@ -58,8 +69,14 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 		collected.append(input, Path{})
 	}
 
+	checked := make(map[string]struct{}, len(collected.byPath))
 	for _, claim := range collected.ordered {
-		claims := collected.byPath[claim.path.String()]
+		pointer := claim.path.String()
+		if _, seen := checked[pointer]; seen {
+			continue
+		}
+		checked[pointer] = struct{}{}
+		claims := collected.byPath[pointer]
 		if len(claims) < 2 || allMappingClaims(claims) {
 			continue
 		}
@@ -67,21 +84,43 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 	}
 
 	root := document.Clone(inputs[0])
+	index := new(mappingIndex)
 	for _, input := range inputs[1:] {
-		mergeDisjointMappings(root, input)
+		mergeDisjointMappings(root, input, index)
 	}
 	applications := make([]historyApplication, 0, len(inputs))
 	for _, input := range inputs {
 		applications = append(applications, historyApplication{layer: LayerBase, value: input})
 	}
+	origins := compositionOrigins(collected.ordered)
+	completeCompositionOrigins(root, Path{}, origins)
 	return &composedDocument{
 		root:         root,
 		claims:       cloneCompositionClaims(collected.ordered),
-		origins:      compositionOrigins(collected.ordered),
+		origins:      origins,
 		applications: applications,
 	}, nil
 }
 
+// Conflict claims treat sequences atomically, but inspection needs origins for
+// every descendant. Preserve composite mapping claims while filling those gaps.
+func completeCompositionOrigins(node *document.Node, path Path, origins map[string]Origin) {
+	pointer := path.String()
+	if _, present := origins[pointer]; !present {
+		origins[pointer] = Origin{Layer: LayerBase, Locations: []Location{publicLocation(node.Location)}}
+	}
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			completeCompositionOrigins(entry.Value, path.Key(entry.Key), origins)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			completeCompositionOrigins(child, path.Index(index), origins)
+		}
+	}
+}
+
 func (c *compositionClaims) append(node *document.Node, path Path) {
 	claim := compositionClaim{
 		path:     path.clone(),
@@ -109,18 +148,18 @@ func allMappingClaims(claims []compositionClaim) bool {
 	return true
 }
 
-func mergeDisjointMappings(target, incoming *document.Node) {
+func mergeDisjointMappings(target, incoming *document.Node, index *mappingIndex) {
 	for _, incomingEntry := range incoming.Mapping {
-		index := documentEntryIndex(target.Mapping, incomingEntry.Key)
-		if index < 0 {
+		position := index.find(target, incomingEntry.Key)
+		if position < 0 {
 			cloned := incomingEntry
 			cloned.Value = document.Clone(incomingEntry.Value)
-			target.Mapping = append(target.Mapping, cloned)
+			index.append(target, 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)
+		mergeDisjointMappings(target.Mapping[position].Value, incomingEntry.Value, index)
 	}
 }
 

internal/engine/compose_test.go

@@ -116,6 +116,31 @@ func TestComposeDocumentsReportsAllClaimantsAtExactLocations(t *testing.T) {
 	assertCompositionConflict(t, diagnostics, "/group", []string{"source-1", "source-2", "source-3"})
 }
 
+func TestComposeDocumentsKeepsFirstConflictAndAppendedEntryOrder(t *testing.T) {
+	first := parseCompositionDocument(t, "source-1", "catalog:\n  first: 1\n  shared: one\n  later: one\n")
+	second := parseCompositionDocument(t, "source-2", "catalog:\n  second: 2\n  shared: two\n  later: two\n")
+	third := parseCompositionDocument(t, "source-3", "catalog:\n  third: 3\n  shared: three\n")
+	_, diagnostics := composeDocuments(first, second, third)
+	assertCompositionConflict(t, diagnostics, "/catalog/shared", []string{"source-1", "source-2", "source-3"})
+
+	composed, diagnostics := composeDocuments(
+		parseCompositionDocument(t, "source-1", "catalog:\n  first: 1\n"),
+		parseCompositionDocument(t, "source-2", "catalog:\n  second: 2\n"),
+		parseCompositionDocument(t, "source-3", "catalog:\n  third: 3\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("composeDocuments() diagnostics = %#v", diagnostics)
+	}
+	entries := composed.root.Mapping[0].Value.Mapping
+	got := make([]string, len(entries))
+	for index, entry := range entries {
+		got[index] = entry.Key
+	}
+	if want := []string{"first", "second", "third"}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("catalog keys = %#v, want %#v", got, want)
+	}
+}
+
 func TestComposeDocumentsTreatsEmptyMappingsAsChildlessContributions(t *testing.T) {
 	empty, diagnostics := composeDocuments(
 		parseCompositionDocument(t, "source-1", "{}\n"),

internal/engine/destination.go

@@ -0,0 +1,127 @@
+package engine
+
+import (
+	"errors"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+var (
+	errInvalidDestination = errors.New("configkit: destination must contain only non-reserved mapping keys")
+	errDestinationLimit   = errors.New("configkit: destination exceeds document limits")
+)
+
+func normalizeDestination(path Path) (Path, error) {
+	if !path.valid() {
+		return Path{}, errInvalidDestination
+	}
+
+	segments := make([]pathSegment, 0, len(path.segments))
+	for _, segment := range path.segments {
+		if segment.kind != pathSegmentKey && segment.kind != pathSegmentParsed {
+			return Path{}, errInvalidDestination
+		}
+		if segment.key == "configkit" || segment.key == "extends" {
+			return Path{}, errInvalidDestination
+		}
+		segments = append(segments, pathSegment{kind: pathSegmentKey, key: segment.key})
+	}
+	return Path{segments: segments}, nil
+}
+
+func invalidDestinationDiagnostic(destination Path) Diagnostic {
+	return Diagnostic{
+		Code: CodeOptionsInvalid, Severity: SeverityError,
+		Path:    destination.clone(),
+		Message: "input destination must contain only non-reserved mapping keys",
+	}
+}
+
+func setSourceDestinations(sources []SourceInfo, destination Path) {
+	for index := range sources {
+		sources[index].Destination = destination.clone()
+	}
+}
+
+// placeDocument checks all per-document bounds before allocating destination
+// wrappers. Aggregate accounting remains the caller's responsibility.
+func placeDocument(body *document.Node, destination Path, sourceRoot document.Location, maxDepth, maxNodes int) (Path, *document.Node, error) {
+	normalized, err := normalizeDestination(destination)
+	if err != nil {
+		return Path{}, nil, err
+	}
+	if body == nil {
+		return normalized, nil, nil
+	}
+	if !destinationWithinLimits(body, normalized, maxDepth, maxNodes) {
+		return normalized, nil, errDestinationLimit
+	}
+
+	placed := body
+	for index := len(normalized.segments) - 1; index >= 0; index-- {
+		key := normalized.segments[index].key
+		placed = &document.Node{
+			Kind:     document.Mapping,
+			Location: sourceRoot,
+			Mapping: []document.Entry{{
+				Key:           key,
+				KeyLocation:   sourceRoot,
+				ValueLocation: sourceRoot,
+				Value:         placed,
+			}},
+		}
+	}
+	return normalized, placed, nil
+}
+
+// destinationWithinLimits expects a normalized mapping-key destination.
+func destinationWithinLimits(body *document.Node, destination Path, maxDepth, maxNodes int) bool {
+	if body == nil {
+		return true
+	}
+	if maxDepth < 0 || maxNodes < 0 || !destination.valid() {
+		return false
+	}
+	segments := len(destination.segments)
+	if segments > maxDepth || segments > maxNodes/2 {
+		return false
+	}
+	wrapperNodes := destinationWrapperNodeCount(destination)
+	for _, segment := range destination.segments {
+		if segment.kind != pathSegmentKey {
+			return false
+		}
+	}
+	if documentDepth(body) > maxDepth-segments {
+		return false
+	}
+	return countInputNodes(body) <= maxNodes-wrapperNodes
+}
+
+func destinationWrapperNodeCount(destination Path) int {
+	return 2 * len(destination.segments)
+}
+
+func documentDepth(node *document.Node) int {
+	if node == nil {
+		return -1
+	}
+	depth := 0
+	var visit func(*document.Node, int)
+	visit = func(current *document.Node, currentDepth int) {
+		if current == nil {
+			return
+		}
+		if currentDepth > depth {
+			depth = currentDepth
+		}
+		for _, entry := range current.Mapping {
+			visit(entry.Value, currentDepth+1)
+		}
+		for _, child := range current.Sequence {
+			visit(child, currentDepth+1)
+		}
+	}
+	visit(node, 0)
+	return depth
+}

internal/engine/destination_accounting_test.go

@@ -0,0 +1,176 @@
+package engine
+
+import (
+	"testing"
+	"testing/fstest"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestDestinationPlacementCountsAgainstSharedNodeBudget(t *testing.T) {
+	t.Parallel()
+
+	limits := defaultImportLimits()
+	destination := Keys("collections", "entry")
+	empty := Mapping("empty", map[string]any{})
+	parsed, diagnostics := parseSource(empty, "fixture", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	perInput := countInputNodes(parsed.document) + destinationWrapperNodeCount(destination)
+
+	for _, test := range []struct {
+		name       string
+		delta      int
+		wantError  bool
+		wantCharge int
+	}{
+		{name: "below aggregate bound", delta: -1, wantCharge: perInput},
+		{name: "at aggregate bound", delta: 0, wantCharge: perInput},
+		{name: "above aggregate bound", delta: 1, wantError: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			accounting := &importAccounting{nodes: limits.nodes - perInput + test.delta}
+			composition, _, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{
+				source: empty, destination: destination,
+			}}}, 1, nil, accounting)
+			if test.wantError {
+				if composition != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSourceLimitExceeded {
+					t.Fatalf("over-budget placement = %#v, %#v", composition, diagnostics)
+				}
+				return
+			}
+			if composition == nil || len(diagnostics) != 0 || composition.inputNodes != test.wantCharge {
+				t.Fatalf("bounded placement = %#v, %#v", composition, diagnostics)
+			}
+		})
+	}
+}
+
+func TestMultipleMountedInputsSharePlacementCharges(t *testing.T) {
+	t.Parallel()
+
+	limits := defaultImportLimits()
+	inputs := []loadInput{
+		{source: Mapping("one", map[string]any{}), destination: Keys("collection", "one")},
+		{source: Mapping("two", map[string]any{}), destination: Keys("collection", "two")},
+		{source: Mapping("three", map[string]any{}), destination: Keys("collection", "three")},
+	}
+	perInput := countInputNodes(&document.Node{Kind: document.Mapping}) + destinationWrapperNodeCount(Keys("collection", "one"))
+	charge := len(inputs) * perInput
+	accounting := &importAccounting{nodes: limits.nodes - charge}
+	composition, _, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: inputs}, 1, nil, accounting)
+	if len(diagnostics) != 0 || composition == nil || accounting.nodes != limits.nodes || composition.inputNodes != charge {
+		t.Fatalf("shared placement accounting = %#v, %#v, accounting %#v; want charge %d", composition, diagnostics, accounting, charge)
+	}
+}
+
+func TestImportedEmptyBodiesChargeEachDestinationWrapper(t *testing.T) {
+	t.Parallel()
+
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":     &fstest.MapFile{Data: []byte("configkit: {imports: [{path: fragment.yml}]}\n")},
+		"fragment.yml": &fstest.MapFile{Data: []byte("{}\n")},
+	})
+	mainSource := filesystem.File("main.yml")
+	mainParsed, diagnostics := parseSource(mainSource, "fixture", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	fragmentParsed, diagnostics := parseSource(Bytes("fragment", []byte("{}\n")), "fixture", defaultParseLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	rawNodes := countInputNodes(mainParsed.document) + countInputNodes(fragmentParsed.document)
+	destination := Keys("collection", "empty")
+	placementNodes := 2 * destinationWrapperNodeCount(destination)
+	limits := defaultImportLimits()
+	for _, test := range []struct {
+		name      string
+		delta     int
+		wantError bool
+	}{
+		{name: "below aggregate bound", delta: -1},
+		{name: "at aggregate bound", delta: 0},
+		{name: "above aggregate bound", delta: 1, wantError: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			accounting := &importAccounting{nodes: limits.nodes - rawNodes - placementNodes + test.delta}
+			composition, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{{
+				source: mainSource, imports: true, destination: destination,
+			}}}, 1, nil, accounting)
+			if test.wantError {
+				if composition != nil || len(diagnostics) == 0 || diagnostics[0].Code != CodeImportLimitExceeded {
+					t.Fatalf("over-budget imports = %#v, %#v", composition, diagnostics)
+				}
+				return
+			}
+			if len(diagnostics) != 0 || composition == nil || len(sources) != 2 {
+				t.Fatalf("bounded imports = %#v, sources %#v, diagnostics %#v", composition, sources, diagnostics)
+			}
+			if composition.inputNodes != rawNodes+placementNodes || accounting.nodes != limits.nodes+test.delta {
+				t.Fatalf("import node charge = %d, accounting %d; want %d and %d", composition.inputNodes, accounting.nodes, rawNodes+placementNodes, limits.nodes+test.delta)
+			}
+			if node, present := documentAtPath(composition.root, Keys("collection", "empty")); !present || node.Kind != document.Mapping || len(node.Mapping) != 0 {
+				t.Fatalf("placed empty import body = %#v, present %v", node, present)
+			}
+		})
+	}
+}
+
+func TestNoSourceAndRootPlacementAddNoWrapperCharges(t *testing.T) {
+	t.Parallel()
+
+	accounting := &importAccounting{}
+	composition, sources, _, diagnostics := acquireUnderlayGroup(loadGroup{inputs: []loadInput{
+		{source: NoSource(), destination: Keys("absent")},
+		{source: Mapping("root", map[string]any{})},
+	}}, 1, nil, accounting)
+	if len(diagnostics) != 0 || composition == nil || len(sources) != 1 || accounting.nodes != 1 || composition.inputNodes != 1 {
+		t.Fatalf("root and absent placement = %#v, sources %#v, diagnostics %#v, accounting %#v", composition, sources, diagnostics, accounting)
+	}
+}
+
+func TestSelectedOverlayAndCLIUsePlacementNodeCharges(t *testing.T) {
+	t.Parallel()
+
+	limits := defaultImportLimits()
+	baseNodes := countInputNodes(&document.Node{Kind: document.Mapping}) + destinationWrapperNodeCount(Keys("mounted"))
+
+	t.Run("selected overlay", func(t *testing.T) {
+		t.Parallel()
+		filesystem := NewFilesystem("bundle", fstest.MapFS{
+			"overlays/test.yml": &fstest.MapFile{Data: []byte("{}\n")},
+		})
+		request := loadRequest[map[string]any]{
+			underlays: []loadGroup{{inputs: []loadInput{{source: Mapping("empty", map[string]any{}), destination: Keys("mounted")}}}},
+			overlay:   "test", overlayFilesystem: &FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem},
+		}
+		accounting := &importAccounting{nodes: limits.nodes - baseNodes}
+		_, diagnostics := assembleLayeredInput(request, assembledInput{}, accounting)
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverlayLimitExceeded {
+			t.Fatalf("overlay diagnostics = %#v", diagnostics)
+		}
+	})
+
+	t.Run("CLI override", func(t *testing.T) {
+		t.Parallel()
+		override := Override{Path: Keys("mounted", "value"), Value: "x", Flag: "--value"}
+		builder := valueBuilder{sourceID: "fixture", active: make(map[valueVisit]bool)}
+		if _, issue := builder.build(override.Value, override.Path, len(override.Path.segments)); issue != nil {
+			t.Fatal(issue)
+		}
+		overrideNodes := builder.nodes + destinationWrapperNodeCount(override.Path)
+		request := loadRequest[map[string]any]{
+			underlays: []loadGroup{{inputs: []loadInput{{source: Mapping("empty", map[string]any{}), destination: Keys("mounted")}}}},
+			overrides: []Override{override},
+		}
+		accounting := &importAccounting{nodes: limits.nodes - baseNodes - overrideNodes + 1}
+		_, diagnostics := assembleLayeredInput(request, assembledInput{}, accounting)
+		if len(diagnostics) != 1 || diagnostics[0].Code != CodeOverrideLimitExceeded {
+			t.Fatalf("override diagnostics = %#v", diagnostics)
+		}
+	})
+}

internal/engine/destination_diagnostic_test.go

@@ -0,0 +1,142 @@
+package engine
+
+import (
+	"path/filepath"
+	"testing"
+	"testing/fstest"
+)
+
+func TestLayeredDestinationDiagnosticsUseEffectivePathsAndAuthoredLocations(t *testing.T) {
+	t.Parallel()
+
+	t.Run("strict conflict retains both source locations", func(t *testing.T) {
+		t.Parallel()
+		loaded, err := LoadLayered(
+			Input{Source: NoSource()},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{
+				{Source: Bytes("left", []byte("section:\n  value: left\n")), Destination: Keys("application")},
+				{Source: Bytes("right", []byte("section:\n  value: right\n")), Destination: Keys("application")},
+			}}}},
+		)
+		if loaded != nil {
+			t.Fatalf("conflicting load returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeComposeConflict || diagnostic.Path.String() != "/application/section/value" {
+			t.Fatalf("conflict diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 2 {
+			t.Fatalf("primary location = %#v", diagnostic.Primary)
+		}
+		if len(diagnostic.Related) != 1 || diagnostic.Related[0].SourceID != "source-2" || diagnostic.Related[0].Line != 2 {
+			t.Fatalf("related locations = %#v", diagnostic.Related)
+		}
+	})
+
+	t.Run("decode failure keeps effective path and content line", func(t *testing.T) {
+		t.Parallel()
+		type section struct {
+			Count int `yaml:"count"`
+		}
+		type config struct {
+			Application struct {
+				Section section `yaml:"section"`
+			} `yaml:"application"`
+		}
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("section:\n  count: invalid\n")), Destination: Keys("application")},
+			testSecretBinding(),
+			LayeredOptions[config]{},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid load returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeDecodeType || diagnostic.Path.String() != "/application/section/count" {
+			t.Fatalf("decode diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 2 {
+			t.Fatalf("decode location = %#v", diagnostic.Primary)
+		}
+	})
+
+	t.Run("imported version failure uses effective path and import source", func(t *testing.T) {
+		t.Parallel()
+		directory := t.TempDir()
+		writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml}]}\n")
+		writeImportFile(t, directory, "fragment.yml", "version: forbidden\n")
+		rule := &VersionRule{Path: Keys("application", "version"), Check: func(Version, *Validation) error { return nil }}
+		loaded, err := LoadLayered(
+			Input{Source: NoSource()},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{
+				Definition: Definition[map[string]any]{Version: rule},
+				Underlays: []Underlay{{Inputs: []Input{{
+					Source: File(filepath.Join(directory, "main.yml")), Imports: true, Destination: Keys("application"),
+				}}}},
+			},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid import returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeImportVersion || diagnostic.Path.String() != "/application/version" {
+			t.Fatalf("import version diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-2" || diagnostic.Primary.Line != 1 {
+			t.Fatalf("import version location = %#v", diagnostic.Primary)
+		}
+	})
+
+	t.Run("invalid destination has no fictional source location", func(t *testing.T) {
+		t.Parallel()
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("value: present\n")), Destination: Keys("configkit")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid destination returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeOptionsInvalid || diagnostic.Path.String() != "/configkit" || diagnostic.Primary != nil {
+			t.Fatalf("invalid destination diagnostic = %#v", diagnostic)
+		}
+	})
+
+	t.Run("malformed source directive keeps its local path and location", func(t *testing.T) {
+		t.Parallel()
+		filesystem := NewFilesystem("bundle", fstest.MapFS{
+			"main.yml": &fstest.MapFile{Data: []byte("configkit: {imports: [true]}\nvalue: present\n")},
+		})
+		loaded, err := LoadLayered(
+			Input{Source: filesystem.File("main.yml"), Imports: true, Destination: Keys("application")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{},
+		)
+		if loaded != nil {
+			t.Fatalf("malformed directive returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeImportInvalid || diagnostic.Path.String() != "/configkit/imports/0" {
+			t.Fatalf("malformed directive diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 1 {
+			t.Fatalf("malformed directive location = %#v", diagnostic.Primary)
+		}
+	})
+}
+
+func firstDestinationDiagnostic(t *testing.T, err error) Diagnostic {
+	t.Helper()
+	loadError, ok := err.(*LoadError)
+	if !ok {
+		t.Fatalf("load error = %#v", err)
+	}
+	diagnostics := loadError.Diagnostics()
+	if len(diagnostics) == 0 {
+		t.Fatal("load error has no diagnostics")
+	}
+	return diagnostics[0]
+}

internal/engine/destination_import_test.go

@@ -0,0 +1,208 @@
+package engine
+
+import (
+	"path/filepath"
+	"testing"
+	"testing/fstest"
+)
+
+type importDestinationConfig struct {
+	Mounted     map[string]string            `yaml:"mounted"`
+	Collections map[string]map[string]string `yaml:"collections"`
+}
+
+func TestLayeredHostImportsInheritInputDestinationTransitively(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: first.yml
+    - path: second.yml
+root: main
+`)
+	firstPath := writeImportFile(t, directory, "first.yml", `configkit:
+  imports:
+    - path: nested/third.yml
+first: one
+`)
+	thirdPath := writeImportFile(t, directory, "nested/third.yml", "nested: three\n")
+	secondPath := writeImportFile(t, directory, "second.yml", "second: two\n")
+
+	loaded, err := LoadLayered(
+		Input{Source: File(mainPath), Imports: true, Destination: Keys("mounted")},
+		testSecretBinding(),
+		LayeredOptions[importDestinationConfig]{},
+	)
+	if err != nil {
+		if loadError, ok := err.(*LoadError); ok {
+			t.Fatalf("LoadLayered diagnostics: %#v", loadError.Diagnostics())
+		}
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	want := map[string]string{"root": "main", "first": "one", "nested": "three", "second": "two"}
+	if len(result.Config.Mounted) != len(want) {
+		t.Fatalf("mounted values = %#v", result.Config.Mounted)
+	}
+	for key, value := range want {
+		if result.Config.Mounted[key] != value {
+			t.Errorf("mounted[%q] = %q, want %q", key, result.Config.Mounted[key], value)
+		}
+	}
+	if _, ok := result.Metadata().Lookup(Keys("mounted", "configkit")); ok {
+		t.Fatal("source-root import directive leaked into application content")
+	}
+	sources := result.Sources()
+	wantPaths := []string{canonicalTestPath(t, mainPath), canonicalTestPath(t, firstPath), canonicalTestPath(t, thirdPath), canonicalTestPath(t, secondPath)}
+	if len(sources) != len(wantPaths) {
+		t.Fatalf("sources = %#v", sources)
+	}
+	for index, source := range sources {
+		if source.Destination.String() != "/mounted" {
+			t.Errorf("source %d destination = %q", index, source.Destination.String())
+		}
+		if source.Path != wantPaths[index] {
+			t.Errorf("source %d path = %q, want %q", index, source.Path, wantPaths[index])
+		}
+		if index > 0 && source.ImportedFrom == nil {
+			t.Errorf("imported source %d has no physical import location", index)
+		}
+	}
+}
+
+func TestLayeredNativeFilesystemImportsInheritInputDestination(t *testing.T) {
+	t.Parallel()
+
+	native := fstest.MapFS{
+		"main.yml":   &fstest.MapFile{Data: []byte("configkit:\n  imports:\n    - path: first.yml\nroot: main\n")},
+		"first.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: nested.yml}]}\nfirst: imported\n")},
+		"nested.yml": &fstest.MapFile{Data: []byte("nested: imported\n")},
+	}
+	namespace := newNativeNamespace("bundle", native)
+	loaded, err := LoadLayered(
+		Input{Source: filesystemFile(namespace, "main.yml"), Imports: true, Destination: Keys("mounted")},
+		testSecretBinding(),
+		LayeredOptions[importDestinationConfig]{},
+	)
+	if err != nil {
+		if loadError, ok := err.(*LoadError); ok {
+			t.Fatalf("LoadLayered diagnostics: %#v", loadError.Diagnostics())
+		}
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	if result.Config.Mounted["root"] != "main" || result.Config.Mounted["first"] != "imported" || result.Config.Mounted["nested"] != "imported" {
+		t.Fatalf("mounted native filesystem values = %#v", result.Config.Mounted)
+	}
+	sources := result.Sources()
+	if len(sources) != 3 || sources[0].Destination.String() != "/mounted" || sources[1].Destination.String() != "/mounted" || sources[2].Destination.String() != "/mounted" {
+		t.Fatalf("native source destinations = %#v", sources)
+	}
+}
+
+func TestLayeredImportOnlyInputMaterializesDestination(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "main.yml", `configkit:
+  imports:
+    - path: optional.yml
+      optional: true
+`)
+	loaded, err := LoadLayered(
+		Input{Source: File(mainPath), Imports: true, Destination: Keys("collections", "empty")},
+		testSecretBinding(),
+		LayeredOptions[importDestinationConfig]{},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	info, ok := result.Metadata().Lookup(Keys("collections", "empty"))
+	if !ok || !info.Explicit {
+		t.Fatalf("import-only body destination = %#v, %v", info, ok)
+	}
+	sources := result.Sources()
+	if len(sources) != 1 || sources[0].Destination.String() != "/collections/empty" {
+		t.Fatalf("sources = %#v", sources)
+	}
+}
+
+func TestLayeredImportedVersionClaimsUseEffectiveDestination(t *testing.T) {
+	t.Parallel()
+
+	for _, test := range []struct {
+		name        string
+		rulePath    Path
+		destination Path
+		wantCode    Code
+	}{
+		{name: "protected effective path", rulePath: Keys("mounted", "version"), destination: Keys("mounted"), wantCode: CodeImportVersion},
+		{name: "ordinary nested domain version", rulePath: Keys("version"), destination: Keys("collections", "entry")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			directory := t.TempDir()
+			writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml}]}\n")
+			writeImportFile(t, directory, "fragment.yml", "version: v2\n")
+			rule := &VersionRule{Path: test.rulePath, Check: func(Version, *Validation) error { return nil }}
+			loaded, err := LoadLayered(
+				Input{Source: NoSource()},
+				testSecretBinding(),
+				LayeredOptions[importDestinationConfig]{
+					Definition: Definition[importDestinationConfig]{Version: rule},
+					Underlays: []Underlay{{Inputs: []Input{{
+						Source: File(filepath.Join(directory, "main.yml")), Imports: true, Destination: test.destination,
+					}}}},
+				},
+			)
+			if test.wantCode != "" {
+				if err == nil {
+					t.Fatal("protected imported version claim was accepted")
+				}
+				loadError, ok := err.(*LoadError)
+				if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != test.wantCode {
+					t.Fatalf("load error = %#v", err)
+				}
+				if test.name == "protected effective path" {
+					sources := loadError.Sources()
+					if len(sources) != 2 || sources[0].Destination.String() != "/mounted" || sources[1].Destination.String() != "/mounted" {
+						t.Fatalf("error source destinations = %#v", sources)
+					}
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("ordinary nested version was rejected: %v", err)
+			}
+			result := testLoaded(loaded)
+			if got := result.Config.Collections["entry"]["version"]; got != "v2" {
+				t.Fatalf("nested version = %q", got)
+			}
+		})
+	}
+}
+
+func TestLayeredDestinationsDoNotChangeRepeatedSourceIdentity(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	writeImportFile(t, directory, "shared.yml", "value: shared\n")
+	importerPath := writeImportFile(t, directory, "importer.yml", "configkit: {imports: [{path: shared.yml}]}\n")
+	_, err := LoadLayered(
+		Input{Source: NoSource()},
+		testSecretBinding(),
+		LayeredOptions[importDestinationConfig]{Underlays: []Underlay{{Inputs: []Input{
+			{Source: File(importerPath), Imports: true, Destination: Keys("imported")},
+			{Source: File(filepath.Join(directory, "shared.yml")), Destination: Keys("direct")},
+		}}}},
+	)
+	if err == nil {
+		t.Fatal("the same physical source was accepted at two destinations in one group")
+	}
+	loadError, ok := err.(*LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != CodeImportRepeated {
+		t.Fatalf("load error = %#v", err)
+	}
+}

internal/engine/destination_integration_test.go

@@ -0,0 +1,463 @@
+package engine
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+func TestLayeredDestinationInheritanceUsesFinalSiblingScopes(t *testing.T) {
+	t.Parallel()
+
+	loaded, err := LoadLayered(
+		Input{Source: Bytes("main", []byte("parent:\n  label: final\n  settings:\n    primary: {height: 4}\ncancelled:\n  extends: null\n  own: retained\n")), Destination: Keys("catalog", "entries")},
+		testSecretBinding(),
+		LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{
+			{Source: Bytes("parent", []byte("parent:\n  label: lower\n  settings:\n    primary: {width: '10'}\n    secondary: {extends: primary}\ncancelled:\n  extends: parent\n  own: retained\n")), Destination: Keys("catalog", "entries")},
+			{Source: Bytes("child", []byte("child:\n  extends: parent\n  settings:\n    primary: {width: '20'}\n    secondary: {extends: primary}\n")), Destination: Keys("catalog", "entries")},
+		}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	entries := layeredMap(t, layeredMap(t, result.Config, "catalog"), "entries")
+	parent := layeredMap(t, entries, "parent")
+	child := layeredMap(t, entries, "child")
+	parentSettings := layeredMap(t, parent, "settings")
+	childSettings := layeredMap(t, child, "settings")
+	if parent["label"] != "final" || child["label"] != "final" {
+		t.Fatalf("higher-tier parent was not inherited after precedence: parent=%#v child=%#v", parent, child)
+	}
+	if layeredMapValue(t, parentSettings, "secondary", "width") != "10" ||
+		fmt.Sprint(layeredMapValue(t, parentSettings, "secondary", "height")) != "4" ||
+		layeredMapValue(t, childSettings, "secondary", "width") != "20" ||
+		fmt.Sprint(layeredMapValue(t, childSettings, "secondary", "height")) != "4" {
+		t.Fatalf("nested sibling scopes were not resolved at their destinations: parent=%#v child=%#v", parentSettings, childSettings)
+	}
+	cancelled := layeredMap(t, entries, "cancelled")
+	if len(cancelled) != 1 || cancelled["own"] != "retained" {
+		t.Fatalf("higher-tier cancellation did not suppress inheritance: %#v", cancelled)
+	}
+	if _, ok := result.Metadata().Lookup(Keys("catalog", "entries", "child", inheritanceKey)); ok {
+		t.Fatal("effective child retained its extends directive")
+	}
+}
+
+func TestLayeredDestinationInheritanceDoesNotSearchOtherCollections(t *testing.T) {
+	t.Parallel()
+
+	_, err := LoadLayered(
+		Input{Source: Bytes("main", []byte("parent: {value: foreign}\n")), Destination: Keys("catalog", "other")},
+		testSecretBinding(),
+		LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{{
+			Source:      Bytes("child", []byte("child: {extends: parent}\n")),
+			Destination: Keys("catalog", "entries"),
+		}}}}},
+	)
+	if got := groupedErrorCode(t, err); got != CodeInheritanceMissing {
+		t.Fatalf("cross-collection parent lookup error = %s, want %s", got, CodeInheritanceMissing)
+	}
+}
+
+func TestLayeredMountedVersionProtectionCoversInheritanceAndUpperTiers(t *testing.T) {
+	t.Parallel()
+
+	for _, test := range []struct {
+		name     string
+		body     string
+		rulePath Path
+	}{
+		{name: "inherited protected value", body: "parent: {version: v2}\nchild: {extends: parent}\n", rulePath: Keys("catalog", "entries", "child", "version")},
+		{name: "inherited protected ancestor", body: "parent: {schema: null}\nchild: {extends: parent}\n", rulePath: Keys("catalog", "entries", "child", "schema", "version")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			_, err := LoadLayered(
+				Input{Source: NoSource()}, testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: &VersionRule{Path: test.rulePath, Check: func(Version, *Validation) error { return nil }}},
+					Underlays:  []Underlay{{Inputs: []Input{{Source: Bytes("underlay", []byte(test.body)), Destination: Keys("catalog", "entries")}}}},
+				},
+			)
+			if got := groupedErrorCode(t, err); got != CodeInheritanceVersion {
+				t.Fatalf("inheritance version protection code = %s", got)
+			}
+		})
+	}
+	t.Run("inheritance consumes a protected directive", func(t *testing.T) {
+		_, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("parent: {value: inherited}\nchild: {extends: parent}\n")), Destination: Keys("catalog", "entries")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Definition: Definition[map[string]any]{Version: &VersionRule{
+				Path: Keys("catalog", "entries", "child", "extends"),
+				Check: func(version Version, _ *Validation) error {
+					if !version.Present || version.Text != "parent" {
+						t.Errorf("mounted protected directive value = %#v", version)
+					}
+					return nil
+				},
+			}}},
+		)
+		if got := groupedErrorCode(t, err); got != CodeInheritanceVersion {
+			t.Fatalf("consumed directive protection code = %s", got)
+		}
+	})
+	t.Run("inherited mapping ancestor without a version claim", func(t *testing.T) {
+		loaded, err := LoadLayered(
+			Input{Source: NoSource()}, testSecretBinding(),
+			LayeredOptions[map[string]any]{
+				Definition: Definition[map[string]any]{Version: &VersionRule{Path: Keys("catalog", "entries", "child", "schema", "version"), Check: func(Version, *Validation) error { return nil }}},
+				Underlays: []Underlay{{Inputs: []Input{{
+					Source: Bytes("underlay", []byte("parent: {schema: {name: ordinary}}\nchild: {extends: parent}\n")), Destination: Keys("catalog", "entries"),
+				}}}},
+			},
+		)
+		if err != nil {
+			t.Fatalf("nonclaiming inherited mapping ancestor was rejected: %v", err)
+		}
+		if got := layeredMapValue(t, testLoaded(loaded).Config, "catalog", "entries", "child", "schema", "name"); got != "ordinary" {
+			t.Fatalf("inherited non-version value = %#v", got)
+		}
+	})
+
+	for _, test := range []struct {
+		name string
+		body string
+	}{
+		{name: "overlay protected value", body: "catalog: {schema: {version: v2}}\n"},
+		{name: "overlay protected ancestor", body: "catalog: {schema: null}\n"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			filesystem := NewFilesystem("overlay", fstest.MapFS{"overlays/test.yml": &fstest.MapFile{Data: []byte(test.body)}})
+			_, err := LoadLayered(
+				Input{Source: Bytes("main", []byte("schema: {version: v1}\n")), Destination: Keys("catalog")},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: &VersionRule{Path: Keys("catalog", "schema", "version"), Check: func(version Version, _ *Validation) error {
+						if !version.Present || version.Text != "v1" {
+							t.Errorf("mounted main version = %#v", version)
+						}
+						return nil
+					}}},
+					Overlay: "test", OverlayFilesystem: &FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem},
+				},
+			)
+			if got := groupedErrorCode(t, err); got != CodeOverlayVersion {
+				t.Fatalf("overlay version protection code = %s", got)
+			}
+		})
+	}
+
+	for _, test := range []struct {
+		name  string
+		path  Path
+		value any
+	}{
+		{name: "override protected value", path: Keys("catalog", "schema", "version"), value: "v2"},
+		{name: "override protected ancestor", path: Keys("catalog", "schema"), value: map[string]any{"version": "v2"}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			_, err := LoadLayered(
+				Input{Source: Bytes("main", []byte("schema: {version: v1}\n")), Destination: Keys("catalog")},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: &VersionRule{Path: Keys("catalog", "schema", "version"), Check: func(Version, *Validation) error { return nil }}},
+					Overrides:  []Override{{Path: test.path, Value: test.value, Flag: "--config"}},
+				},
+			)
+			if got := groupedErrorCode(t, err); got != CodeOverrideVersion {
+				t.Fatalf("override version protection code = %s", got)
+			}
+		})
+	}
+
+	t.Run("unrelated inheritance under the protected mapping ancestor", func(t *testing.T) {
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("schema: {version: v1}\nentries:\n  parent: {value: inherited}\n  child: {extends: parent}\n")), Destination: Keys("catalog")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Definition: Definition[map[string]any]{Version: &VersionRule{Path: Keys("catalog", "schema", "version"), Check: func(Version, *Validation) error { return nil }}}},
+		)
+		if err != nil {
+			t.Fatalf("unrelated mounted inheritance was rejected: %v", err)
+		}
+		child := layeredMapValue(t, testLoaded(loaded).Config, "catalog", "entries", "child", "value")
+		if child != "inherited" {
+			t.Fatalf("unrelated inheritance value = %#v", child)
+		}
+	})
+}
+
+type destinationDecodeProbe string
+
+var destinationDecodeProbeValues []string
+
+func (value *destinationDecodeProbe) UnmarshalText(text []byte) error {
+	destinationDecodeProbeValues = append(destinationDecodeProbeValues, string(text))
+	*value = destinationDecodeProbe(text)
+	return nil
+}
+
+func (value destinationDecodeProbe) MarshalText() ([]byte, error) {
+	return []byte(value), nil
+}
+
+type destinationLifecycleConfig struct {
+	Defaulted string `yaml:"defaulted"`
+	Tenant    struct {
+		Tagged     string                 `yaml:"tagged" configkit:"secret"`
+		Dynamic    string                 `yaml:"dynamic"`
+		Credential Secret                 `yaml:"credential"`
+		Derived    string                 `yaml:"derived"`
+		Decoded    destinationDecodeProbe `yaml:"decoded"`
+	} `yaml:"tenant"`
+}
+
+func TestLayeredDestinationLifecycleAndSensitiveInspectionUseFinalShape(t *testing.T) {
+	const (
+		oldTagged   = "SUPERSEDED-TAGGED-CANARY"
+		newTagged   = "CURRENT-TAGGED-CANARY"
+		oldDynamic  = "SUPERSEDED-DYNAMIC-CANARY"
+		newDynamic  = "CURRENT-DYNAMIC-CANARY"
+		credential  = "RESOLVED-CREDENTIAL-CANARY"
+		secretName  = "DESTINATION_SECRET_NAME"
+		decodeValue = "decoded-once"
+	)
+	type calls struct{ defaults, resolve, lookup, validate int }
+	newLoad := func(mounted bool, observed *calls) (*testResult[destinationLifecycleConfig], error) {
+		mainBody := "tagged: " + newTagged + "\ndynamic: " + newDynamic + "\ncredential: " + secretName + "\ndecoded: " + decodeValue + "\n"
+		underlayBody := "tagged: " + oldTagged + "\ndynamic: " + oldDynamic + "\ncredential: OLD_DESTINATION_SECRET_NAME\ndecoded: old\n"
+		if !mounted {
+			mainBody = "tenant:\n" + indentDestinationBody(mainBody)
+			underlayBody = "tenant:\n" + indentDestinationBody(underlayBody)
+		}
+		main := loadInput{source: Bytes("main", []byte(mainBody))}
+		underlay := loadInput{source: Bytes("underlay", []byte(underlayBody))}
+		if mounted {
+			main.destination = Keys("tenant")
+			underlay.destination = Keys("tenant")
+		}
+		definition := Definition[destinationLifecycleConfig]{
+			Defaults: func() destinationLifecycleConfig {
+				observed.defaults++
+				return destinationLifecycleConfig{Defaulted: "from-default"}
+			},
+			Resolve: func(configuration *destinationLifecycleConfig, resolution *Resolution) error {
+				observed.resolve++
+				configuration.Tenant.Derived = configuration.Tenant.Dynamic
+				resolution.Record(Keys("tenant", "derived"), ChangeNormalization, "derive from mounted input", Keys("tenant", "dynamic"))
+				return nil
+			},
+			Validate: func(configuration *destinationLifecycleConfig, _ *Validation) {
+				observed.validate++
+				if configuration.Tenant.Decoded != destinationDecodeProbe(decodeValue) || configuration.Tenant.Dynamic != newDynamic {
+					t.Errorf("validation saw a non-final mounted configuration: %#v", configuration.Tenant)
+				}
+			},
+			Secret: func(path Path) bool { return path.String() == "/tenant/dynamic" },
+		}
+		loaded, err := loadGroupedWithSecretLookup(loadRequest[destinationLifecycleConfig]{
+			main: main, underlays: []loadGroup{{inputs: []loadInput{underlay}}}, definition: definition,
+		}, testSecretBinding(), func(name string) (string, bool) {
+			observed.lookup++
+			if name != secretName {
+				return "", false
+			}
+			return credential, true
+		})
+		return testLoaded(loaded), err
+	}
+
+	var observed calls
+	destinationDecodeProbeValues = nil
+	mounted, err := newLoad(true, &observed)
+	if err != nil {
+		if loadError, ok := err.(*LoadError); ok {
+			t.Fatalf("mounted load: %#v", loadError.Diagnostics())
+		}
+		t.Fatalf("mounted load: %v", err)
+	}
+	if observed != (calls{defaults: 1, resolve: 1, lookup: 1, validate: 1}) {
+		t.Fatalf("mounted lifecycle counts = %#v", observed)
+	}
+	sawFinalDecode := false
+	for _, value := range destinationDecodeProbeValues {
+		if value == decodeValue {
+			sawFinalDecode = true
+		} else if value != "" {
+			t.Fatalf("decoder observed a non-final mounted value %q: %#v", value, destinationDecodeProbeValues)
+		}
+	}
+	if !sawFinalDecode {
+		t.Fatal("final mounted value was not decoded")
+	}
+	if mounted.Config.Defaulted != "from-default" || mounted.Config.Tenant.Tagged != newTagged || mounted.Config.Tenant.Dynamic != newDynamic ||
+		mounted.Config.Tenant.Derived != newDynamic || mounted.Config.Tenant.Decoded != destinationDecodeProbe(decodeValue) ||
+		mounted.Config.Tenant.Credential.Value() != credential {
+		t.Fatalf("mounted final configuration = %#v", mounted.Config)
+	}
+	for _, path := range []Path{Keys("tenant", "tagged"), Keys("tenant", "dynamic"), Keys("tenant", "derived")} {
+		if !mounted.Inspection.secrets[path.String()] {
+			t.Errorf("effective sensitivity missing at %s: %#v", path, mounted.Inspection.secrets)
+		}
+	}
+	for _, path := range []Path{Keys("tenant", "tagged"), Keys("tenant", "dynamic"), Keys("tenant", "derived")} {
+		explanation, ok := mounted.Explain(path)
+		if !ok || !explanation.Effective.Redacted {
+			t.Errorf("effective explanation at %s = %#v, %v", path, explanation, ok)
+			continue
+		}
+		for _, change := range explanation.History {
+			if strings.Contains(change.Before.YAML+change.After.YAML, oldTagged) ||
+				strings.Contains(change.Before.YAML+change.After.YAML, newTagged) ||
+				strings.Contains(change.Before.YAML+change.After.YAML, oldDynamic) ||
+				strings.Contains(change.Before.YAML+change.After.YAML, newDynamic) {
+				t.Errorf("classified history leaked a canary at %s: %#v", path, change)
+			}
+		}
+		var rendered bytes.Buffer
+		if err := mounted.WriteExplanation(&rendered, path); err != nil || strings.Contains(rendered.String(), "CANARY") {
+			t.Errorf("safe explanation at %s = %q, %v", path, rendered.String(), err)
+		}
+	}
+	var effective bytes.Buffer
+	if err := mounted.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	for _, canary := range []string{oldTagged, newTagged, oldDynamic, newDynamic, credential} {
+		if strings.Contains(effective.String(), canary) {
+			t.Fatalf("effective rendering exposed %q: %s", canary, effective.String())
+		}
+	}
+	if serializedConfig := fmt.Sprintf("%#v", mounted.Config); strings.Contains(serializedConfig, credential) {
+		t.Fatalf("configuration formatting exposed the resolved credential: %s", serializedConfig)
+	}
+	for _, path := range []Path{Keys("tenant", "tagged"), Keys("tenant", "dynamic")} {
+		if history := fmt.Sprintf("%#v", mounted.Metadata().History(path)); strings.Contains(history, "CANARY") {
+			t.Fatalf("metadata history at %s exposed a canary: %s", path, history)
+		}
+	}
+
+	var authoredCalls calls
+	destinationDecodeProbeValues = nil
+	authored, err := newLoad(false, &authoredCalls)
+	if err != nil {
+		t.Fatalf("explicitly wrapped load: %v", err)
+	}
+	if authoredCalls != (calls{defaults: 1, resolve: 1, lookup: 1, validate: 1}) {
+		t.Fatalf("authored lifecycle counts = %#v", authoredCalls)
+	}
+	sawFinalDecode = false
+	for _, value := range destinationDecodeProbeValues {
+		if value == decodeValue {
+			sawFinalDecode = true
+		} else if value != "" {
+			t.Fatalf("authored decoder observed a non-final value %q: %#v", value, destinationDecodeProbeValues)
+		}
+	}
+	if !sawFinalDecode {
+		t.Fatal("final authored value was not decoded")
+	}
+	if !reflect.DeepEqual(mounted.Config, authored.Config) {
+		t.Fatalf("mounted config %#v differs from explicitly wrapped config %#v", mounted.Config, authored.Config)
+	}
+	differences, err := Diff(mounted.Inspection, authored.Inspection)
+	if err != nil || len(differences) != 0 {
+		t.Fatalf("mounted versus authored diff = %#v, %v", differences, err)
+	}
+}
+
+func TestLayeredDestinationDiffUsesBilateralSensitivity(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Tenant struct {
+			Dynamic string `yaml:"dynamic"`
+			Derived string `yaml:"derived"`
+		} `yaml:"tenant"`
+	}
+	load := func(value string, classify bool) *testResult[config] {
+		t.Helper()
+		loaded, err := loadGroupedWithSecretLookup(loadRequest[config]{
+			main: loadInput{source: Bytes("input", []byte("dynamic: "+value+"\n")), destination: Keys("tenant")},
+			definition: Definition[config]{
+				Resolve: func(configuration *config, resolution *Resolution) error {
+					configuration.Tenant.Derived = configuration.Tenant.Dynamic
+					resolution.Record(Keys("tenant", "derived"), ChangeNormalization, "derive from dynamic input", Keys("tenant", "dynamic"))
+					return nil
+				},
+				Secret: func(path Path) bool { return classify && path.String() == "/tenant/dynamic" },
+			},
+		}, testSecretBinding(), func(string) (string, bool) { return "", false })
+		if err != nil {
+			t.Fatalf("LoadLayered: %v", err)
+		}
+		return testLoaded(loaded)
+	}
+	left := load("LEFT-DIFF-CANARY", true)
+	right := load("RIGHT-DIFF-CANARY", false)
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 2 {
+		t.Fatalf("differences = %#v", differences)
+	}
+	seen := make(map[string]bool)
+	for _, difference := range differences {
+		path := difference.Path.String()
+		seen[path] = true
+		if !difference.Left.Redacted || !difference.Right.Redacted || !difference.HiddenChanged ||
+			strings.Contains(difference.Left.YAML+difference.Right.YAML, "DIFF-CANARY") {
+			t.Errorf("bilaterally classified difference at %s = %#v", path, difference)
+		}
+	}
+	for _, path := range []string{"/tenant/dynamic", "/tenant/derived"} {
+		if !seen[path] || !left.Inspection.secrets[path] || right.Inspection.secrets[path] {
+			t.Errorf("classification/difference at %s: seen=%v left=%v right=%v", path, seen[path], left.Inspection.secrets[path], right.Inspection.secrets[path])
+		}
+	}
+}
+
+func TestLayeredDestinationDecodeFailureReturnsNoResultOrLaterCallbacks(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Tenant struct {
+			Count int `yaml:"count"`
+		} `yaml:"tenant"`
+	}
+	calls := struct{ defaults, resolve, lookup, validate int }{}
+	loaded, err := loadGroupedWithSecretLookup(loadRequest[config]{
+		main: loadInput{source: Bytes("invalid", []byte("count: not-an-integer\n")), destination: Keys("tenant")},
+		definition: Definition[config]{
+			Defaults: func() config { calls.defaults++; return config{} },
+			Resolve:  func(*config, *Resolution) error { calls.resolve++; return nil },
+			Validate: func(*config, *Validation) { calls.validate++ },
+		},
+	}, testSecretBinding(), func(string) (string, bool) { calls.lookup++; return "", false })
+	if loaded != nil || err == nil {
+		t.Fatalf("mounted decode failure returned a usable result: %#v, %v", loaded, err)
+	}
+	if calls.defaults != 1 || calls.resolve != 0 || calls.lookup != 0 || calls.validate != 0 {
+		t.Fatalf("callbacks after mounted decode failure = %#v", calls)
+	}
+	if got := groupedErrorCode(t, err); got != CodeDecodeType {
+		t.Fatalf("mounted decode failure code = %s", got)
+	}
+}
+
+func indentDestinationBody(body string) string {
+	var result strings.Builder
+	for _, line := range strings.Split(strings.TrimSuffix(body, "\n"), "\n") {
+		result.WriteString("  ")
+		result.WriteString(line)
+		result.WriteByte('\n')
+	}
+	return result.String()
+}

internal/engine/destination_load_test.go

@@ -0,0 +1,118 @@
+package engine
+
+import (
+	"bytes"
+	"testing"
+)
+
+type destinationLoadConfig struct {
+	Main        map[string]string            `yaml:"main"`
+	Defaults    map[string]string            `yaml:"defaults"`
+	Collections map[string]map[string]string `yaml:"collections"`
+	Value       string                       `yaml:"value"`
+}
+
+func TestLayeredLoadPlacesNonImportMainAndUnderlayInputs(t *testing.T) {
+	t.Parallel()
+
+	destination := Keys("main")
+	main := Input{Source: Bytes("main", []byte("value: primary\n")), Destination: destination}
+	underlay := Input{Source: Bytes("defaults", []byte("value: fallback\n")), Destination: Keys("defaults")}
+	loaded, err := LoadLayered(main, testSecretBinding(), LayeredOptions[destinationLoadConfig]{
+		Underlays: []Underlay{{Inputs: []Input{underlay}}},
+	})
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	if result.Config.Main["value"] != "primary" || result.Config.Defaults["value"] != "fallback" {
+		t.Fatalf("mounted configuration = %#v", result.Config)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Destination.String() != "/main" || sources[1].Destination.String() != "/defaults" {
+		t.Fatalf("source destinations = %#v", sources)
+	}
+
+	destination.segments[0].key = "caller mutation"
+	sources[0].Destination.segments[0].key = "record mutation"
+	if got := result.Sources()[0].Destination.String(); got != "/main" {
+		t.Fatalf("source destination was not independently owned: %q", got)
+	}
+}
+
+func TestLayeredLoadOmittedAndEmptyDestinationsKeepRootPlacement(t *testing.T) {
+	t.Parallel()
+
+	inputs := []Input{
+		{Source: Bytes("omitted", []byte("value: same\n"))},
+		{Source: Bytes("empty", []byte("value: same\n")), Destination: Keys()},
+	}
+	for _, input := range inputs {
+		loaded, err := LoadLayered(input, testSecretBinding(), LayeredOptions[destinationLoadConfig]{})
+		if err != nil {
+			t.Fatalf("LoadLayered: %v", err)
+		}
+		result := testLoaded(loaded)
+		if result.Config.Value != "same" {
+			t.Fatalf("root value = %q", result.Config.Value)
+		}
+		sources := result.Sources()
+		if len(sources) != 1 || sources[0].Destination.String() != "" {
+			t.Fatalf("root source destination = %#v", sources)
+		}
+	}
+}
+
+func TestLayeredLoadDistinguishesEmptyBodyFromNoSource(t *testing.T) {
+	t.Parallel()
+
+	loaded, err := LoadLayered(
+		Input{Source: NoSource(), Destination: Keys("absent")},
+		testSecretBinding(),
+		LayeredOptions[destinationLoadConfig]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: Bytes("empty", []byte("{}\n")), Destination: Keys("collections", "one"),
+		}}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	info, ok := result.Metadata().Lookup(Keys("collections", "one"))
+	if !ok || !info.Explicit || info.Origin.Layer != LayerUnderlay {
+		t.Fatalf("empty mounted mapping = %#v, %v", info, ok)
+	}
+	if _, ok := result.Metadata().Lookup(Keys("absent")); ok {
+		t.Fatal("NoSource materialized its destination")
+	}
+	sources := result.Sources()
+	if len(sources) != 1 || sources[0].Label != "empty" || sources[0].Destination.String() != "/collections/one" {
+		t.Fatalf("sources = %#v", sources)
+	}
+}
+
+func TestLayeredLoadPreflightsDestinationsBeforeReadingSources(t *testing.T) {
+	t.Parallel()
+
+	reader := bytes.NewBufferString("value: unused\n")
+	_, err := LoadLayered(
+		Input{Source: NoSource()},
+		testSecretBinding(),
+		LayeredOptions[destinationLoadConfig]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: Reader("reader", reader), Destination: Keys("configkit"),
+		}}}}},
+	)
+	if err == nil {
+		t.Fatal("reserved destination was accepted")
+	}
+	if reader.Len() == 0 {
+		t.Fatal("source reader was consumed before destination preflight")
+	}
+	loadError, ok := err.(*LoadError)
+	if !ok || len(loadError.Diagnostics()) != 1 {
+		t.Fatalf("error = %#v", err)
+	}
+	diagnostic := loadError.Diagnostics()[0]
+	if diagnostic.Code != CodeOptionsInvalid || diagnostic.Path.String() != "/configkit" || diagnostic.Primary != nil {
+		t.Fatalf("destination diagnostic = %#v", diagnostic)
+	}
+}

internal/engine/destination_provenance_test.go

@@ -0,0 +1,181 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"path"
+	"reflect"
+	"testing"
+)
+
+func TestLayeredDestinationsRetainOriginsHistoryAndDetachedRecords(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	sharedPath := writeImportFile(t, directory, "shared.yml", "section:\n  shared: lower\n  branch:\n    child: old\n    kept: retained\n")
+	shared := File(sharedPath)
+	load := func() (*Loaded[map[string]any], error) {
+		return LoadLayered(
+			Input{Source: NoSource()}, testSecretBinding(),
+			LayeredOptions[map[string]any]{Underlays: []Underlay{
+				{Inputs: []Input{
+					{Source: shared, Destination: Keys("catalog")},
+					{Source: Bytes("peer", []byte("section:\n  peer: present\n")), Destination: Keys("catalog")},
+				}},
+				{Inputs: []Input{{Source: shared, Destination: Keys("backup")}}},
+				{Inputs: []Input{{Source: Bytes("replacement", []byte("section:\n  branch: replaced\n")), Destination: Keys("catalog")}}},
+			}},
+		)
+	}
+	loaded, err := load()
+	if err != nil {
+		t.Fatalf("LoadLayered: %v, diagnostics %#v", err, err.(*LoadError).Diagnostics())
+	}
+	result := testLoaded(loaded)
+	metadata := result.Metadata()
+
+	container, ok := metadata.Lookup(Keys("catalog"))
+	if !ok || !container.Explicit || len(container.Origin.Locations) < 3 {
+		t.Fatalf("mounted container origin = %#v, %v", container, ok)
+	}
+	containerSources := make(map[string]bool)
+	for _, location := range container.Origin.Locations {
+		containerSources[location.SourceID] = true
+		if location.Line < 1 {
+			t.Fatalf("introduced container lacks its source-root fallback: %#v", location)
+		}
+	}
+	for _, sourceID := range []string{"source-1", "source-2", "source-4"} {
+		if !containerSources[sourceID] {
+			t.Fatalf("container contributors = %#v, missing %s", container.Origin.Locations, sourceID)
+		}
+	}
+
+	sharedField, ok := metadata.Lookup(Keys("catalog", "section", "shared"))
+	if !ok || len(sharedField.Origin.Locations) != 1 || sharedField.Origin.Locations[0].SourceID != "source-1" || sharedField.Origin.Locations[0].Line != 2 {
+		t.Fatalf("authored leaf origin = %#v, %v", sharedField, ok)
+	}
+	peerField, ok := metadata.Lookup(Keys("catalog", "section", "peer"))
+	if !ok || len(peerField.Origin.Locations) != 1 || peerField.Origin.Locations[0].SourceID != "source-2" {
+		t.Fatalf("peer leaf origin = %#v, %v", peerField, ok)
+	}
+	if _, ok := metadata.Lookup(Keys("catalog", "section", "branch", "child")); ok {
+		t.Fatal("replaced descendant remains effective")
+	}
+	removed := metadata.History(Keys("catalog", "section", "branch", "child"))
+	if len(removed) < 2 || removed[0].Origin.Locations[0].SourceID != "source-1" {
+		t.Fatalf("removed descendant history = %#v", removed)
+	}
+
+	backup, ok := metadata.Lookup(Keys("backup", "section", "shared"))
+	if !ok || len(backup.Origin.Locations) != 1 || backup.Origin.Locations[0].SourceID != "source-3" {
+		t.Fatalf("second-tier leaf origin = %#v, %v", backup, ok)
+	}
+	sources := result.Sources()
+	if len(sources) != 4 || sources[0].Path != sources[2].Path || sources[0].Destination.String() != "/catalog" || sources[2].Destination.String() != "/backup" {
+		t.Fatalf("same source in separate tiers = %#v", sources)
+	}
+	if sources[0].Layer != LayerUnderlay || sources[0].UnderlayIndex != 1 || sources[2].Layer != LayerUnderlay || sources[2].UnderlayIndex != 2 {
+		t.Fatalf("tier source records = %#v", sources)
+	}
+
+	other, err := load()
+	if err != nil {
+		t.Fatalf("independent load: %v", err)
+	}
+	differences, err := Diff(loaded.Inspection, other.Inspection)
+	if err != nil || len(differences) != 0 {
+		t.Fatalf("equivalent inspection diff = %#v, %v", differences, err)
+	}
+
+	sources[0].Destination.segments[0].key = "mutated"
+	sharedField.Origin.Locations[0].SourceID = "mutated"
+	removed[0].Path.segments[0].key = "mutated"
+	if got := result.Sources()[0].Destination.String(); got != "/catalog" {
+		t.Fatalf("source record copy changed after inspection: %q", got)
+	}
+	fresh, ok := result.Metadata().Lookup(Keys("catalog", "section", "shared"))
+	if !ok || fresh.Origin.Locations[0].SourceID != "source-1" {
+		t.Fatalf("effective origin changed after mutating returned records: %#v, %v", fresh, ok)
+	}
+	if got := result.Metadata().History(Keys("catalog", "section", "branch", "child")); len(got) < 2 || got[0].Path.String() != "/catalog/section/branch/child" {
+		t.Fatalf("historical path changed after mutating returned records: %#v", got)
+	}
+	if got := other.Inspection.sources[0].Destination.String(); got != "/catalog" {
+		t.Fatalf("independent load destination changed: %q", got)
+	}
+}
+
+func TestLayeredImportedInheritedValuesRetainResourceContexts(t *testing.T) {
+	t.Parallel()
+
+	type entry struct {
+		Extends  string `yaml:"extends"`
+		Resource string `yaml:"resource"`
+		Count    int    `yaml:"count"`
+	}
+	type config struct {
+		Catalog struct {
+			Entries map[string]entry `yaml:"entries"`
+		} `yaml:"catalog"`
+	}
+	directory := t.TempDir()
+	mainPath := writeImportFile(t, directory, "config/main.yml", "child:\n  extends: parent\n")
+	inputPath := writeImportFile(t, directory, "shared/input.yml", "configkit: {imports: [{path: parent.yml}]}\n")
+	writeImportFile(t, directory, "shared/parent.yml", "parent:\n  resource: icon.txt\n  count: 1\n")
+	writeImportFile(t, directory, "shared/icon.txt", "parent-resource")
+	var resolutionContext ResourceContext
+	loaded, err := LoadLayered(
+		Input{Source: File(mainPath), Destination: Keys("catalog", "entries")},
+		testSecretBinding(),
+		LayeredOptions[config]{
+			Underlays: []Underlay{{Inputs: []Input{{
+				Source: File(inputPath), Imports: true, Destination: Keys("catalog", "entries"),
+			}}}},
+			Definition: Definition[config]{Resolve: func(configuration *config, resolution *Resolution) error {
+				info, ok := resolution.InputInfo(Keys("catalog", "entries", "child", "resource"))
+				if !ok || len(info.Origin.Locations) != 1 || len(info.Origin.Inheritance) != 1 {
+					return errors.New("inherited resource lacks its source origin")
+				}
+				var found bool
+				resolutionContext, found = resolution.state.input.sourceContext(info.Origin.Locations[0].SourceID)
+				if !found || resolutionContext.BaseDir != "." || resolutionContext.FS == nil {
+					return errors.New("resolver could not find the declaring resource context")
+				}
+				data, err := fs.ReadFile(resolutionContext.FS, path.Join(resolutionContext.BaseDir, configuration.Catalog.Entries["child"].Resource))
+				if err != nil || string(data) != "parent-resource" {
+					return errors.New("resolver could not read the inherited relative resource")
+				}
+				return nil
+			}},
+		},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v, diagnostics %#v", err, err.(*LoadError).Diagnostics())
+	}
+	result := testLoaded(loaded)
+	child := result.Config.Catalog.Entries["child"]
+	if child.Resource != "icon.txt" || child.Count != 1 || child.Extends != "" {
+		t.Fatalf("inherited child = %#v", child)
+	}
+	field, ok := result.Metadata().Lookup(Keys("catalog", "entries", "child", "resource"))
+	if !ok || len(field.Origin.Locations) != 1 || len(field.Origin.Inheritance) != 1 {
+		t.Fatalf("inherited resource origin = %#v, %v", field, ok)
+	}
+	context, ok := result.Inspection.SourceContext(field.Origin.Locations[0].SourceID)
+	if !ok || context.BaseDir != "." || context.FS == nil || resolutionContext.BaseDir != context.BaseDir {
+		t.Fatalf("completed resource contexts = %#v, %#v, %v", resolutionContext, context, ok)
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, child.Resource))
+	if err != nil || string(data) != "parent-resource" {
+		t.Fatalf("completed source-relative resource = %q, %v", data, err)
+	}
+	if !reflect.DeepEqual(result.Config.Catalog.Entries["parent"], entry{Resource: "icon.txt", Count: 1}) {
+		t.Fatalf("surviving lower-tier parent = %#v", result.Config.Catalog.Entries["parent"])
+	}
+	for _, source := range result.Sources() {
+		if source.Layer == LayerUnderlay && source.Destination.String() != "/catalog/entries" {
+			t.Errorf("underlay destination = %#v", source)
+		}
+	}
+}

internal/engine/destination_test.go

@@ -0,0 +1,178 @@
+package engine
+
+import (
+	"errors"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestNormalizeDestinationUsesLiteralMappingKeys(t *testing.T) {
+	t.Parallel()
+
+	parsed, err := ParsePath("/a~1b/0/~0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	normalized, err := normalizeDestination(parsed)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := normalized.String(), "/a~1b/0/~0"; got != want {
+		t.Fatalf("normalized path = %q, want %q", got, want)
+	}
+	for index, want := range []string{"a/b", "0", "~"} {
+		segment := normalized.segments[index]
+		if segment.kind != pathSegmentKey || segment.key != want {
+			t.Fatalf("segment %d = %#v, want key %q", index, segment, want)
+		}
+	}
+
+	original := Keys("owned", "path")
+	owned, err := normalizeDestination(original)
+	if err != nil {
+		t.Fatal(err)
+	}
+	owned.segments[0].key = "changed"
+	if got := original.String(); got != "/owned/path" {
+		t.Fatalf("normalizing shared path storage mutated its input: %q", got)
+	}
+}
+
+func TestNormalizeDestinationRejectsIndicesAndReservedKeys(t *testing.T) {
+	t.Parallel()
+
+	for name, path := range map[string]Path{
+		"explicit index":   Keys("items").Index(0),
+		"negative index":   Keys("items").Index(-1),
+		"invalid path":     Path{invalid: true},
+		"configkit":        Keys("configkit"),
+		"extends":          Keys("extends"),
+		"nested configkit": Keys("outer", "configkit"),
+		"nested extends":   Keys("outer", "extends"),
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			if _, err := normalizeDestination(path); !errors.Is(err, errInvalidDestination) {
+				t.Fatalf("normalizeDestination(%q) error = %v, want invalid destination", path.String(), err)
+			}
+		})
+	}
+
+	for _, key := range []string{"myextends", "configkit2", "", ".", "..", "../x", "a.b", "a/b", "~"} {
+		t.Run("ordinary key "+key, func(t *testing.T) {
+			t.Parallel()
+			if _, err := normalizeDestination(Keys(key)); err != nil {
+				t.Fatalf("normalizeDestination(%q): %v", key, err)
+			}
+		})
+	}
+}
+
+func TestPlaceDocumentWrapsEmptyMappingAndPreservesSourceCoordinates(t *testing.T) {
+	t.Parallel()
+
+	rootLocation := document.Location{SourceID: "source-a", Line: 1, Column: 1}
+	body := &document.Node{
+		Kind:     document.Mapping,
+		Location: document.Location{SourceID: "source-a", Line: 4, Column: 1},
+	}
+	path, placed, err := placeDocument(body, Keys("a", "b"), rootLocation, 2, 5)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := path.String(), "/a/b"; got != want {
+		t.Fatalf("normalized destination = %q, want %q", got, want)
+	}
+	if placed == body || len(placed.Mapping) != 1 || placed.Mapping[0].Key != "a" {
+		t.Fatalf("outer destination mapping was not created: %#v", placed)
+	}
+	inner := placed.Mapping[0].Value
+	if inner.Kind != document.Mapping || len(inner.Mapping) != 1 || inner.Mapping[0].Key != "b" {
+		t.Fatalf("inner destination mapping was not created: %#v", inner)
+	}
+	if inner.Mapping[0].Value != body || len(body.Mapping) != 0 {
+		t.Fatal("placement mutated or copied the authored body")
+	}
+	if placed.Location != rootLocation || inner.Location != rootLocation || placed.Mapping[0].KeyLocation != rootLocation || placed.Mapping[0].ValueLocation != rootLocation {
+		t.Fatal("introduced structure did not use the source-root coordinate fallback")
+	}
+	if body.Location != (document.Location{SourceID: "source-a", Line: 4, Column: 1}) {
+		t.Fatalf("authored body location changed: %#v", body.Location)
+	}
+	if got, want := countInputNodes(placed), countInputNodes(body)+4; got != want {
+		t.Fatalf("placed node count = %d, want body count plus four synthetic nodes (%d)", got, want)
+	}
+}
+
+func TestPlaceDocumentPreservesAuthoredEntryCoordinates(t *testing.T) {
+	t.Parallel()
+
+	rootLocation := document.Location{SourceID: "source-b", Line: 1, Column: 1}
+	bodyLocation := document.Location{SourceID: "source-b", Line: 3, Column: 1}
+	keyLocation := document.Location{SourceID: "source-b", Line: 3, Column: 3}
+	valueLocation := document.Location{SourceID: "source-b", Line: 4, Column: 5}
+	value := &document.Node{Kind: document.String, Location: valueLocation, Text: "value"}
+	body := &document.Node{
+		Kind:     document.Mapping,
+		Location: bodyLocation,
+		Mapping: []document.Entry{{
+			Key:           "leaf",
+			KeyLocation:   keyLocation,
+			ValueLocation: valueLocation,
+			Value:         value,
+		}},
+	}
+	_, placed, err := placeDocument(body, Keys("mounted"), rootLocation, 2, 5)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if placed.Mapping[0].KeyLocation != rootLocation || placed.Mapping[0].ValueLocation != rootLocation {
+		t.Fatal("introduced destination entry did not use the source-root fallback")
+	}
+	if placed.Mapping[0].Value != body || body.Location != bodyLocation {
+		t.Fatal("placement changed the authored body node or location")
+	}
+	entry := body.Mapping[0]
+	if entry.KeyLocation != keyLocation || entry.ValueLocation != valueLocation || entry.Value != value || value.Location != valueLocation {
+		t.Fatal("placement changed authored entry or value coordinates")
+	}
+}
+
+func TestPlaceDocumentRootAndBounds(t *testing.T) {
+	t.Parallel()
+
+	body := &document.Node{
+		Kind: document.Mapping,
+		Mapping: []document.Entry{{
+			Key:   "leaf",
+			Value: &document.Node{Kind: document.String, Text: "value"},
+		}},
+	}
+	path, placed, err := placeDocument(body, Path{}, body.Location, 1, 3)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if path.String() != "" || placed != body {
+		t.Fatal("root placement must preserve the existing body")
+	}
+
+	destination := Keys("a", "b")
+	if !destinationWithinLimits(body, destination, 3, 7) {
+		t.Fatal("exact depth and node limits should admit the placed document")
+	}
+	if destinationWithinLimits(body, destination, 2, 7) {
+		t.Fatal("placement exceeding the depth limit was accepted")
+	}
+	if destinationWithinLimits(body, destination, 3, 6) {
+		t.Fatal("placement exceeding the node limit was accepted")
+	}
+	_, placed, err = placeDocument(body, destination, body.Location, 3, 6)
+	if !errors.Is(err, errDestinationLimit) || placed != nil {
+		t.Fatalf("bounded placement returned node %p and error %v", placed, err)
+	}
+	_, absent, err := placeDocument(nil, destination, document.Location{}, 0, 0)
+	if err != nil || absent != nil {
+		t.Fatalf("absent body should not consume placement limits: node %p, error %v", absent, err)
+	}
+}

internal/engine/destination_tiers_test.go

@@ -0,0 +1,347 @@
+package engine
+
+import (
+	"path/filepath"
+	"reflect"
+	"testing"
+	"testing/fstest"
+)
+
+func TestLayeredDestinationStrictCompositionBoundaries(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		inputs   []Input
+		wantPath []string
+		want     any
+		alsoPath []string
+		alsoWant any
+	}{
+		{
+			name: "disjoint peers at one destination",
+			inputs: []Input{
+				{Source: Bytes("left", []byte("left: one\n")), Destination: Keys("application")},
+				{Source: Bytes("right", []byte("right: two\n")), Destination: Keys("application")},
+			},
+			wantPath: []string{"application", "right"},
+			want:     "two",
+			alsoPath: []string{"application", "left"},
+			alsoWant: "one",
+		},
+		{
+			name: "different destinations",
+			inputs: []Input{
+				{Source: Bytes("left", []byte("value: shared\n")), Destination: Keys("left")},
+				{Source: Bytes("right", []byte("value: shared\n")), Destination: Keys("right")},
+			},
+			wantPath: []string{"right", "value"},
+			want:     "shared",
+		},
+		{
+			name: "ancestor and descendant destinations",
+			inputs: []Input{
+				{Source: Bytes("parent", []byte("branch: {base: one}\n")), Destination: Keys("application")},
+				{Source: Bytes("child", []byte("leaf: two\n")), Destination: Keys("application", "branch")},
+			},
+			wantPath: []string{"application", "branch", "leaf"},
+			want:     "two",
+			alsoPath: []string{"application", "branch", "base"},
+			alsoWant: "one",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			loaded, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), LayeredOptions[map[string]any]{
+				Underlays: []Underlay{{Inputs: test.inputs}},
+			})
+			if err != nil {
+				t.Fatalf("LoadLayered: %v", err)
+			}
+			result := testLoaded(loaded)
+			if got := layeredMapValue(t, result.Config, test.wantPath...); got != test.want {
+				t.Fatalf("value at %v = %#v, want %#v", test.wantPath, got, test.want)
+			}
+			if len(test.alsoPath) != 0 {
+				if got := layeredMapValue(t, result.Config, test.alsoPath...); got != test.alsoWant {
+					t.Fatalf("value at %v = %#v, want %#v", test.alsoPath, got, test.alsoWant)
+				}
+			}
+		})
+	}
+
+	for _, test := range []struct {
+		name  string
+		left  string
+		right string
+	}{
+		{name: "equal values conflict", left: "value: same\n", right: "value: same\n"},
+		{name: "unequal values conflict", left: "value: left\n", right: "value: right\n"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			inputs := []Input{
+				{Source: Bytes("left", []byte(test.left)), Destination: Keys("application")},
+				{Source: Bytes("right", []byte(test.right)), Destination: Keys("application")},
+			}
+			loaded, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), LayeredOptions[map[string]any]{
+				Underlays: []Underlay{{Inputs: inputs}},
+			})
+			if loaded != nil || groupedErrorCode(t, err) != CodeComposeConflict {
+				t.Fatalf("strict composition result = %#v, %v", loaded, err)
+			}
+		})
+	}
+
+	t.Run("strict conflict is not hidden by an upper override", func(t *testing.T) {
+		inputs := []Input{
+			{Source: Bytes("left", []byte("value: left\n")), Destination: Keys("application")},
+			{Source: Bytes("right", []byte("value: right\n")), Destination: Keys("application")},
+		}
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("value: upper\n")), Destination: Keys("application")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: inputs}}},
+		)
+		if loaded != nil || groupedErrorCode(t, err) != CodeComposeConflict {
+			t.Fatalf("strict composition result = %#v, %v", loaded, err)
+		}
+	})
+}
+
+func TestLayeredDestinationMatchesExplicitlyWrappedInput(t *testing.T) {
+	t.Parallel()
+
+	load := func(input Input) map[string]any {
+		t.Helper()
+		loaded, err := LoadLayered(Input{Source: NoSource()}, testSecretBinding(), LayeredOptions[map[string]any]{
+			Underlays: []Underlay{{Inputs: []Input{input}}},
+		})
+		if err != nil {
+			t.Fatalf("LoadLayered: %v", err)
+		}
+		return testLoaded(loaded).Config
+	}
+
+	placed := load(Input{Source: Bytes("placed", []byte("field: value\n")), Destination: Keys("application")})
+	authored := load(Input{Source: Bytes("authored", []byte("application: {field: value}\n"))})
+	if !reflect.DeepEqual(placed, authored) {
+		t.Fatalf("placed configuration %#v differs from explicitly wrapped %#v", placed, authored)
+	}
+}
+
+func TestEmptyMountedMappingRetainsLowerChildren(t *testing.T) {
+	t.Parallel()
+
+	destination := Keys("collections", "weather")
+	loaded, err := LoadLayered(
+		Input{Source: Bytes("empty-main", []byte("{}\n")), Destination: destination},
+		testSecretBinding(),
+		LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: Bytes("lower", []byte("prompt: {title: retained}\n")), Destination: destination,
+		}}}}},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	if got := layeredMapValue(t, testLoaded(loaded).Config, "collections", "weather", "prompt", "title"); got != "retained" {
+		t.Fatalf("lower child beneath empty mounted mapping = %#v, want retained", got)
+	}
+}
+
+func TestLayeredDestinationOverlayRetainsOrdinaryValueSemantics(t *testing.T) {
+	t.Parallel()
+
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"overlays/test.yml": &fstest.MapFile{Data: []byte("settings:\n  metadata:\n    version: overlay\n")},
+	})
+	loaded, err := LoadLayered(
+		Input{Source: Bytes("main", []byte("mapping: {shared: main}\nsequence: [9]\ncleared: null\nenabled: false\nmetadata: {version: main}\n")), Destination: Keys("settings")},
+		testSecretBinding(),
+		LayeredOptions[map[string]any]{
+			Underlays: []Underlay{{Inputs: []Input{{
+				Source:      Bytes("underlay", []byte("mapping: {retained: lower, shared: lower}\nsequence: [1, 2]\ncleared: {old: true}\nenabled: true\ncount: 7\nmetadata: {version: lower, label: retained}\n")),
+				Destination: Keys("settings"),
+			}}}},
+			Overlay:           "test",
+			OverlayFilesystem: &FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem},
+			Overrides:         []Override{{Path: Keys("settings", "count"), Value: 0, Flag: "--count"}},
+		},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	settings := layeredMap(t, result.Config, "settings")
+	mapping := layeredMap(t, settings, "mapping")
+	if mapping["retained"] != "lower" || mapping["shared"] != "main" {
+		t.Fatalf("recursive mapping overlay = %#v", mapping)
+	}
+	sequence, ok := settings["sequence"].([]any)
+	if !ok || len(sequence) != 1 || sequence[0] != int64(9) {
+		t.Fatalf("sequence overlay = %#v", settings["sequence"])
+	}
+	if value, present := settings["cleared"]; !present || value != nil {
+		t.Fatalf("explicit null = %#v, present %v", value, present)
+	}
+	if value, ok := settings["enabled"].(bool); !ok || value {
+		t.Fatalf("explicit false = %#v", settings["enabled"])
+	}
+	if value, ok := settings["count"].(int64); !ok || value != 0 {
+		t.Fatalf("explicit zero override = %#v", settings["count"])
+	}
+	metadata := layeredMap(t, settings, "metadata")
+	if metadata["version"] != "overlay" || metadata["label"] != "retained" {
+		t.Fatalf("version-only overlay = %#v", metadata)
+	}
+	if _, present := settings["settings"]; present {
+		t.Fatalf("upper tiers were incorrectly nested under the main destination: %#v", result.Config)
+	}
+}
+
+func TestLayeredDestinationChecksVersionOnceOnComposedMain(t *testing.T) {
+	t.Parallel()
+
+	type appConfig struct {
+		Version  int8   `yaml:"version"`
+		Count    int    `yaml:"count"`
+		Main     string `yaml:"main"`
+		Imported string `yaml:"imported"`
+		Lower    string `yaml:"lower"`
+	}
+	type config struct {
+		App appConfig `yaml:"app"`
+	}
+	filesystem := NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":     &fstest.MapFile{Data: []byte("configkit: {imports: [{path: fragment.yml}]}\nversion: 6\ncount: 4\nmain: main\n")},
+		"fragment.yml": &fstest.MapFile{Data: []byte("imported: included\n")},
+	})
+	checks := 0
+	loaded, err := LoadLayered(
+		Input{Source: filesystem.File("main.yml"), Imports: true, Destination: Keys("app")},
+		testSecretBinding(),
+		LayeredOptions[config]{
+			Definition: Definition[config]{Version: &VersionRule{Path: Keys("app", "version"), Check: func(version Version, _ *Validation) error {
+				checks++
+				if !version.Present || version.Text != "6" {
+					t.Errorf("nested main version = %#v", version)
+				}
+				return nil
+			}}},
+			Underlays: []Underlay{{Inputs: []Input{{
+				Source: Bytes("lower", []byte("lower: underlay\n")), Destination: Keys("app"),
+			}}}},
+		},
+	)
+	if err != nil {
+		t.Fatalf("LoadLayered: %v", err)
+	}
+	result := testLoaded(loaded)
+	if checks != 1 {
+		t.Fatalf("version hook calls = %d, want 1", checks)
+	}
+	if result.Config.App.Version != 6 || result.Config.App.Count != 4 || result.Config.App.Main != "main" || result.Config.App.Imported != "included" || result.Config.App.Lower != "underlay" {
+		t.Fatalf("nested app = %#v", result.Config.App)
+	}
+}
+
+func TestLayeredDestinationVersionClaimsUsePlacedApplicationPaths(t *testing.T) {
+	t.Parallel()
+
+	rule := &VersionRule{Path: Keys("app", "schema", "version"), Check: func(Version, *Validation) error { return nil }}
+	for _, test := range []struct {
+		name      string
+		body      string
+		wantCode  Code
+		wantValue any
+	}{
+		{name: "underlay direct claim", body: "schema: {version: v2}\n", wantCode: CodeUnderlayVersion},
+		{name: "underlay ancestor claim", body: "schema: null\n", wantCode: CodeUnderlayVersion},
+		{name: "underlay mapping ancestor without claim", body: "schema: {name: ordinary}\n", wantValue: "ordinary"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			loaded, err := LoadLayered(
+				Input{Source: NoSource()},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: rule},
+					Underlays: []Underlay{{Inputs: []Input{{
+						Source: Bytes("underlay", []byte(test.body)), Destination: Keys("app"),
+					}}}},
+				},
+			)
+			if test.wantCode != "" {
+				if loaded != nil || groupedErrorCode(t, err) != test.wantCode {
+					t.Fatalf("version claim result = %#v, %v", loaded, err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("nonclaiming mapping ancestor was rejected: %v", err)
+			}
+			result := testLoaded(loaded)
+			if got := layeredMapValue(t, result.Config, "app", "schema", "name"); got != test.wantValue {
+				t.Fatalf("nested ordinary value = %#v", got)
+			}
+		})
+	}
+
+	for _, test := range []struct {
+		name     string
+		body     string
+		wantCode Code
+	}{
+		{name: "imported direct claim", body: "schema: {version: v2}\n", wantCode: CodeImportVersion},
+		{name: "imported ancestor claim", body: "schema: null\n", wantCode: CodeImportVersion},
+		{name: "imported mapping ancestor without claim", body: "schema: {name: ordinary}\n"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			directory := t.TempDir()
+			writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml}]}\n")
+			writeImportFile(t, directory, "fragment.yml", test.body)
+			loaded, err := LoadLayered(
+				Input{Source: NoSource()},
+				testSecretBinding(),
+				LayeredOptions[map[string]any]{
+					Definition: Definition[map[string]any]{Version: rule},
+					Underlays: []Underlay{{Inputs: []Input{{
+						Source: File(filepath.Join(directory, "main.yml")), Imports: true, Destination: Keys("app"),
+					}}}},
+				},
+			)
+			if test.wantCode != "" {
+				if loaded != nil || groupedErrorCode(t, err) != test.wantCode {
+					t.Fatalf("version claim result = %#v, %v", loaded, err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("nonclaiming imported mapping ancestor was rejected: %v", err)
+			}
+			result := testLoaded(loaded)
+			if got := layeredMapValue(t, result.Config, "app", "schema", "name"); got != "ordinary" {
+				t.Fatalf("nested ordinary value = %#v", got)
+			}
+		})
+	}
+}
+
+func layeredMap(t *testing.T, value map[string]any, key string) map[string]any {
+	t.Helper()
+	result, ok := value[key].(map[string]any)
+	if !ok {
+		t.Fatalf("%q = %#v, want mapping", key, value[key])
+	}
+	return result
+}
+
+func layeredMapValue(t *testing.T, value map[string]any, keys ...string) any {
+	t.Helper()
+	for _, key := range keys[:len(keys)-1] {
+		value = layeredMap(t, value, key)
+	}
+	return value[keys[len(keys)-1]]
+}

internal/engine/diagnostic.go

@@ -85,15 +85,28 @@ func canonicalDiagnostics(diagnostics []Diagnostic) []Diagnostic {
 	for i := range canonical {
 		slices.SortFunc(canonical[i].Related, compareLocation)
 	}
-	slices.SortFunc(canonical, compareDiagnostic)
+	type orderedDiagnostic struct {
+		diagnostic Diagnostic
+		pointer    string
+	}
+	ordered := make([]orderedDiagnostic, len(canonical))
+	for i, diagnostic := range canonical {
+		ordered[i] = orderedDiagnostic{diagnostic: diagnostic, pointer: diagnostic.Path.String()}
+	}
+	slices.SortFunc(ordered, func(left, right orderedDiagnostic) int {
+		return compareDiagnosticWithPaths(left.diagnostic, right.diagnostic, left.pointer, right.pointer)
+	})
+	for i, item := range ordered {
+		canonical[i] = item.diagnostic
+	}
 	return canonical
 }
 
-func compareDiagnostic(left, right Diagnostic) int {
+func compareDiagnosticWithPaths(left, right Diagnostic, leftPath, rightPath string) int {
 	if order := compareOptionalLocation(left.Primary, right.Primary); order != 0 {
 		return order
 	}
-	if order := cmp.Compare(left.Path.String(), right.Path.String()); order != 0 {
+	if order := cmp.Compare(leftPath, rightPath); order != 0 {
 		return order
 	}
 	if order := cmp.Compare(left.Code, right.Code); order != 0 {

internal/engine/diagnostic_test.go

@@ -90,6 +90,26 @@ func TestDiagnosticsAreCanonicalAndDetached(t *testing.T) {
 	}
 }
 
+func TestCanonicalDiagnosticsSortEscapedPaths(t *testing.T) {
+	primary := &Location{SourceID: "source-1", Line: 1, Column: 1}
+	diagnostics := []Diagnostic{
+		{Code: "same", Severity: SeverityError, Path: Keys("a/b"), Message: "same", Primary: primary},
+		{Code: "same", Severity: SeverityError, Path: Keys("a~b"), Message: "same", Primary: primary},
+		{Code: "z", Severity: SeverityError, Path: Keys("same"), Message: "same", Primary: primary},
+		{Code: "a", Severity: SeverityError, Path: Keys("same"), Message: "same", Primary: primary},
+	}
+	canonical := canonicalDiagnostics(diagnostics)
+	want := []string{"/a~0b", "/a~1b", "/same", "/same"}
+	for i, diagnostic := range canonical {
+		if got := diagnostic.Path.String(); got != want[i] {
+			t.Fatalf("diagnostic[%d] path = %q, want %q", i, got, want[i])
+		}
+	}
+	if canonical[2].Code != "a" || canonical[3].Code != "z" {
+		t.Fatalf("same-path code order = %q, %q", canonical[2].Code, canonical[3].Code)
+	}
+}
+
 func TestWriteDiagnosticsEscapesControls(t *testing.T) {
 	t.Parallel()
 

internal/engine/diff.go

@@ -0,0 +1,98 @@
+package engine
+
+import (
+	"bytes"
+	"errors"
+	"io"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+var errDiffInspectionUnavailable = errors.New("configkit: diff inspection unavailable")
+var errDiffSnapshotUnavailable = errors.New("configkit: diff snapshot unavailable")
+
+// Difference describes one canonical changed region with safe snapshots.
+type Difference struct {
+	Path          Path
+	Left          ValueSnapshot
+	Right         ValueSnapshot
+	HiddenChanged bool
+}
+
+type diffRegion struct {
+	path  Path
+	left  *document.Node
+	right *document.Node
+}
+
+func prepareDiffRegions(left, right *Inspection) ([]diffRegion, error) {
+	if left == nil || left.final == nil || right == nil || right.final == nil {
+		return nil, errDiffInspectionUnavailable
+	}
+
+	deltas := compareConfigurationValues(left.final, right.final)
+	regions := make([]diffRegion, 0, len(deltas))
+	for _, delta := range deltas {
+		leftNode, leftPresent := documentAtPathWithLookup(left.final, delta.path, left.immutableLookup)
+		if !leftPresent {
+			leftNode = nil
+		}
+		rightNode, rightPresent := documentAtPathWithLookup(right.final, delta.path, right.immutableLookup)
+		if !rightPresent {
+			rightNode = nil
+		}
+		regions = append(regions, diffRegion{path: delta.path, left: leftNode, right: rightNode})
+	}
+	return regions, nil
+}
+
+// Diff compares the final frozen values retained by two successful inspections.
+// Values are serialized only after applying the combined sensitivity of both loads.
+func Diff(left, right *Inspection) ([]Difference, error) {
+	regions, err := prepareDiffRegions(left, right)
+	if err != nil {
+		return nil, err
+	}
+	sensitivity := newDiffSensitivity(left.secrets, right.secrets)
+	differences := make([]Difference, 0, len(regions))
+	for _, region := range regions {
+		paired, supported := preparePairedDiffSnapshot(region.left, region.right, region.path, sensitivity)
+		if !supported {
+			return nil, errDiffSnapshotUnavailable
+		}
+		leftSnapshot, err := serializeDiffSnapshot(paired.left, paired.leftRedacted)
+		if err != nil {
+			return nil, errDiffSnapshotUnavailable
+		}
+		rightSnapshot, err := serializeDiffSnapshot(paired.right, paired.rightRedacted)
+		if err != nil {
+			return nil, errDiffSnapshotUnavailable
+		}
+		differences = append(differences, Difference{
+			Path:          region.path,
+			Left:          leftSnapshot,
+			Right:         rightSnapshot,
+			HiddenChanged: paired.hiddenChanged,
+		})
+	}
+	return differences, nil
+}
+
+func serializeDiffSnapshot(node *document.Node, redacted bool) (ValueSnapshot, error) {
+	if node == nil {
+		return ValueSnapshot{}, nil
+	}
+	var output bytes.Buffer
+	if err := writeDiffDocument(&output, node); err != nil {
+		return ValueSnapshot{}, errDiffSnapshotUnavailable
+	}
+	return ValueSnapshot{Present: true, Redacted: redacted, YAML: output.String()}, nil
+}
+
+func writeDiffDocument(output io.Writer, node *document.Node) error {
+	if err := yamladapter.Write(output, node); err != nil {
+		return errDiffSnapshotUnavailable
+	}
+	return nil
+}

internal/engine/diff_hidden_test.go

@@ -0,0 +1,130 @@
+package engine
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestPreparePairedDiffSnapshotReportsWhollyMaskedChanges(t *testing.T) {
+	left := &document.Node{Kind: document.String, Text: "private-before"}
+	right := &document.Node{Kind: document.String, Text: "private-after"}
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Keys("credential"), newDiffSensitivity(nil, map[string]bool{"/credential": true}),
+	)
+	if !supported || !paired.hiddenChanged || !paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("masked change = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, paired.left, "[REDACTED]")
+	assertDiffStringNode(t, paired.right, "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotDetectsRawChangesBehindIdenticalMarkers(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "token: old-canary\n")
+	right := parseCompositionDocument(t, "right", "token: new-canary\n")
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "token"), mappingValue(t, right, "token"), Keys("token"),
+		newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.hiddenChanged || paired.left.Text != paired.right.Text {
+		t.Fatalf("identically rendered masked values lost raw change: %#v, supported=%v", paired, supported)
+	}
+}
+
+func TestPreparePairedDiffSnapshotCombinesPublicAndHiddenChanges(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "public: before\ntoken: old\n")
+	right := parseCompositionDocument(t, "right", "public: after\ntoken: new\n")
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Path{}, newDiffSensitivity(nil, map[string]bool{"/token": true}),
+	)
+	if !supported || !paired.hiddenChanged {
+		t.Fatalf("mixed public and hidden change = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.left, "public"), "before")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "public"), "after")
+	assertDiffStringNode(t, mappingValue(t, paired.left, "token"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "token"), "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotDoesNotFlagUnchangedMaskedValues(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "public: before\ntoken: unchanged\n")
+	right := parseCompositionDocument(t, "right", "public: after\ntoken: unchanged\n")
+	regions, err := prepareDiffRegions(diffTestInspection(left), diffTestInspection(right))
+	if err != nil || len(regions) != 1 || regions[0].path.String() != "/public" {
+		t.Fatalf("changed regions = %#v, err = %v; want only /public", regions, err)
+	}
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Path{}, newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.leftRedacted || !paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("unchanged masked values affected hidden-change state: %#v, supported=%v", paired, supported)
+	}
+
+	leftSequence := parseCompositionDocument(t, "left-sequence", "items: [unchanged, before]\n")
+	rightSequence := parseCompositionDocument(t, "right-sequence", "items: [unchanged, after]\n")
+	sequence, supported := preparePairedDiffSnapshot(
+		mappingValue(t, leftSequence, "items"), mappingValue(t, rightSequence, "items"), Keys("items"),
+		newDiffSensitivity(nil, map[string]bool{"/items/0": true}),
+	)
+	if !supported || !sequence.leftRedacted || !sequence.rightRedacted || sequence.hiddenChanged {
+		t.Fatalf("unchanged masked sequence member affected hidden-change state: %#v, supported=%v", sequence, supported)
+	}
+}
+
+func TestPreparePairedDiffSnapshotReportsMaskedAdditionAndRemoval(t *testing.T) {
+	value := &document.Node{Kind: document.String, Text: "private"}
+	tests := []struct {
+		name        string
+		left, right *document.Node
+		leftHidden  bool
+		rightHidden bool
+	}{
+		{name: "addition", right: value, rightHidden: true},
+		{name: "removal", left: value, leftHidden: true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			paired, supported := preparePairedDiffSnapshot(
+				test.left, test.right, Keys("token"), newDiffSensitivity(nil, map[string]bool{"/token": true}),
+			)
+			if !supported || !paired.hiddenChanged || paired.leftRedacted != test.leftHidden || paired.rightRedacted != test.rightHidden {
+				t.Fatalf("masked presence change = %#v, supported=%v", paired, supported)
+			}
+			if test.left == nil && paired.left != nil || test.right == nil && paired.right != nil {
+				t.Fatalf("absence became a value: %#v", paired)
+			}
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotReportsHiddenSequenceLengthChange(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "items: [public]\n")
+	right := parseCompositionDocument(t, "right", "items: [public, private]\n")
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "items"), mappingValue(t, right, "items"), Keys("items"),
+		newDiffSensitivity(nil, map[string]bool{"/items/1": true}),
+	)
+	if !supported || !paired.hiddenChanged || paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("masked sequence length change = %#v, supported=%v", paired, supported)
+	}
+	if len(paired.left.Sequence) != 1 || len(paired.right.Sequence) != 2 {
+		t.Fatalf("safe sequence lengths = %d, %d; want 1, 2", len(paired.left.Sequence), len(paired.right.Sequence))
+	}
+	assertDiffStringNode(t, paired.right.Sequence[1], "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotClassificationAloneDoesNotCreateHiddenChange(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "token: same\n")
+	right := parseCompositionDocument(t, "right", "token: same\n")
+	regions, err := prepareDiffRegions(diffTestInspection(left), diffTestInspection(right))
+	if err != nil || len(regions) != 0 {
+		t.Fatalf("classification-independent changed regions = %#v, err = %v", regions, err)
+	}
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "token"), mappingValue(t, right, "token"), Keys("token"),
+		newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.leftRedacted || !paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("classification alone reported hidden change: %#v, supported=%v", paired, supported)
+	}
+}

internal/engine/diff_operation_test.go

@@ -0,0 +1,282 @@
+package engine
+
+import (
+	"bytes"
+	"errors"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type diffIntegrationConfig struct {
+	Defaulted  string         `yaml:"defaulted"`
+	Dynamic    map[string]any `yaml:"dynamic"`
+	Credential Secret         `yaml:"credential" configkit:"secret"`
+	Derived    string         `yaml:"derived"`
+}
+
+type diffOptionalSecretConfig struct {
+	Optional *Secret `yaml:"optional" configkit:"secret"`
+}
+
+func TestDiffUsesFinalValuesAndDependencyDerivedSensitivity(t *testing.T) {
+	var resolveCalls int
+	options := Options[diffIntegrationConfig]{Definition: Definition[diffIntegrationConfig]{
+		Defaults: func() diffIntegrationConfig {
+			return diffIntegrationConfig{Defaulted: "from-default", Dynamic: map[string]any{"defaulted": "retained"}}
+		},
+		Resolve: func(configuration *diffIntegrationConfig, resolution *Resolution) error {
+			resolveCalls++
+			configuration.Derived = configuration.Dynamic["visible"].(string)
+			resolution.Record(Keys("derived"), ChangeNormalization, "derive from effective values", Keys("credential"))
+			return nil
+		},
+	}}
+	input := func(value string) Source {
+		return Bytes("effective.yml", []byte("dynamic: {visible: "+value+"}\ncredential: DIFF_ENGINE_SECRET\n"))
+	}
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-left-canary")
+	left, err := testLoad(input("before"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-right-canary")
+	right, err := testLoad(input("after"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if left.Config.Defaulted != "from-default" || right.Config.Defaulted != "from-default" {
+		t.Fatalf("defaulted values = %q, %q", left.Config.Defaulted, right.Config.Defaulted)
+	}
+	if !left.Inspection.secrets["/credential"] || !left.Inspection.secrets["/derived"] ||
+		!right.Inspection.secrets["/credential"] || !right.Inspection.secrets["/derived"] {
+		t.Fatalf("typed or dependency-derived sensitivity was not retained: left=%#v right=%#v", left.Inspection.secrets, right.Inspection.secrets)
+	}
+	if node, present := documentAtPathWithLookup(left.Inspection.final, Keys("derived"), left.Inspection.immutableLookup); !present || node.Text != "before" {
+		t.Fatalf("left frozen derived value = %#v, present=%v", node, present)
+	}
+
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 2 || differences[0].Path.String() != "/derived" || differences[1].Path.String() != "/dynamic/visible" {
+		t.Fatalf("differences = %#v", differences)
+	}
+	if !differences[0].Left.Redacted || !differences[0].Right.Redacted || !differences[0].HiddenChanged ||
+		differences[0].Left.YAML != "'[REDACTED]'\n" || differences[0].Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("dependency-derived difference = %#v", differences[0])
+	}
+	if differences[1].Left.Redacted || differences[1].Right.Redacted || differences[1].HiddenChanged ||
+		differences[1].Left.YAML != "before\n" || differences[1].Right.YAML != "after\n" {
+		t.Fatalf("public dynamic difference = %#v", differences[1])
+	}
+	for _, value := range differences {
+		if strings.Contains(value.Left.YAML+value.Right.YAML, "credential-") {
+			t.Fatal("diff snapshot exposed a resolved credential")
+		}
+	}
+
+	var effective bytes.Buffer
+	if err := left.Inspection.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String(), "credential-left-canary") {
+		t.Fatal("standalone effective rendering exposed a credential after diff")
+	}
+	explanation, ok := left.Inspection.Explain(Keys("derived"))
+	if !ok || !explanation.Effective.Redacted || explanation.Effective.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("explanation changed after diff: %#v, ok=%v", explanation, ok)
+	}
+	if _, ok := left.Inspection.Metadata().Lookup(Keys("dynamic", "visible")); !ok {
+		t.Fatal("metadata lookup failed after diff")
+	}
+	if resolveCalls != 2 {
+		t.Fatalf("resolution callbacks ran during diff: %d calls", resolveCalls)
+	}
+
+	// Returned records can be modified without affecting a later comparison.
+	differences[0].Path.segments[0].key = "changed-by-caller"
+	differences[0].Left.YAML = "changed-by-caller"
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-after-load-canary")
+	repeated, err := Diff(left.Inspection, right.Inspection)
+	if err != nil || len(repeated) != 2 || repeated[0].Path.String() != "/derived" || repeated[0].Left.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("repeated diff = %#v, err = %v", repeated, err)
+	}
+	if resolveCalls != 2 {
+		t.Fatalf("repeated diff reran resolution: %d calls", resolveCalls)
+	}
+
+	independent, err := testLoad(input("before"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resolveCalls != 3 {
+		t.Fatalf("independent load did not retain normal resolution behavior: %d calls", resolveCalls)
+	}
+	if _, err := Diff(left.Inspection, independent.Inspection); err != nil {
+		t.Fatalf("independent load comparison = %v", err)
+	}
+}
+
+func TestDiffUsesReferenceNamesAndFrozenSecretState(t *testing.T) {
+	loadReference := func(name string) *testResult[diffOptionalSecretConfig] {
+		t.Helper()
+		result, err := testLoad[diffOptionalSecretConfig](Bytes("secret-reference.yml", []byte("optional: "+name+"\n")))
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+	t.Setenv("DIFF_ROTATING_SECRET", "first-credential-canary")
+	left := loadReference("DIFF_ROTATING_SECRET")
+	t.Setenv("DIFF_ROTATING_SECRET", "second-credential-canary")
+	right := loadReference("DIFF_ROTATING_SECRET")
+	t.Setenv("DIFF_ROTATING_SECRET", "changed-after-load-canary")
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil || len(differences) != 0 {
+		t.Fatalf("credential rotation behind one reference = %#v, err = %v", differences, err)
+	}
+	if node, present := documentAtPathWithLookup(left.Inspection.final, Keys("optional"), left.Inspection.immutableLookup); !present || node.Text != "DIFF_ROTATING_SECRET" {
+		t.Fatalf("frozen reference = %#v, present=%v", node, present)
+	}
+	if node, present := documentAtPathWithLookup(right.Inspection.final, Keys("optional"), right.Inspection.immutableLookup); !present || node.Text != "DIFF_ROTATING_SECRET" {
+		t.Fatalf("right frozen reference = %#v, present=%v", node, present)
+	}
+
+	t.Setenv("DIFF_OTHER_SECRET", "other-credential-canary")
+	other := loadReference("DIFF_OTHER_SECRET")
+	differences, err = Diff(left.Inspection, other.Inspection)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/optional" {
+		t.Fatalf("reference-name change = %#v, err = %v", differences, err)
+	}
+	if !differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Left.Redacted ||
+		!differences[0].Right.Redacted || !differences[0].HiddenChanged ||
+		differences[0].Left.YAML != "'[REDACTED]'\n" || differences[0].Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("classified reference-name diff = %#v", differences[0])
+	}
+
+	absent, err := testLoad[diffOptionalSecretConfig](Bytes("absent-secret.yml", []byte("{}\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if absent.Config.Optional != nil {
+		t.Fatalf("missing optional reference decoded as %#v", absent.Config.Optional)
+	}
+	if node, present := documentAtPathWithLookup(absent.Inspection.final, Keys("optional"), absent.Inspection.immutableLookup); !present || node.Kind != document.Null {
+		t.Fatalf("optional nil projection = %#v, present=%v", node, present)
+	}
+	differences, err = Diff(absent.Inspection, left.Inspection)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/optional" {
+		t.Fatalf("optional absence diff = %#v, err = %v", differences, err)
+	}
+	if !differences[0].Left.Present || !differences[0].Left.Redacted ||
+		!differences[0].Right.Present || !differences[0].Right.Redacted || !differences[0].HiddenChanged {
+		t.Fatalf("optional reference change snapshots = %#v", differences[0])
+	}
+	for _, value := range differences {
+		if strings.Contains(value.Left.YAML+value.Right.YAML, "credential-canary") {
+			t.Fatal("reference diff exposed a resolved credential")
+		}
+	}
+}
+
+func TestDiffProtectsHistoricalSensitiveRemovalAndReplacement(t *testing.T) {
+	type configuration struct {
+		Dynamic map[string]any `yaml:"dynamic"`
+	}
+	secretPath := Keys("dynamic", "token")
+	secretSelector := func(path Path) bool { return path.String() == secretPath.String() }
+	load := func(input string, resolve func(*configuration, *Resolution) error) *testResult[configuration] {
+		t.Helper()
+		result, err := testLoad(Bytes("historical-secret.yml", []byte(input)), Options[configuration]{Definition: Definition[configuration]{
+			Secret:  secretSelector,
+			Resolve: resolve,
+		}})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+
+	t.Run("removed historical value", func(t *testing.T) {
+		left := load("dynamic: {token: removed-canary}\n", func(configuration *configuration, resolution *Resolution) error {
+			delete(configuration.Dynamic, "token")
+			resolution.Record(secretPath, ChangeNormalization, "remove token")
+			return nil
+		})
+		right := load("dynamic: {token: current-canary}\n", nil)
+		if !left.Inspection.secrets[secretPath.String()] {
+			t.Fatal("removed historical selector was not retained")
+		}
+		if _, present := documentAtPathWithLookup(left.Inspection.final, secretPath, left.Inspection.immutableLookup); present {
+			t.Fatal("removed token remains in the effective left tree")
+		}
+		differences, err := Diff(left.Inspection, right.Inspection)
+		if err != nil || len(differences) != 1 || differences[0].Path.String() != secretPath.String() {
+			t.Fatalf("historical removal diff = %#v, err = %v", differences, err)
+		}
+		if differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Right.Redacted || !differences[0].HiddenChanged {
+			t.Fatalf("historical removal snapshots = %#v", differences[0])
+		}
+		if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "canary") {
+			t.Fatal("historical removal diff exposed a sensitive value")
+		}
+	})
+
+	t.Run("replaced historical value", func(t *testing.T) {
+		left := load("dynamic: {token: original-canary}\n", func(configuration *configuration, resolution *Resolution) error {
+			configuration.Dynamic["token"] = "replacement-canary"
+			resolution.Record(secretPath, ChangeNormalization, "replace token")
+			return nil
+		})
+		right := load("dynamic: {token: other-canary}\n", nil)
+		if !left.Inspection.secrets[secretPath.String()] {
+			t.Fatal("replaced historical selector was not retained")
+		}
+		differences, err := Diff(left.Inspection, right.Inspection)
+		if err != nil || len(differences) != 1 || differences[0].Path.String() != secretPath.String() {
+			t.Fatalf("historical replacement diff = %#v, err = %v", differences, err)
+		}
+		if !differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Left.Redacted ||
+			!differences[0].Right.Redacted || !differences[0].HiddenChanged {
+			t.Fatalf("historical replacement snapshots = %#v", differences[0])
+		}
+		if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "canary") {
+			t.Fatal("historical replacement diff exposed a sensitive value")
+		}
+	})
+}
+
+func TestDiffReturnsSafeErrorsWithoutRecords(t *testing.T) {
+	valid := diffTestInspection(&document.Node{Kind: document.String, Text: "safe"})
+	for _, test := range []struct {
+		name        string
+		left, right *Inspection
+	}{
+		{name: "nil left", right: valid},
+		{name: "nil right", left: valid},
+		{name: "missing final tree", left: &Inspection{}, right: valid},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			differences, err := Diff(test.left, test.right)
+			if differences != nil || err == nil || err != errDiffInspectionUnavailable || strings.Contains(err.Error(), "safe") {
+				t.Fatalf("differences, err = %#v, %v", differences, err)
+			}
+		})
+	}
+}
+
+func TestDiffSerializationErrorDoesNotExposeWriterDetails(t *testing.T) {
+	err := writeDiffDocument(diffErrorWriter{}, &document.Node{Kind: document.String, Text: "safe"})
+	if err != errDiffSnapshotUnavailable || strings.Contains(err.Error(), "CANARY") {
+		t.Fatalf("serialization error = %v", err)
+	}
+}
+
+type diffErrorWriter struct{}
+
+func (diffErrorWriter) Write([]byte) (int, error) {
+	return 0, errors.New("CANARY_WRITER_ERROR")
+}