Prepare the ConfigKit v0.10.0 release

README.md

@@ -45,6 +45,11 @@ ConfigKit does not search conventional application paths implicitly. See the
 and [ADR 0015](docs/adr/0015-shared-dependency-ready-ordering.md) and
 [ADR 0016](docs/adr/0016-independent-import-controls.md).
 
+Main-file discovery is explicit too: applications choose and order `Search`
+descriptions and pass `SearchConfig` to the existing load APIs. See the
+[discovery contract](docs/config.md#configuration-discovery), [consumer guide](docs/consumers/README.md#explicit-main-file-discovery),
+and [ADR 0019](docs/adr/0019-explicit-configuration-discovery.md).
+
 ```go
 type Config struct {
     Enabled bool `yaml:"enabled"`

discovery.go

@@ -0,0 +1,90 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+
+// Search is an immutable ordered description of possible host-file inputs.
+// It is consumed only when passed to SearchConfig and used by a load.
+type Search struct{ value engine.Search }
+
+// FilenameGroup selects filenames in order for a directory search.
+// Construct groups with NamedFilenames, GenericFilenames, or CustomFilenames.
+type FilenameGroup struct{ value engine.FilenameGroup }
+
+// SearchOptions controls whether a search must select a file.
+type SearchOptions struct {
+	// Required reports a missing-source error if the complete search is exhausted.
+	Required bool
+}
+
+// DirectorySearchOptions selects the filename groups used by SearchDirectory.
+type DirectorySearchOptions struct {
+	// Filenames concatenates groups in order. A nil slice uses Named then Generic;
+	// a non-nil empty slice selects no filenames.
+	Filenames []FilenameGroup
+}
+
+// NamedFilenames selects <application>.yml followed by <application>.yaml.
+func NamedFilenames() FilenameGroup { return FilenameGroup{value: engine.NamedFilenames()} }
+
+// GenericFilenames selects config.yml, config.yaml, main.yml, and main.yaml.
+func GenericFilenames() FilenameGroup { return FilenameGroup{value: engine.GenericFilenames()} }
+
+// CustomFilenames selects literal filename components in the supplied order.
+// Names cannot be empty or contain a directory component.
+func CustomFilenames(names ...string) FilenameGroup {
+	return FilenameGroup{value: engine.CustomFilenames(names...)}
+}
+
+// SearchDirectory describes a search within directory. Omitted options or a nil
+// Filenames slice uses Named then Generic groups. Explicitly empty Filenames
+// selects no names and does not resolve the directory base.
+func SearchDirectory(directory string, supplied ...DirectorySearchOptions) Search {
+	options := make([]engine.DirectorySearchOptions, len(supplied))
+	for index, option := range supplied {
+		if option.Filenames == nil {
+			continue
+		}
+		options[index].Filenames = make([]engine.FilenameGroup, len(option.Filenames))
+		for groupIndex, group := range option.Filenames {
+			options[index].Filenames[groupIndex] = group.value
+		}
+	}
+	return Search{value: engine.SearchDirectory(directory, options...)}
+}
+
+// Paths describes literal host-file candidates in the supplied order.
+func Paths(paths ...string) Search { return Search{value: engine.Paths(paths...)} }
+
+// Searches concatenates descriptions in order and snapshots their entries.
+func Searches(searches ...Search) Search {
+	values := make([]engine.Search, len(searches))
+	for index, search := range searches {
+		values[index] = search.value
+	}
+	return Search{value: engine.Searches(values...)}
+}
+
+// UnixSystemDirectories describes the conventional Unix system directories.
+// Loading on an unsupported platform fails if this preset is reached.
+func UnixSystemDirectories() Search { return Search{value: engine.UnixSystemDirectories()} }
+
+// UserConfigDirectory describes searches rooted at the Go user-config directory.
+func UserConfigDirectory() Search { return Search{value: engine.UserConfigDirectory()} }
+
+// HomeDirectory describes searches rooted at the Go home directory.
+func HomeDirectory() Search { return Search{value: engine.HomeDirectory()} }
+
+// WorkingDirectory describes application-named files in the current directory.
+func WorkingDirectory() Search { return Search{value: engine.WorkingDirectory()} }
+
+// SearchConfig creates a source that selects the first matching host file when
+// consumed by Load or LoadLayered. Without Required, exhausting the search uses
+// the absent-input lifecycle. Description and option errors are reported while
+// loading.
+func SearchConfig(application string, search Search, supplied ...SearchOptions) Source {
+	options := make([]engine.SearchOptions, len(supplied))
+	for index, option := range supplied {
+		options[index] = engine.SearchOptions{Required: option.Required}
+	}
+	return Source{value: engine.SearchConfig(application, search.value, options...)}
+}

docs/adr/0019-explicit-configuration-discovery.md

@@ -0,0 +1,68 @@
+# ADR 0019: Keep Configuration Discovery Explicit and Inside Loading
+
+## Status
+
+Accepted.
+
+## Date
+
+2026-10-24
+
+## Context
+
+Applications often need a small ordered convention for choosing a main
+configuration file. Keeping that choice entirely in each application repeats
+path selection and file-loading plumbing. Moving the convention into ConfigKit
+without an explicit declaration would make ordinary loads search paths that a
+consumer did not choose, obscuring both precedence and filesystem access.
+
+ConfigKit already owns synchronous source acquisition, parsing, composition,
+decoding, validation, and inspection. A selected host file can therefore enter
+that lifecycle as the existing file source, preserving its source identity,
+resource context, and diagnostics. Optional exhaustion can use the same absent
+main lifecycle as an explicitly absent source.
+
+## Decision
+
+Consumers choose and order the main-file conventions they want to use. They may
+describe literal paths, directory filename groups, and supported directory
+presets, and may mark the complete search as required. Search descriptions are
+explicit inputs to loading; no preset or convention is activated implicitly.
+
+During synchronous loading, ConfigKit resolves the description to one real host
+file or to absence before parsing. The selected file follows the existing source
+acquisition and processing rules. A selected-file access, parse, version,
+composition, decode, or validation failure is terminal and does not resume the
+search. Optional exhaustion shares the absent-main lifecycle; required
+exhaustion reports a missing-source diagnostic. No candidate-attempt history is
+added to public metadata.
+
+This keeps convention choice with applications while moving repeated selection
+and loading plumbing into the library's established lifecycle. It preserves
+explicit control, ordinary source behavior, and complete-on-return ownership.
+
+## Alternatives Considered
+
+- **Keep all search and fallback loading in each application.** This preserves
+  consumer ownership but repeats the file-selection and load coordination that
+  ConfigKit already owns, and risks divergent handling of selected-file errors.
+- **Search conventional locations implicitly during every load.** This reduces
+  setup, but activates filesystem access and precedence the consumer did not
+  select and makes ordinary source behavior less predictable.
+- **Return candidate paths for the application to load one by one.** This keeps
+  selection outside the load lifecycle and encourages repeated complete loads,
+  potentially rerunning defaults, callbacks, and validation during fallback.
+
+## Consequences
+
+The public search description must be immutable and root-owned, with private
+engine conversion and selection. The contract documents candidate ordering,
+selection, exhaustion, context, and provenance; the API declaration documents
+the public symbols. Dynamic bases and file availability are observed when each
+load consumes the description. Applications remain responsible for selecting
+conventions and for their schema and domain requirements.
+
+The [configuration contract](../config.md) owns implemented behavior, and the
+[component overview](../internal/overview.md) describes implementation ownership.
+The completed implementation plan and remediation record are preserved in Git
+history at `0369111` under `docs/roadmap/`.

docs/config.md

@@ -12,6 +12,7 @@ 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. |
+| `SearchConfig(application, search, options...)` | Creates an explicit, reusable search source. Loading selects one host file or follows the absent-input lifecycle when an optional search is exhausted. |
 | `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. |
@@ -28,6 +29,79 @@ 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.
 
+### Configuration discovery
+
+Discovery is opt-in. Construct `Search` from literal `Paths`, directory searches,
+or the supported directory presets, then pass it to `SearchConfig`. Search
+descriptions are immutable and reusable; each load resolves dynamic directories
+and checks current candidate files again. `Searches` is ordered concatenation.
+It preserves the supplied description order and does not deduplicate candidates.
+
+`NamedFilenames` selects `<application>.yml`, then `<application>.yaml`.
+`GenericFilenames` selects `config.yml`, `config.yaml`, `main.yml`, then
+`main.yaml`. `SearchDirectory` defaults to `[Named, Generic]` when options are
+omitted or `Filenames` is nil. A non-nil empty filename list contributes no
+candidates and does not resolve its directory. `CustomFilenames` supplies
+literal single filename components; it does not expand application-name
+placeholders. An application slug is required only when a selected directory or
+filename group uses it. Literal custom paths are not expanded for `~`, variables,
+or wildcards; relative paths are resolved from the working directory at load time.
+The zero `Search`, `Searches()`, `Paths()`, and an empty custom filename group
+contribute no candidates. An explicitly supplied empty path or filename is
+invalid. Slugs and custom filenames must be usable single host filename
+components; ConfigKit preserves their case and spelling.
+
+Search expansion is ordered concatenation. For each directory search, ConfigKit
+tries its filename groups and names before advancing to the next directory.
+When multiple directories use the same filename list, candidates follow the
+directory-first Cartesian product: every filename in one directory is tried
+before the next directory. Explicit file candidates stay at their declared
+positions among directory searches.
+
+The presets expand to these ordered directories and filename groups:
+
+| Preset | Directory, in order | Filename groups |
+| --- | --- | --- |
+| `UnixSystemDirectories` | `/usr/local/etc/<application>`, `/etc/<application>` | Named, then Generic |
+| `UnixSystemDirectories` | `/etc` | Named only |
+| `UserConfigDirectory` | Go user-config directory / `<application>` | Named, then Generic |
+| `UserConfigDirectory` | Go user-config directory | Named only |
+| `HomeDirectory` | Go home directory / `.<application>` | Named, then Generic |
+| `HomeDirectory` | Go home directory | Named only |
+| `WorkingDirectory` | Current working directory | Named only |
+
+The Unix system preset is supported on aix, android, darwin, dragonfly, freebsd,
+hurd, illumos, ios, linux, netbsd, openbsd, and solaris; reaching it on another
+platform is an option error. Other presets use Go's user-config, home, and
+working-directory helpers. A reached
+directory-helper error terminates selection. Bases for later searches are not
+resolved after an earlier candidate is selected. Missing candidates continue;
+the first existing candidate is selected and then follows ordinary file loading
+rules. Errors while acquiring or processing a selected file are terminal and do
+not resume the search.
+
+Searches are optional by default. Exhaustion uses the same absent-main lifecycle
+as `NoSource()`: there is no main document, source record, source ID, or explicit
+presence contribution. Imports may be enabled but there are no main declarations
+to traverse. Defaults, version checks, overlays, CLI overrides, resolution, and
+validation retain their normal lifecycle. `SearchOptions{Required: true}` makes
+exhaustion an error; it does not change candidate or selected-file error handling.
+
+For an ordinary active overlay after optional exhaustion, provide an explicit
+filesystem context. A host context with only `Root` is sufficient; ConfigKit
+loads the overlay at its normal root-relative location and does not infer a root
+from attempted search directories. The same root-only context can accompany a
+search that selects a file: the selected file determines its base and must be
+inside `Root`. A selected host file rejects an explicit `BaseDir` or native
+filesystem context. Invalid context fails without resuming selection. Layered
+overlays keep their independent `OverlayFilesystem` requirement.
+
+When a search selects a file, inspection reports that actual file with ordinary
+source IDs, paths, placement, metadata origins, and resource context. Missing
+candidates create no records. `SearchConfig` can be used as the main source of
+ordinary or layered loading, including an underlay input, and does not add a new
+precedence tier.
+
 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
@@ -78,9 +152,14 @@ non-root destination. Unknown modes, invalid destinations, and invalid or
 conflicting sequence registrations fail before source acquisition. Overlay and
 CLI coordinates remain rooted at the application root. Ordinary `Load` recognizes
 source-root collection declarations whether or not imports are enabled; an
-`imports` declaration still requires imports to be enabled. `NoSource` rejects
-imports, active overlays, and filesystem context as invalid combinations but
-accepts overrides and remains an absent source rather than an empty document.
+`imports` declaration still requires imports to be enabled. `NoSource` may enable
+imports, but it has no main declarations to traverse. It accepts an explicit
+filesystem context: a host context requires an absolute existing `Root` and may
+omit `BaseDir` to use that root as its internal import base; a native context
+requires its namespace, `BaseDir`, and `Root`. Supplied context is validated even
+when imports are disabled. An active ordinary overlay requires this explicit
+context. `NoSource` accepts overrides and remains absent rather than contributing
+an empty document.
 
 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
@@ -149,7 +228,8 @@ 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.
+Ordinary `Load` infers overlay context from sources that provide one; with
+`NoSource`, it requires the explicit `Options.Filesystem` context.
 
 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
@@ -969,6 +1049,12 @@ Every other name resolves only to `<configuration-root>/overlays/<name>.yml`, wi
 no extension fallback, case normalization, inheritance between overlay files,
 or overlay stack. A selected missing overlay is an error.
 
+With `NoSource`, the main input stays absent and the explicit filesystem context
+supplies the overlay root. The overlay is recorded as the first acquired source
+when no other source exists. Host contexts may use the root-only form described
+under [single-source loading](#single-source-loading); native contexts retain
+their complete namespace, base, and root requirements.
+
 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. Overlay bytes, nodes, and source records count toward the same

docs/consumers/README.md

@@ -55,6 +55,21 @@ both without redefining their syntax. ConfigKit overlays are configuration overl
 they do not select or replace an application's LLM, model, pipeline, report, or
 execution profile concepts.
 
+## Explicit main-file discovery
+
+Applications choose the search conventions and their precedence, then pass the
+resulting `SearchConfig` source to the same `Load` or `LoadLayered` entry point.
+For example, the maintained sequence catalog puts a caller-selected file before
+a custom directory fallback while keeping its underlay and overlay choices
+independent. See [`sequence_catalog.go`](../../examples/consumers/sequence_catalog.go)
+and its [behavior tests](../../examples/consumers/sequence_catalog_test.go), plus
+the [working-directory executable example](../../examples/consumers/discovery_test.go).
+Together, the root-owned [discovery API](../../discovery.go) and [configuration
+discovery contract](../config.md#configuration-discovery) define ordered
+concatenation, directory-first Cartesian expansion, optional exhaustion, and
+selected-file behavior. [ADR 0019](../adr/0019-explicit-configuration-discovery.md)
+records why discovery is explicitly selected.
+
 ## Import defaults and selection inspection
 
 `Options.ImportDefaults` and `LayeredOptions.ImportDefaults` apply one

docs/development.md

@@ -63,6 +63,7 @@ Accepted decisions:
 - [Separate import selection controls](adr/0016-independent-import-controls.md)
 - [Define identity and merge rules for each collection](adr/0017-collection-sequence-policy.md)
 - [Apply CLI overrides after inheritance and decoding](adr/0018-cli-after-structural-resolution.md)
+- [Keep main-file discovery explicit and composable](adr/0019-explicit-configuration-discovery.md)
 
 For cross-cutting changes, follow every applicable row. Completed implementation
 and remediation plans are preserved in Git history at `cb5b435`; use current
@@ -167,6 +168,27 @@ The subsequent import-integration audit and completed remediation record are
 preserved at `6903c9e:docs/roadmap/remediation.md`; retrieve them with
 `git show 6903c9e:docs/roadmap/remediation.md`.
 
+The completed sequence-identity/collection-policy roadmap and implementation
+plan are preserved in Git history at `d0a605a`:
+
+```sh
+git show d0a605a:docs/roadmap/sequences.md
+git show d0a605a:docs/roadmap/implementation.md
+```
+
+Current behavior belongs to the configuration contract's
+[strict composition](config.md#strict-document-composition),
+[sibling inheritance](config.md#sibling-inheritance), and
+[CLI overrides](config.md#explicit-cli-overrides) sections.
+The completed remediation record, including the algorithm-audit findings, is
+preserved at `ea46001:docs/roadmap/remediation.md`.
+
+The completed configuration-discovery roadmap, implementation plan, and
+remediation record are preserved in Git history at `0369111`, for example
+`git show 0369111:docs/roadmap/remediation.md`. Current behavior belongs to
+[configuration discovery](config.md#configuration-discovery); the public
+[discovery declarations](../discovery.go) own the Go API.
+
 Potential enhancements are tracked in [future features](roadmap/future.md).
 
 Install the Go version required by `go.mod` or a compatible newer toolchain.
@@ -216,6 +238,7 @@ 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), [relative destinations](../internal/engine/destination_import_test.go), [globs](../internal/engine/glob_test.go) | `go test ./internal/engine -run 'Source\|Import\|Glob\|Filesystem\|Nonregular\|RegularFile'` |
+| Explicit main-file discovery and absent-input loading | [Candidate selection](../internal/engine/discovery_test.go), [load integration](../internal/engine/discovery_load_test.go), [public facade](../facade_discovery_test.go), and maintained [catalog](../examples/consumers/sequence_catalog_test.go) and [preset](../examples/consumers/discovery_test.go) examples | `go test ./internal/engine -run 'Discovery|Search'`, `go test . -run 'Search'`, and `go test ./examples/consumers -run 'WorkingDirectoryPreset|SequenceCatalog'` |
 | Import defaults, empty-selection evidence, and dependency-ready ordering | [Defaults and facade evidence](../facade_import_defaults_test.go), [engine import evidence](../internal/engine/import_evidence_test.go), [scheduler](../internal/engine/dependency_order_test.go), [scheduling integrations](../internal/engine/imports_test.go) | `go test ./internal/engine -run 'Import|ScheduleDependencies|Inheritance'`, `go test . -run 'ImportDefaults|FacadeImport'`, and `go test ./examples/consumers` |
 | 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'` |

docs/internal/overview.md

@@ -7,10 +7,13 @@ private state; public signatures and fields do not expose engine types.
 
 Implemented component owners:
 
-- `internal/engine` owns host and native-filesystem source acquisition,
-  grouped strict composition, recursive precedence assembly, per-input mapping
-  and sequence-item placement, YAML collection declarations and operations for
-  identified sequences, and application identity registrations for strict
+- `internal/engine` owns host and native-filesystem source acquisition and
+  explicitly selected configuration discovery. A search resolves once during
+  shared acquisition; the selected concrete file or absent-input state then
+  follows the existing composition, metadata, and resource-context lifecycle.
+  It also owns grouped strict composition, recursive precedence assembly,
+  per-input mapping and sequence-item placement, YAML collection declarations
+  and operations for identified sequences, and application identity registrations for strict
   composition, inheritance, and final validation. It applies the selected
   collection operation at precedence boundaries and owns dependency-ready,
   destination-local inheritance before typed decoding. CLI assignment follows
@@ -31,9 +34,9 @@ Implemented component owners:
   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 conversion files explicitly cross the path, record, callback, secret,
+  search-description, 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.
@@ -53,7 +56,9 @@ these constraints and rejects exported aliases, fields, or signatures involving
 internal types.
 
 The [configuration contract](../config.md) owns implemented YAML and lifecycle
-semantics, including [input destinations](../config.md#input-destinations),
+semantics, including
+[configuration discovery](../config.md#configuration-discovery),
+[input destinations](../config.md#input-destinations),
 [shared source-version checks](../config.md#shared-source-version-checks), and
 [effective configuration diffs](../config.md#effective-configuration-diffs).
 The [strict composition contract](../config.md#strict-document-composition)
@@ -78,5 +83,7 @@ records the shared dependency-ready ordering decision; [ADR
 0016](../adr/0016-independent-import-controls.md) records the independent
 selection controls. [ADR 0017](../adr/0017-collection-sequence-policy.md) and
 [ADR 0018](../adr/0018-cli-after-structural-resolution.md) record collection
-operations and CLI ordering. The [import internals](imports.md) document candidate and
-namespace identities, discovery and scheduling, and the bounded algorithms.
+operations and CLI ordering. [ADR 0019](../adr/0019-explicit-configuration-discovery.md)
+records explicit main-file discovery. The [import internals](imports.md)
+document candidate and namespace identities, discovery and scheduling, and the
+bounded algorithms.

docs/policy/architecture.md

@@ -144,8 +144,12 @@ remains in the roadmap until implemented.
 ConfigKit owns configuration input processing and the coordination of decoding,
 resolution, validation, and inspection. Applications own their configuration
 types, field meanings, defaults, normalization, semantic validation, and schema
-version policy. Applications also own main-file search conventions, CLI wiring,
-remote acquisition, runtime initialization, and resource checks.
+version policy. Applications choose and order their main-file search conventions;
+ConfigKit provides explicitly selected search descriptions that resolve during
+loading. Applications also own CLI wiring, remote acquisition, runtime
+initialization, and resource checks. See the [configuration discovery
+contract](../config.md#configuration-discovery) and [ADR
+0019](../adr/0019-explicit-configuration-discovery.md).
 
 The engine applies each input's mapping destination within the existing source
 assembly, applies immediate-parent-relative YAML destinations to imports, and
@@ -157,11 +161,13 @@ placement, and [ADR 0012](../adr/0012-relative-import-destinations.md) records
 the import extension.
 
 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`.
+runtime resources. It must not perform implicit network access, implicitly
+discover application-specific paths, or change process-global state during
+ordinary `Load`.
 Import discovery is available only through declarations in selected sources:
 explicit `path`, `dir`, and `glob` selectors may search their declared scope, but
-ConfigKit does not search conventional application paths or discover a main
+ConfigKit does not search conventional application paths unless the application
+explicitly supplies a discovery description, and it never discovers a main
 configuration file implicitly. Imports and sibling inheritance share
 dependency-ready scheduling within their existing scopes; acquisition order and
 precedence-tier boundaries remain distinct. The [configuration contract](../config.md#explicit-imports)

docs/releases/v0.10.0.md

@@ -0,0 +1,77 @@
+# ConfigKit v0.10.0
+
+This minor release adds explicit, composable discovery of a configuration file.
+Applications choose the search conventions and their order; ConfigKit selects one
+file during loading and returns the final configuration with its normal metadata.
+Optional searches that find no file share the expanded `NoSource()` lifecycle.
+
+## Changes
+
+- Add `SearchConfig`, immutable `Search` and `FilenameGroup` descriptions, and
+  search-wide requiredness through `SearchOptions`. Compose explicit `Paths`,
+  custom `SearchDirectory` descriptions, and built-in Unix system, user-config,
+  home, or working-directory presets with `Searches`. See the
+  [public declarations](../../discovery.go) and
+  [discovery contract](../config.md#configuration-discovery).
+- Expand searches in supplied order, trying a directory's filename groups before
+  moving to the next directory. Named and generic groups provide conventional
+  YAML filenames; custom groups provide literal names. Directory searches
+  distinguish omitted filename selection from an explicitly empty selection.
+  Constructors perform no filesystem access, and each load observes current
+  directory bases and file availability.
+- Select the first existing candidate. Access errors and failures while loading
+  the selected file terminate the load rather than trying another configuration.
+  Selected files retain physical source identity, placement, provenance, imports,
+  and relative-resource context. Discovery works with ordinary loading and
+  layered inputs, including underlays, without adding a precedence tier.
+- Permit enabled imports with an absent main input: there are no main import
+  declarations to traverse, while actual underlay sources retain normal imports.
+  Support ordinary selected overlays with an absent main and explicit filesystem
+  context, including a root-only host context. Layered overlays retain their
+  independent `OverlayFilesystem` requirement. See
+  [single-source loading](../config.md#single-source-loading) and
+  [layered loading](../config.md#layered-loading).
+- Preserve ordinary file-context behavior after discovery and include the
+  failing candidate's file identity in access diagnostics. Use Go's
+  platform-aware Windows component rules and native drive-root-relative path
+  resolution. Isolate user-config tests according to the host Go conventions.
+
+## Compatibility and upgrade
+
+Existing explicit source constructors and loading APIs remain available.
+`File(path)` remains an exclusive selection that fails for a missing file;
+discovery is activated only by explicitly supplying `SearchConfig`.
+
+Searches are optional by default. Set `SearchOptions{Required: true}` when the
+application must find a file. An exhausted optional search contributes no main
+document or source record; application version checks, defaults, resolution,
+and validation still run under their existing rules and may reject incomplete
+configuration. A selected overlay is still required to exist.
+
+The former `NoSource()` rejections of enabled imports and explicit filesystem
+context are relaxed. An active ordinary overlay with no main file now loads
+when explicit context is supplied. Applications that relied on those combinations
+being rejected should review their option handling. Selected host files derive
+their own base directory; do not supply an explicit `BaseDir` or native filesystem
+context for a search that selects a host file.
+
+Applications may replace their own fallback-loading loops with the new search
+descriptions. The maintained [consumer examples](../consumers/README.md#explicit-main-file-discovery)
+show simple preset selection and custom-first discovery alongside existing
+configuration tiers. Composition, collection operations, inheritance, CLI
+precedence, and the complete-on-return ownership model retain their existing
+semantics.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.10.0
+```
+
+The consumer minimum remains Go 1.26.0. Release validation uses Go 1.26.10.
+The YAML dependency remains `go.yaml.in/yaml/v4 v4.0.0-rc.6`, and the maintainer
+tool remains ReleaseKit v0.4.1. 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/v0.10.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.10.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.10.0/docs/oss/attribution.md)

docs/releases/v0.9.0.md

@@ -0,0 +1,111 @@
+# ConfigKit v0.9.0
+
+This minor release expands import selection, adds author-declared collection
+identity and operations, and supports inheritance between identified records.
+It also moves CLI assignments after inheritance and typed decoding. These changes
+include breaking API and configuration behavior changes from v0.8.0; review the
+migration guidance below before upgrading.
+
+## Changes
+
+- Extend imports with named-file and directory lists, directory filename patterns,
+  exclusions, recursion and depth controls, and explicit hidden-file and symlink
+  discovery controls. Add `ImportDefaults` to ordinary and layered options, with
+  authored declarations taking precedence over application defaults. Imports are
+  optional by default. `SourceInfo.ImportSelections` exposes completed optional
+  declarations that selected no sources. See the
+  [import contract](../config.md#explicit-imports) and
+  [consumer guidance](../consumers/README.md#import-defaults-and-selection-inspection).
+- Use shared dependency-ready ordering for import composition and sibling
+  inheritance. Dependencies complete before dependents, with deterministic
+  selection among currently eligible contributions. Source acquisition and
+  callback order remain separate from assembly order. See the
+  [ordering decision](../adr/0015-shared-dependency-ready-ordering.md).
+- Add root `configkit.sequences` declarations for collection identity and
+  `replace`, `append`, or `upsert` operations, including in selected overlays.
+  Strict peers combine distinct identities and reject duplicates; precedence
+  and inheritance apply the destination's collection operation. Upsert replaces
+  a whole matching record at its retained position. Identity supports compound
+  field sets and scalar strings, numbers, booleans, and null. See the
+  [collection contract](../config.md#strict-document-composition).
+- Add `Definition.SequenceIdentities`, exact `SequenceIdentityAt` registrations,
+  and dynamic `SequenceIdentityMatching` selectors. Application registrations
+  govern structural collection operations and check existing matching final
+  typed sequences, including values from defaults, CLI assignments, and
+  application resolution. YAML-only declarations govern structural operations
+  without adding that final-result check.
+- Support record inheritance within an identified sequence using an `extends`
+  mapping containing the parent's complete identity. The child retains its own
+  identity, relationships use surviving assembled records, and output sequence
+  order is preserved. See [sibling inheritance](../config.md#sibling-inheritance)
+  and the maintained [sequence catalog](../consumers/README.md#identified-prompt-sequences).
+- Apply explicit CLI overrides to the decoded configuration before application
+  resolution. Mappings retain omitted fields and their presence metadata;
+  sequences replace whole lists independently of collection operations. See the
+  [CLI contract](../config.md#explicit-cli-overrides).
+- Correct native import alias and contained-symlink handling, source-version
+  path projection, and provenance/history through collection movement,
+  replacement, inheritance, and CLI assignment. Reduce repeated scans,
+  accumulated policy copying, and sparse-projection storage during construction.
+
+## Compatibility and upgrade
+
+This pre-v1 minor release intentionally changes the v0.8.0 contracts:
+
+1. **Migrate import requiredness.** The YAML `optional` field is removed and is
+   rejected. Replace `optional: true` with `required: false`; omission is equivalent
+   when the application does not default imports to required. Replace
+   `optional: false` with `required: true`. Imports that previously relied on
+   implicit requiredness now need `required: true`, or an application
+   `ImportDefaults.Required` default set to true. Review discovery filters:
+   `follow` and `hidden` default to true, and directory searches select all
+   filenames unless a pattern is supplied.
+2. **Replace sequence registrations.** `SequenceComposition` and
+   `Definition.SequenceCompositions` are removed without compatibility aliases.
+   For a previous registration at `prompts` using `id`, use:
+
+   ```go
+   rules.SequenceIdentities = []configkit.SequenceIdentity{
+       configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id"),
+   }
+   ```
+
+   These registrations now also enforce identity in final typed sequences. Fix
+   missing or duplicate identities introduced by defaults, CLI assignments, or
+   application resolution. Registration does not require a collection to exist
+   or determine its allowed shape; schema and application validation retain
+   those responsibilities.
+3. **Review collection precedence and ordering.** An identified collection now
+   defaults to upsert across layers and inheritance when no explicit mode
+   applies, rather than unconditional whole-sequence replacement. Declare
+   `mode: replace` at the relevant destination where replacement is intended.
+   With append or upsert, `[]` contributes no records and does not clear the
+   collection; null is invalid. Use `mode: replace` with `[]` to clear it.
+   An explicit mode persists at its destination until changed; a parent's mode
+   does not transfer to a different inheriting destination. Unidentified
+   sequences retain strict peer conflicts and whole-value precedence
+   replacement. Dependency-ready assembly can change sequence positions and
+   diagnostic or history order compared with the former traversal order.
+4. **Review CLI recovery and inheritance assumptions.** CLI overrides can no
+   longer repair values that fail earlier decoding, bypass source or inheritance
+   errors, or add or cancel inheritance relationships. Supply structural
+   directives in source configuration. Application resolution still receives
+   explicit CLI values and may record subsequent normalization or derivation.
+5. **Update affected Go literals and diagnostic handling.** Use keyed literals
+   for expanded option and source-information structs. Collection validation adds
+   `CodeSequenceInvalid` (`sequence.invalid`) and `CodeSequenceConflict`
+   (`sequence.conflict`); update diagnostic handling where applicable.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.9.0
+```
+
+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 v0.4.1. 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/v0.9.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.9.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.9.0/docs/oss/attribution.md)

docs/roadmap/complexity.md

@@ -161,9 +161,10 @@ syntax, dependencies, or complexity-based gates.
 
 ## Relationship To Remediation And Other Work
 
-[Remediation](remediation.md) owns the identified correctness and algorithmic
-efficiency findings. Several overlap these files, particularly item-scope
-application matching and inheritance policy localization.
+The completed remediation record at `ea46001:docs/roadmap/remediation.md`
+documents the identified correctness and algorithmic efficiency findings.
+Several overlap these files, particularly item-scope application matching and
+inheritance policy localization.
 
 Use the documented contract and completed remediation as the behavioral baseline.
 Keep corrective changes reviewable under their findings, and retain their
@@ -172,9 +173,9 @@ overlapping work and select its starting revision rather than duplicating the
 remediation plan here. These structural changes do not close a finding merely
 because its function moved or became smaller.
 
-The pending [discovery roadmap](discovery.md) remains separate feature work.
-Import cleanup should leave clear private boundaries for future use without
-implementing discovery behavior as part of this refactor.
+[Configuration discovery](../config.md#configuration-discovery) is implemented.
+Import cleanup should preserve its source-acquisition boundary and behavior as
+part of this refactor.
 
 ## Work Needed And Validation
 

docs/roadmap/discovery.md

@@ -1,381 +0,0 @@
-# Composable Configuration Discovery Roadmap
-
-Status: proposal with OD-1 through OD-5 and the final review clarifications
-settled and integrated below.
-This document describes future work, not an implemented contract. A staged
-implementation plan will follow review of the complete roadmap.
-
-## Purpose And Ownership
-
-Make ordinary file-based loading concise for applications that want conventional
-configuration locations. ConfigKit supplies useful built-in candidate lists and
-an ordered selection operation. Consumers continue to own main-file search
-conventions: they choose which lists apply, their order, custom candidates, and
-whether a configuration file is required.
-
-A consumer can select one preset for a simple application or compose several
-presets and custom path lists. The application supplies an explicit slug where
-needed to construct conventional paths. ConfigKit does not infer application
-identity from the executable name or enable discovery for existing explicit
-sources.
-
-Discovery selects one source for the existing loading lifecycle. It does not
-create additional configuration tiers or merge every matching file. Existing
-layered loading remains available when consumers intentionally combine sources.
-
-## Current Baseline
-
-The [source contract](../config.md#sources) provides explicit host files, native
-filesystem files, bytes, readers, mappings, and `NoSource()`. A missing explicit
-file fails. The zero source is invalid. There is no conventional main-file search.
-
-[Ordinary loading](../config.md#single-source-loading) infers a selected overlay's
-filesystem context from the main source. `NoSource()` currently rejects enabled
-imports, active overlays, and filesystem context. In
-[layered loading](../config.md#layered-loading), an absent main input can coexist
-with underlays; a selected overlay requires an explicit `OverlayFilesystem`.
-These distinctions matter when an optional search finds nothing.
-
-The [architecture policy](../policy/architecture.md#responsibility-boundaries)
-assigns search conventions to consumers and currently excludes implicit main-file
-discovery. The addition should clarify that consumers may explicitly select
-library-provided search conventions. Consumer ownership remains unchanged.
-
-## Agreed Direction
-
-### Filename Groups, Directory Searches, And Search Presets
-
-Organize discovery around three composable descriptions. These are ordered lists:
-their order determines precedence.
-
-**Filename groups** provide reusable, ordered filename templates. The built-in
-groups are:
-
-| Group | Filenames, in order |
-| --- | --- |
-| Named | `<appname>.yml`, `<appname>.yaml` |
-| Generic | `config.yml`, `config.yaml`, `main.yml`, `main.yaml` |
-
-`<appname>` is the application slug supplied by the consumer.
-
-**Directory searches** pair a directory with an ordered list of filename groups.
-The default selection is `[Named, Generic]`, producing the six filenames above
-in that order. A directory search may select different groups or reverse their
-precedence, such as `[Generic, Named]`, while retaining each group's internal
-order. Consumers can also supply custom filenames, subject to the literal
-single-component rules under OD-5 below.
-
-**Search presets** bundle ordered directory searches. Consumers can select a
-complete preset for the simple case or compose presets, custom directory searches,
-and explicit file candidates in their desired order. Explicit file candidates
-are literal paths and receive no filename expansion.
-
-### Ordered Expansion And Concatenation
-
-The selection model is **ordered expansion and concatenation**, followed by
-first-match selection:
-
-1. Expand the selected presets into directory searches in their supplied order.
-2. For each directory search, concatenate its selected filename groups in order,
-   substitute the application slug where needed, and join each filename to that
-   directory.
-3. Concatenate the resulting candidate lists, preserving the positions of custom
-   directory searches and explicit file candidates.
-4. Try candidates in order, stopping at the first selected file or an error as
-   described below.
-
-Directory priority comes before filename priority: exhaust one directory's
-filenames before trying the next directory. When several directories share a
-filename list, their expansion is a Cartesian product traversed directory first.
-Different directories may select different lists, so ordered expansion and
-concatenation describes the complete model. Set union does not express its
-precedence semantics.
-
-The eventual documentation should explain this model using the applicable
-mathematical nomenclature: ordered lists, ordered concatenation, and Cartesian
-product where a shared filename list applies. Retain these names in relevant
-implementation comments where they help readers understand the operations. The
-model describes ordering without requiring eager construction of every candidate.
-
-### Unix System Preset
-
-The agreed Unix system preset is:
-
-| Directory, in search order | Filename groups, in order |
-| --- | --- |
-| `/usr/local/etc/<appname>` | `[Named, Generic]` |
-| `/etc/<appname>` | `[Named, Generic]` |
-| `/etc` | `[Named]` |
-
-This produces 14 candidates: six in each application-specific directory, then
-`/etc/<appname>.yml` and `/etc/<appname>.yaml`. For example,
-`/usr/local/etc/<appname>/main.yaml` precedes
-`/etc/<appname>/<appname>.yml`. Selecting only `Named` for `/etc` is an ordinary
-directory-search description; the selection algorithm needs no special exception.
-
-### Additional Presets (OD-1 — Settled)
-
-Provide the following independently selectable presets alongside Unix system.
-Within each preset, search directories in the order shown:
-
-| Preset | Directory | Filename groups, in order |
-| --- | --- | --- |
-| User configuration | `<user-config-dir>/<appname>` | `[Named, Generic]` |
-| User configuration | `<user-config-dir>` | `[Named]` |
-| Home directory | `<home>/.<appname>` | `[Named, Generic]` |
-| Home directory | `<home>` | `[Named]` |
-| Working directory | Current working directory | `[Named]` |
-
-Resolve the user-configuration base with
-[`os.UserConfigDir`](https://pkg.go.dev/os#UserConfigDir), the home base with
-[`os.UserHomeDir`](https://pkg.go.dev/os#UserHomeDir), and the working-directory
-base with [`os.Getwd`](https://pkg.go.dev/os#Getwd). Adopt the Go helpers'
-platform-specific directory and environment behavior, including their errors;
-the settled OD-2 rules below govern how discovery handles resolution errors.
-
-The goal is to support familiar conventions through idiomatic Go. This feature
-does not add a separate XDG compatibility layer or XDG system preset. A selected
-user-configuration base has no additional hard-coded `<home>/.config` fallback.
-The Home directory preset is a separate, explicit choice.
-
-The working-directory preset searches only that directory and uses application-named
-files; it does not walk ancestor directories. The consumer chooses precedence
-between all presets. Selecting one does not automatically select any others.
-
-### Illustrative Consumer API
-
-Names below are illustrative and are not implemented Go API:
-
-```go
-// A simple system-wide application.
-source := configkit.SearchConfig("myapp", configkit.UnixSystemDirectories())
-```
-
-```go
-// A consumer-selected working-directory, user, then system search.
-source := configkit.SearchConfig(
-    "myapp",
-    configkit.WorkingDirectory(),
-    configkit.UserConfigDirectory(),
-    configkit.UnixSystemDirectories(),
-)
-```
-
-```go
-// Explicit custom candidates followed by a selected conventional fallback.
-source := configkit.SearchConfig(
-    "myapp",
-    configkit.Paths("/run/myapp/config.yml", "/opt/myapp/config.yaml"),
-    configkit.UnixSystemDirectories(),
-)
-```
-
-Use the resulting source with ordinary loading or as a layered main input. Custom
-searches and file lists participate at their specified position; they do not
-silently append unselected presets. Reordering presets or their directory
-searches changes search priority.
-
-### Selection And Failure Behavior
-
-| Situation | Selected behavior |
-| --- | --- |
-| Candidate is absent | Continue to the next candidate. |
-| A candidate file is selected | Stop searching and load that file. |
-| Candidate is inaccessible, unreadable, or not an accepted file type | Fail using normal source diagnostics. |
-| Selected file fails parsing, version checks, composition, decoding, or validation | Fail without trying another candidate. |
-| Entire search has no match | Absence is permitted by default; follow the shared absent-main lifecycle settled under OD-3 below. |
-| Entire search has no match and is required | Fail with a diagnostic explaining the unsuccessful search. |
-| Caller explicitly uses `File(path)` | Retain existing exclusive-file behavior, including errors for a missing file. |
-
-Requiredness applies to the complete search, not to each preset or directory.
-Permitting an absent file does not waive application version, schema, or domain
-requirements; the remaining lifecycle may still reject an incomplete configuration.
-
-Perform discovery before parsing the chosen main file. Do not implement fallback
-by repeatedly running complete loads. Defaults, application resolution, secret
-lookup, and final validation retain their existing lifecycle execution rules.
-Once a file has been selected, later failure does not restart discovery.
-
-### Construction And Filesystem Behavior
-
-Constructors describe the search without filesystem access. Resolve dynamic base
-directories and access candidate files during loading, so reusable descriptions
-can observe changes on subsequent loads. Do not change the process working
-directory or environment.
-
-Reuse the host-file acquisition rules for selected files, including regular-file
-checks, symlink handling, resource ownership, and source limits. The feature does
-not require a new filesystem traversal engine, recursive directory search, or
-new file-size limits. Candidate filenames and groups are bounded by their
-explicit descriptions, rather than by scanning unrelated directories.
-
-The exact Go names, constructors, and representation of search-wide options
-belong in the implementation plan. Keep the public surface root-owned and keep
-the single-preset case concise.
-
-### Unavailable Bases And Platform Applicability (OD-2 — Settled)
-
-Resolve directory bases only as the ordered search reaches them. Once an earlier
-candidate is selected, later searches do not resolve their bases. Static invalid
-search arguments still fail ordinary option validation before source acquisition.
-An explicitly empty filename selection skips directory resolution as specified
-under OD-4; it has no candidates to locate.
-
-If a selected directory's base cannot be resolved, stop and report the error.
-A missing directory or candidate file beneath a resolved base allows the search
-to continue; a directory-resolution error does not. This distinction also applies
-to optional searches: permitting no match does not suppress errors encountered
-while searching. Preserve the agreed errors for inaccessible or unreadable
-candidates.
-
-Treat using the Unix system preset on an unsupported platform as an explicit
-configuration error when that preset is reached. Specify its supported platforms
-in the implementation plan. Do not translate the Unix preset into a different
-platform's convention. User, home, and working-directory presets use their native
-Go directory helpers.
-
-This keeps search priority predictable when directory resolution fails. Consumers
-select the presets appropriate to their application and environment; this proposal
-does not add an option to skip unavailable bases or unsupported presets.
-
-### No Match, Imports, And Selected Overlays (OD-3 — Settled)
-
-Use a shared absent-main lifecycle for optional no-match and explicit `NoSource()`.
-Allow imports to be enabled when the main file is absent: there are no main-file
-import declarations to process, and enablement does not introduce automatic
-directory scanning. Other actual sources retain their normal import behavior.
-In layered loads, process underlays normally and retain the existing explicit
-`OverlayFilesystem` requirement for an active overlay.
-
-For ordinary loads with an active overlay and no main file, require an explicit
-filesystem context to locate the overlay. Do not infer an overlay root from the
-first or last searched directory. A missing selected overlay remains an error;
-absence of a main file does not silently disable a requested overlay.
-
-For an absent main input, a host filesystem context specifying only `Root` is
-sufficient to locate the overlay; no main-file `BaseDir` is needed. If discovery
-selects a host file, retain the normal file-context rules: the selected file
-determines its base directory, an explicit `BaseDir` is rejected, and the file
-must be contained within any supplied root. This allows one root-only context
-to work with both selected and absent main inputs, subject to those existing
-root and containment checks. An invalid context is an error, not a reason to
-resume discovery.
-
-Defaults, CLI overrides, application resolution, and validation continue through
-the normal lifecycle. Main-version checking receives an absent main input and
-may reject it under the application's existing policy. Create no synthetic empty
-document or source record for the absent main file.
-
-This deliberately relaxes the current `NoSource()` restrictions on imports and
-on overlays with explicit context. Both ways of obtaining an absent main input
-have the same behavior. Implementation must cover this lifecycle change as well
-as candidate selection.
-
-### Empty Composition And Implicit Preset Selection (OD-4 — Settled)
-
-Require explicit preset or search selection to activate a convention. Supplying
-no presets or custom searches produces no candidates and activates no implicit
-fallback. Empty custom path lists and explicitly empty filename-group selections
-also contribute no candidates. An empty component does not prevent later
-components from contributing candidates.
-
-A directory search with an explicitly empty filename selection does not resolve
-its directory base. Static argument validation still applies. This gives empty
-selections their no-candidate meaning without introducing directory-resolution
-errors for a search that cannot select a file.
-
-If the complete search is empty, an optional search follows the absent-main
-lifecycle settled under OD-3; a required search fails as exhausted. Empty lists
-are valid, supporting consumers that construct their searches programmatically.
-
-Distinguish omission from an explicitly empty filename selection for a directory
-search. Omitting the selection uses the agreed `[Named, Generic]` default;
-explicitly selecting an empty list means no filenames. The eventual API must
-represent both intentions clearly. Presets retain their specified filename groups.
-
-Selecting a single preset keeps the simple case concise. Search-wide requiredness
-provides the check for consumers that must find a configuration file.
-
-### Slugs And Custom Path Interpretation (OD-5 — Settled)
-
-Require a nonempty single-component application slug whenever a selected search
-uses it to construct a directory or filename. Reject path separators, `.` and
-`..`, and forms unusable as that component on the target platform. Preserve
-spelling and case; do not trim, lowercase, or impose a kebab-case naming scheme.
-Validation keeps the slug from changing the intended directory structure without
-imposing a separate naming style.
-
-Searches that do not use the slug do not require one. Examples include explicit
-file candidates and a custom directory searched with only Generic filenames.
-
-Treat custom paths as literal host paths. Resolve relative paths against the
-working directory at load time. Do not expand shell variables, `~`, or wildcard
-syntax. Consumers can construct expanded paths themselves; User and Home presets
-provide the common directory-resolution conveniences. These rules keep custom
-paths consistent with explicit file loading.
-
-Custom filenames within directory searches are literal single filename
-components. They cannot be absolute paths or contain subdirectory components;
-use custom directory searches or explicit file candidates for those layouts.
-They receive no application-name placeholder expansion. The Named group supplies
-application-name expansion, while consumers may construct custom filenames
-themselves before supplying them.
-
-### Placement, Provenance, And Existing Features
-
-The selected file participates as the actual file source. Preserve its physical
-identity, effective placement, source origin, and passive resource context.
-Relative imports and relative application resources use the selected file's
-normal context. Discovery does not relocate overlay or CLI coordinates.
-
-Missing candidates are not acquired sources and do not create source IDs,
-configuration history, or explicit input presence. An entirely absent search
-contributes no synthetic empty configuration file. Diagnostics should distinguish
-an exhausted required search from a failed candidate. Existing selected-source
-inspection remains the foundation; a new public per-attempt trace is not needed
-for this proposal.
-
-Preserve collection policies, inheritance, source-version rules, CLI ordering,
-final validation, and frozen inspection after selection. Search-list order does
-not change precedence among underlays, the main input, an overlay, and CLI values.
-An explicitly selected overlay is not another main-file fallback candidate.
-
-## Work Needed From The Baseline
-
-The subsequent implementation plan should cover:
-
-- Define filename-group, directory-search, preset, and search-source representations,
-  slug validation, and a search-wide requiredness option. Keep construction
-  separate from acquisition and the simple preset selection concise. Distinguish
-  omitted and explicitly empty directory-search filename selections.
-- Implement ordered expansion and concatenation for presets, filename groups,
-  custom searches, and explicit file candidates, followed by first-match selection
-  through existing file access. Resolve each source once per consumption before
-  entering its parse and composition lifecycle.
-- Integrate selected and absent outcomes with ordinary and layered main inputs,
-  placement, imports, application version checking, and overlay contexts according
-  to OD-3, including root-only host contexts for absent main inputs and normal
-  file-context checks when a file is selected. Preserve ownership and close
-  acquired handles on success and failure.
-- Preserve real selected-source metadata and safe diagnostics without treating
-  absent candidates as configuration contributions.
-- Exercise preset, directory, filename-group, and filename ordering, reversed
-  group precedence, the Unix system preset's 14 candidates and named-only `/etc`
-  search, custom-list composition, both filename extensions, no-match behavior,
-  empty components and searches, omitted versus empty filename selections,
-  skipped directory resolution for empty filename selections, slug-dependent and
-  slug-independent searches, literal custom paths and filename components,
-  required searches, unavailable preset bases, errors that must stop fallback,
-  repeated loads, and untouched explicit `File`
-  behavior. Cover imports, overlays, underlays, defaults, CLI, source versions,
-  source contexts, and source-list/inspection evidence at their boundaries.
-- Clarify the architecture policy's consumer-selected discovery boundary. Record
-  any warranted durable ownership decision in an ADR; keep preset filenames and
-  API mechanics in their canonical contract when implemented. Update consumer
-  examples and the configuration contract when the feature lands. Explain the
-  three-part discovery model and its mathematical terminology in the eventual
-  documentation, with simple preset selection and custom composition examples.
-
-This roadmap does not change implementation, policy, or ADRs by itself. Coordinate
-implementation with the ongoing sequence integration audit rather than assuming
-its in-progress changes are part of a fixed implementation baseline.

docs/roadmap/remediation.md

@@ -1,905 +0,0 @@
-# Sequence Implementation Review Remediation
-
-Status: findings 1–17 are complete. The algorithm audit below identifies six
-additional open findings, numbered 18–23. This record introduces no new release
-gate.
-
-Reviewed commit: `0be7cdb00eff576fda6b30c0600b2134d4e2e086`.
-Implementation range: `bc6b977..0be7cdb`.
-Authority: the [sequence roadmap](sequences.md),
-[implementation plan](implementation.md),
-[ADR 0017](../adr/0017-collection-sequence-policy.md), and
-[ADR 0018](../adr/0018-cli-after-structural-resolution.md).
-
-The public facade, shared identity/composition machinery, scoped policies,
-identified-record inheritance, post-decode CLI phase, and final registered
-identity validation are present. The findings below concern remaining
-correctness, contract-alignment, documentation, and scaling gaps.
-
-Validation on the reviewed commit:
-
-- `./scripts/check.sh` passed with Go 1.26.10 on Linux.
-- `go test -race ./...` passed.
-- Public-API probes outside the repository reproduced findings 1–9. The probes
-  used labeled byte sources, defaults, and an in-memory filesystem; they did not
-  change implementation files or use external services.
-- Finding 10 follows directly from the maintained text; finding 11 follows from
-  the prefix-scan loop, not a timing comparison or performance threshold.
-
-Validation after the initial remediation: `./scripts/check.sh` and
-`go test -race ./...` passed with Go 1.26.10 on Linux. The original public-API
-probes for findings 1–9 now demonstrate the required corrected behavior.
-
-Validation after integration remediation: the everyday check, full race suite,
-and local documentation/public-boundary checks passed with the same toolchain.
-The original and follow-up public-API probes demonstrate corrected behavior for
-findings 1–9 and 12–15, including unrelated underlay payload at an old version
-index. The accumulated-copy correction is verified by code inspection and
-ownership regressions; the contract corrections are verified against the
-implemented operations and replay. Each additional finding has its own completion
-record and commit.
-
-P1 denotes the highest-priority correctness issue here; P2 denotes normal-priority
-work needed to complete the selected behavior. Preserve the agreed scope while
-fixing these issues. In particular, do not resolve failures by restricting valid
-identity fields, requiring repeated declarations, changing merge semantics, or
-adding limits.
-
-## 1. Preserve Retained Explicit Values In Nested CLI Mapping Assignments — P1
-
-Status: [x] Complete.
-
-Completion: override metadata now reads the actual target subtree through one
-shared lookup. Ordinary/layered regressions preserve retained values, including
-zero and false, their explicit origins, and absence-based resolver behavior.
-Verified `go test ./internal/engine -run 'Override|CLI' -count=1` and
-`./scripts/check.sh`.
-
-Owner: [`applyDecodedOverrides`](../../internal/engine/override_decode.go),
-especially lines 59–60 and the presence/origin update helpers.
-
-The helpers receive the complete configuration projection as `existing`, but
-interpret it as the subtree at the override path. A partial nested mapping
-therefore treats retained input children as absent and drops their presence and
-origins. `fillDefaultOrigins` subsequently labels those retained values as defaults.
-
-Reproduction: load the following into ordinary nested Go structs:
-
-```yaml
-settings:
-  nested:
-    a: 1
-    b: 2
-```
-
-Apply an override at `[settings]` with value
-`map[string]any{"nested": map[string]any{"a": 3}}`. The typed value of `b`
-initially remains 2, but `Metadata.Lookup([settings,nested,b])` reports
-`Explicit: false` and a default origin. Calling `Default` for that field in
-application resolution then changes the explicitly supplied 2 to the fallback
-value. This violates presence-based defaulting, not merely presentation.
-
-Required remediation: use the actual existing target subtree when updating
-presence/origins, preserving retained descendants and removing genuinely replaced
-ones. Keep override batches detached and preserve the selected post-decode order.
-Add regressions for nested partial mappings, retained zero/false values, metadata,
-and an absence-based resolver default that must not overwrite retained input.
-
-## 2. Read Upsert Origins At Input Paths And Write Them At Output Paths — P2
-
-Status: [x] Complete.
-
-Completion: sequence origin copying now separates incoming and effective paths
-for precedence and inheritance. A regression covers reordered matches, appended
-records, composite origins, and actual resource reads from distinct declaring
-directories. Verified focused origin/inheritance/overlay/provenance tests,
-`go test -race ./internal/engine`, and `./scripts/check.sh`.
-
-Owners: [`applyOverlaySequenceEvidence`](../../internal/engine/overlay.go),
-particularly lines 369 and 381, and the analogous sequence-origin copying in
-[`inheritance_collection.go`](../../internal/engine/inheritance_collection.go).
-
-The overlay helper supplies an output-index path to an origin lookup whose keys
-are still incoming-index paths. When those positions differ and another incoming
-record occupies the output index, metadata takes that other record's source
-location. The same defect was reproduced through mapping-sibling inheritance;
-its helper uses the same single-path copying pattern.
-
-Reproduction: an underlay supplies IDs `[a,b]`; the main source declares `key: id`
-and supplies `[b*,a*]`. The result is correctly `[a*,b*]`, but the value origin
-for final `a*` is the source line of `b*`, and vice versa. In the probe the two
-value lines were 7 and 9; final index 0 incorrectly reported line 7 instead of 9.
-With separately authored records this can select a different source/context.
-
-Required remediation: use the operation's input-to-output projection for origin
-subtree reads as well as writes. Preserve composite contributing origins, not
-just node-location fallbacks. Test reordered upserts and appended records with
-distinct source files/locations, including nested fields, inheritance operations,
-and declaring resource contexts.
-
-## 3. Apply Tier Sequence Operations To Historical Contributions — P2
-
-Status: [x] Complete.
-
-Completion: per-source history now composes strict item projections with the
-tier operation, including nested application prefixes and local sequence replay.
-Regressions cover all three operations, multiple peers, retained records, removed
-fields, source attribution, and a subsequent overlay without a false decoding
-event. Verified focused history tests, `go test -race ./internal/engine`, and
-`./scripts/check.sh`.
-
-Owner: [`applyOverlayWithCollectionPolicy`](../../internal/engine/overlay.go),
-lines 76–84, and existing sequence-history projection/replay.
-
-For composed tiers, `applications` is nonempty. This branch appends the tier's
-original strict-composition applications without incorporating the newly computed
-precedence sequence descriptors. Replay consequently performs a whole-list
-replacement or strict append rather than the actual cross-tier upsert/append.
-The later decoding reconciliation conceals the mismatch by attributing a
-structural combination to typed defaults/decoding.
-
-Reproduction: lower `[a,b]` and higher `[b*,a*]` correctly yield `[a*,b*]`.
-`Explain([items])` instead shows the main contribution producing `[b*,a*]`, then
-a `decoding` event reordering it to `[a*,b*]` with the reason "applied retained
-defaults and canonical typed decoding". No decoder performed that reordering.
-
-Required remediation: project each retained per-source application through the
-tier's selected operation while keeping peer composition distinct from tier
-precedence. Do not collapse authored sources or fake decoding events. Cover
-upsert, append, unmatched retained records, reordered matches, removed fields,
-multiple higher-tier peers, and a subsequent selected overlay. Replay should
-agree with structural assembly before real decoding reconciliation.
-
-## 4. Reproject Nested Source Destinations After Cross-Tier Item Movement — P2
-
-Status: [x] Complete.
-
-Completion: layered assembly projects incoming source destinations and version
-classification evidence with the same item assignments used for values/history.
-Earlier contributions retain their historical coordinates, physical identities
-and contexts. Main/underlay regressions cover reordered upserts, append, replace,
-nested helpers, and source-local version evidence. Verified focused destination,
-origin and source-version tests, focused race coverage, and `./scripts/check.sh`.
-
-Owners: [`projectAcquiredDestinations`](../../internal/engine/imports.go) and the
-sequence-operation handoff in [`assembleLayeredInput`](../../internal/engine/load_request.go).
-
-Source destinations are projected while composing their own tier, but are not
-reprojected when that tier's records move to accumulated-list positions. Nested
-helper source records can therefore point at a different final record.
-
-Reproduction: lower `items` contains `[a,b]`. Main imports a single `b` record
-with `placement: sequence-item` into `[items]`; that record imports a helper
-mapping supplying its value. Upsert retains `a` at index 0 and places the higher
-`b` at index 1. `Sources()` nevertheless reports the surviving helper's destination
-as `/items/0`, its tier-local position, rather than `/items/1`.
-
-Required remediation: carry the sequence operation's projection through nested
-source-destination/evidence updates at precedence boundaries. Keep physical
-source identity and passive resource contexts unchanged, and preserve historical
-meaning for displaced contributions rather than relabeling them as survivors.
-Test a retained lower record followed by an imported higher record, reordered
-matches, and nested helpers. Check version-evidence projection where applicable.
-
-## 5. Honor Identity Reset On Actual Inheritance Replacement — P2
-
-Status: [x] Complete.
-
-Completion: inheritance resolves identity against actual child-value presence,
-discarding parent YAML identity for an explicit replacement while preserving
-agreement for retained values and authoritative application constraints. Added
-policy-state and public-load regressions for changed/unkeyed/non-sequence
-replacements, application agreement/conflict, and mode-only/append/upsert cases.
-Verified focused inheritance tests and `./scripts/check.sh`.
-
-Owners: [`inheritanceCollectionResolution` and `mergeInheritanceCollectionPolicies`](../../internal/engine/inheritance_collection.go),
-especially lines 79–80 and 121.
-
-These paths reconcile the parent's YAML identity before accounting for actual
-replacement, and treat the parent's collection as retained even when the child
-supplies a replacing value. This is stronger than the selected identity lifetime
-and differs from source-tier replacement.
-
-Reproduction:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [base, items]
-      key: id
-    - destination: [child, items]
-      key: name
-      mode: replace
-base:
-  items:
-    - id: a
-      value: parent
-child:
-  extends: base
-  items:
-    - name: b
-      value: child
-```
-
-Loading fails with `sequence.conflict` at `/child/items`. It should succeed:
-actual whole-sequence replacement may discard the parent's YAML identity and
-establish `name`. Authoritative application constraints must still apply.
-
-Required remediation: distinguish actual replacement from a mode-only declaration
-before identity agreement and transfer. Test a changed key, replacement with no
-new key becoming unkeyed, permitted non-sequence replacement, and an agreeing or
-conflicting application registration. Preserve rejection of identity changes
-without a replacement value and during append/upsert.
-
-## 6. Enforce Collection Operations Beneath Newly Added Ancestors — P2
-
-Status: [x] Complete.
-
-Completion: copying a mapping now visits nested collection operations with absent
-lower values, preserving ordinary ancestor removal and whole-record semantics.
-Incoming record-owned policy transfers through the same operation. Regressions
-cover absent/empty/null ancestors, identity and kind failures, empty and valid
-initialization, and later inheritance using an imported item's nested identity.
-Corrected an older test that deferred the required error until a later tier.
-Verified focused merge/collection/inheritance tests, focused ownership race
-coverage, and `./scripts/check.sh`.
-
-Owners: [`mergeRecursiveMappings`](../../internal/engine/recursive_merge.go) and
-[`mergeOverlayMappingsWithCollectionPolicy`](../../internal/engine/overlay.go).
-
-When an incoming ancestor mapping is absent below, the recursive merger clones
-the entire subtree and invokes only its ancestor-level added hook. Nested
-collection resolutions are not visited. This bypasses their operation rules;
-the subsequent declaration-only policy update does not replace those checks.
-
-Reproduction: combine a lower `{}` with this main tier:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [group, items]
-      mode: append
-group:
-  items:
-    - id: a
-```
-
-Loading succeeds despite append having no identity definition. Placing the same
-sequence directly at `[items]` correctly fails. The validity of an operation
-must not depend on whether an enclosing mapping already existed.
-
-Required remediation: account for applicable nested collection policies when
-adding/copying an ancestor subtree, including owned-policy transfer. Test absent
-and present enclosing mappings, append/upsert without identity, explicit empty
-lists, invalid non-sequence contributions, and valid identified initialization.
-Do not turn this into a general path-existence or shape constraint: ordinary
-ancestor replacement/removal retains its documented meaning.
-
-## 7. Validate Final Typed Identity Fields As Payload, Not Directives — P2
-
-Status: [x] Complete.
-
-Completion: shared identity lookup now distinguishes structural records from
-effective payload/reference mappings. Final validation includes literal `extends`
-fields while structural source checks still exclude actual directives. Facade
-regressions cover defaults and recorded resolution, string/null components, and
-duplicate rejection. Verified those regressions and existing structural identity
-and inheritance coverage through `./scripts/check.sh`.
-
-Owners: [`validateFinalSequenceIdentities`](../../internal/engine/final_identity.go)
-and [`newSequenceMappingIndex`](../../internal/engine/sequence_identity.go).
-
-The final validator reuses structural record lookup, which excludes every
-`extends` entry. At this phase there is no structural processing: typed defaults
-and application resolution can produce ordinary payload with that literal name.
-The roadmap expressly distinguishes structural-control exclusion from final
-typed-payload validation; this reuse introduces an extra restriction.
-
-Reproduction: a defaults-only `Catalog` with `Items []map[string]any` containing
-`{"extends": "application-data"}`, registered with
-`SequenceIdentityAt(Keys("items"), "extends")`, fails with a missing identity
-field at `/items/0/extends`, even though the effective field is present.
-
-Required remediation: distinguish structural-payload lookup from final payload
-lookup. Preserve rejection when a source tries to use an actual consumed
-directive as identity. Test final fields supplied by defaults and recorded
-resolution, including a null component, without adding a broader name ban.
-
-## 8. Retain Nonfatal Main-Version Diagnostics In Layered Loads — P2
-
-Status: [x] Complete.
-
-Completion: layered assembly returns nonfatal main-version diagnostics on success
-and alongside later failures; the shared caller stops only for errors. Regression
-coverage verifies ordinary/layered warning parity, exactly one checker call, and
-warnings retained with later structural or decoding errors. Verified through
-`./scripts/check.sh`, including existing source-version ordering coverage.
-
-Owner: [`assembleLayeredInput`](../../internal/engine/load_request.go), lines 143–152.
-
-The moved default-mode version-check call returns its diagnostics only when they
-contain errors. Warning-only diagnostics are discarded. Ordinary loading still
-retains the same warnings.
-
-Reproduction: a `VersionRule.Check` that calls
-`validation.Warning(Keys("version"), "test warning")` and returns nil yields one
-`version.warning` through ordinary `Load`, but zero diagnostics through
-`LoadLayered` with the same main input and default source-version mode.
-
-Required remediation: carry warnings from the single main-tier check into the
-shared diagnostic accumulator on successful layered loads. Preserve checker
-ownership, invocation count, and the selected acquisition/composition ordering.
-Add ordinary/layered parity and warning-plus-later-error regressions.
-
-## 9. Identify Conflicting Application Registrations In Diagnostics — P2
-
-Status: [x] Complete.
-
-Completion: definition validation retains original one-based registration
-positions and names both positions in each overlap diagnostic. No input path or
-new public diagnostic structure is invented. A mixed-selector regression verifies
-multiple conflicts even after an earlier invalid registration is skipped.
-Verified registration/facade coverage through `./scripts/check.sh`.
-
-Owner: [`compileSequenceIdentityPolicy`](../../internal/engine/sequence_registration.go),
-lines 119–128.
-
-Incompatible dynamic overlaps produce only a generic `options.invalid` message,
-an empty configuration path, and no related information. Neither registration
-is identified. The roadmap requires definition-time errors to identify the
-conflicting registrations without inventing a concrete input destination.
-
-Reproduction: register `collections -> AnyMappingKey -> items` using `id`, and
-`collections -> foo -> items` using `name`. Even with `NoSource`, the error is
-only "overlapping sequence registrations must use the same identity fields".
-With multiple registrations the caller cannot identify the conflicting pair.
-
-Required remediation: retain original registration positions and identify both
-in safe definition diagnostics, for example through positional message context.
-No new public diagnostic structure is required solely for this. Test mixed exact
-and dynamic registrations and multiple conflicts, without asserting complete
-English wording or exposing identity values.
-
-## 10. Remove Stale Public Contract Descriptions — P2
-
-Status: [x] Complete.
-
-Completion: the public definition comment now describes final registered-identity
-validation and its shape/lifetime boundaries. Selected-overlay prose explicitly
-separates ordinary replacement from collection policy and links to the canonical
-operation/presence tables. Verified against the implementation, local-link tests,
-and `git diff --check`; no runtime behavior changed.
-
-Owners: [`Definition.SequenceIdentities`](../../lifecycle.go), lines 18–21, and
-the [selected-overlay contract](../config.md#selected-overlays).
-
-The exported definition-field comment still says registrations do not validate
-identities introduced by defaults, CLI, or callbacks. Final validation is now
-implemented and the registration constructors correctly document it.
-
-The selected-overlay table and following paragraph also still unconditionally
-describe sequences as atomic replacements and `[]` as clearing. A later paragraph
-describes collection operations, but does not make the earlier unconditional
-claims accurate: identified collections infer upsert, and `[]` then contributes
-no records rather than clearing.
-
-Required remediation: make the symbol comment describe its implemented final
-guarantee and boundaries, and qualify the overlay table/paragraph for collection
-policy. Link to the canonical collection tables rather than duplicating them.
-Keep YAML-only enforcement distinct from registered final validation.
-
-## 11. Remove The Quadratic Prefix Scan In Item Policy Projection — P2
-
-Status: [x] Complete.
-
-Completion: assigned sequence ranges now supply item offsets directly. Handoffs
-without a range lazily accumulate prefix lengths, visiting each preceding
-contribution once rather than rescanning every prefix. Regression coverage keeps
-inline and policy-less contributions in the correct positions with complete,
-partial, or absent ranges. Existing ownership/placement tests remain intact.
-Verified focused item-policy tests, `./scripts/check.sh`, `go test -race ./...`,
-and inspection of the remaining error-only prefix-scan callers. No new benchmark
-fixture or performance threshold was needed for this bounded change.
-
-Owner: [`projectNestedCollectionPolicies`](../../internal/engine/imports.go),
-especially line 968; the scanned helper is `priorSequenceLength` in the same file.
-
-For every item with local collection policy, the loop scans all preceding
-contributions to calculate `itemIndex`, then overwrites that result when the
-already-indexed sequence range is available. For N such items this performs
-`0 + 1 + ... + (N-1)` contribution visits, excluding path lookup costs, despite
-the common case already having the exact assigned index. This is introduced
-quadratic work in the feature's independently authored-record path.
-
-Required remediation: use the existing assigned range first. If a genuine
-fallback is necessary, maintain per-destination prefix lengths in one pass
-rather than rescanning every prefix. Preserve scope and ordering semantics;
-do not add limits, pooling, or a new public cache. Keep ownership/placement
-regressions and add a representative many-item/local-policy benchmark only if
-needed to exercise the path. A wall-clock threshold is not required.
-
-## Integration Audit Baseline And Scope
-
-Reviewed commit: `8d40983271de5239312eb0a2896737be88c5abdc`.
-
-This follow-up reviews the interaction of collection policy, identified-record
-inheritance, dynamic registrations, and post-decode CLI assignment with existing
-source placement, source-version checking, typed decoding, inspection, resource
-contexts, and ownership boundaries. The accepted ADRs and current configuration
-contract remain the authority; the findings require integration corrections,
-not additional consumer restrictions or new feature choices.
-
-Validation:
-
-- `./scripts/check.sh` and `go test -race ./...` pass with Go 1.26.10 on Linux.
-- Isolated public-API probes outside the repository reproduce findings 12–15.
-  They use labeled byte sources and an in-memory filesystem, with no external
-  services, real credentials, or repository implementation changes.
-- Finding 16 follows from the per-item deep-copy loop; no timing claim or
-  performance threshold is needed. Finding 17 follows from maintained contract
-  text that contradicts the implemented collection operations.
-- Positive comparisons include sequence-valued identity replacement, ancestor
-  retirement through source tiers, quoted versions after item movement, and an
-  append overlay that preserves the main record's version.
-
-## 12. Complete Identity Retirement Across Replacement Shapes — P2
-
-Status: [x] Complete.
-
-Completion: source composition and inheritance now prepare eligible lower policy
-from actual replacement paths before agreement. Removed identity and record state
-cannot constrain a new pending key or reappear in later inheritance. Shared mode
-selection preserves explicit-mode persistence and authoritative registrations.
-Regressions cover null/scalar/mapping/list replacement, ancestor removal with a
-new key, reintroduction, and retained/application-constrained conflicts. Verified
-focused retirement/inheritance tests, engine ownership race coverage, and
-`./scripts/check.sh`.
-
-Owners: [`resolveStrictCollectionPolicies`](../../internal/engine/compose.go),
-especially its `sequencePresent` reset condition;
-[`mergeInheritanceCollectionPolicies`](../../internal/engine/inheritance_collection.go);
-and ancestor-replacement handling in
-[`applyInheritanceScopeTemplates`](../../internal/engine/inheritance_schedule.go).
-
-Policy agreement and retirement still treat some actual replacements as retained
-collections. This has three observable effects under the same selected lifetime
-rule:
-
-1. Source precedence accepts a replacement sequence with a new YAML key, but
-   rejects the equivalent replacement by null or a scalar. For example, a lower
-   tier declaring `key: id` for `[items]` with `items: [{id: old}]`, followed by
-   a main tier declaring `key: name, mode: replace` with `items: null`, fails with
-   `sequence.conflict` at `/items`. Replacing null with `42` also fails; replacing
-   it with `[{name: new}]` succeeds. Null/scalar replacement should discard the
-   lower identity and retain the newly declared key as pending policy, subject
-   to the application's schema and constraints.
-2. Removing an ancestor while declaring a new pending key for its removed
-   descendant fails before the replacement occurs. A lower
-   `group: {items: [{id: old}]}` with key `id`, followed by `group: null` and a
-   root declaration of key `name` for `[group, items]`, fails at `/group/items`.
-   The analogous `base`/`child` inheritance case fails at `/child/group/items`.
-   The old collection is being removed rather than retained.
-3. Inheritance can resurrect removed identity even without a conflicting new
-   declaration. This input fails with a missing `id` at
-   `/grandchild/group/items/0/id`:
-
-   ```yaml
-   configkit:
-     sequences: [{destination: [base, group, items], key: id}]
-   base: {group: {items: [{id: old}]}}
-   child:
-     extends: base
-     group: null
-   grandchild:
-     extends: child
-     group: {items: [{name: new}]}
-   ```
-
-   The same remove/reintroduce operations through source tiers succeed. The
-   inherited descendant identity remains in policy state after its enclosing
-   mapping is removed, so the grandchild incorrectly receives the old key.
-
-Required remediation: account for actual value and ancestor replacement before
-deciding which lower identity participates in agreement, and retire removed
-descendant/record-owned state during inheritance as during source precedence.
-Retained inheritance templates must not reestablish discarded identity later.
-Preserve authoritative application constraints, independently owned mode
-persistence, pending declarations, and rejection of key changes when values are
-genuinely retained. Cover direct non-sequence replacement, ancestor removal with
-a new pending key, and removal followed by reintroduction across both contexts.
-Do not require repeated keys or narrow permitted null/scalar shapes to avoid
-these cases.
-
-## 13. Keep Metadata History In Post-Decode CLI Order — P2
-
-Status: [x] Complete.
-
-Completion: metadata history is assembled from the pre-CLI source view, with CLI
-events appended separately; inheritance is projected before CLI and resolution.
-Effective fields still describe post-CLI values. Regressions verify chronological
-origins and resolver dependencies for ordinary/layered mapping and record
-inheritance, including equal-valued assignments. Verified history/override
-coverage and `./scripts/check.sh`.
-
-Owners: [`historyWithInheritance`](../../internal/engine/inheritance_history.go)
-and the source/CLI metadata handoff in
-[`preparePostDecodeOverrideContext`](../../internal/engine/override_context.go).
-
-The value explanation follows the new lifecycle, but `Metadata.History` still
-inserts inheritance after stored CLI changes. `historyWithInheritance` splits
-only at `LayerResolution`, reflecting the former CLI-before-inheritance order.
-The reconstructed source view can also append an authored source origin after
-the CLI event.
-
-Reproduction: load `base: {value: parent}` and
-`child: {extends: base, value: authored}`, then override `[child, value]` with
-`"cli"`. The final value and `Metadata.Lookup` correctly report CLI provenance.
-`Explain` reports source → inheritance → CLI. `Metadata.History` instead reports
-source → CLI → source → inheritance and ends with a source/inheritance origin.
-This violates chronological metadata history and disagrees with the effective
-origin and the value explanation.
-
-Required remediation: assemble metadata history in the implemented source,
-inheritance, CLI, and resolution order, preserving the real origins and
-dependencies of those events. Cover ordinary/layered loading, mapping and record
-inheritance, equal-valued CLI writes, and a later recorded resolver change at the
-appropriate existing test owners. Do not impose a new exact event-count or
-message-format contract.
-
-## 14. Carry Main-Version Coordinates Through Sequence Movement — P2
-
-Status: [x] Complete.
-
-Completion: the effective version rule now follows the main declaration through
-strict placement, tier combination, and inheritance without changing source-local
-checker/evidence paths. Underlay and overlay ownership checks use assigned record
-positions, including whole-record removal; integer conversion remains confined
-to the actual main header. Regressions cover mounted/indexed paths, appended and
-matched records, integer/string parity, unrelated version payload, imported
-underlay claims, and inheritance. Corrected an older ownership fixture to target
-the actual matching main record. Verified source-version regressions,
-`./scripts/check.sh`, and `go test -race ./internal/engine`.
-
-Owners: [`effectiveSourceVersionRuleForPlacement`](../../internal/engine/source_version.go),
-the main-tier projection handoff in
-[`assembleLayeredInput`](../../internal/engine/load_request.go), and use of
-the effective rule in [`executeLoadRequest`](../../internal/engine/load.go).
-
-Source destinations and version-evidence sensitivity paths follow item movement,
-but the rule used for integer-version decoding retains the original item index.
-Consequently an accepted integer header can fail decoding after an otherwise
-valid upsert placement.
-
-Reproduction with the public API:
-
-- Main input is `id: main` and `version: 4`, placed as one sequence item at
-  `Keys("items")`.
-- A lower input contributes the sequence item `id: lower` at the same destination.
-- Register identity `id` for `[items]`, select `SourceVersionModeIfPresent`, and
-  use `VersionRule.Path = Keys("version")` with an accepting checker.
-- Decode into records with `ID string` and `Version string`.
-
-The checker accepts `4` exactly once. Upsert places the main record at index 1,
-but decoding fails with `decode.type` at `/items/1/version`. The integer-to-string
-exception is still tied to `/items/0/version`. Quoting the identical version as
-`'4'` succeeds. A source-local indexed header path, without item placement,
-reproduces the same problem when its record moves during upsert.
-
-Required remediation: carry the main declaration's effective coordinates through
-the existing item projections into decoding and audit the associated ownership
-protection handoffs for the same stale-coordinate assumption. Preserve
-source-local checker/evidence paths, checker invocation count, main-only
-ownership, and the narrow integer-to-string exception. Cover moved existing and
-newly appended main records, integer/string parity, and protection of the actual
-main header after movement. Do not prohibit supported indexed paths or mounted
-main records as a workaround.
-
-## 15. Exclude Parent Identity From Inherited History Payload — P2
-
-Status: [x] Complete.
-
-Completion: retained record-inheritance operations now exclude parent identity
-from their payload and origin projection. Replay preserves the child's own
-identity before taking field, record, sequence, or root snapshots, without
-inventing inheritance events for retained identity fields. Regressions cover
-single and compound identities, explicit child provenance, ordinary inherited
-payload, and the parent's original history. Verified inheritance/inspection
-coverage, `./scripts/check.sh`, and `go test -race ./internal/engine`.
-
-Owners: inheritance operation construction in
-[`applyInheritanceScopeTemplates`](../../internal/engine/inheritance_schedule.go),
-[`appendInheritanceExplanation`](../../internal/engine/inheritance_explain.go),
-and transferred-origin projection in
-[`inheritance_history.go`](../../internal/engine/inheritance_history.go).
-
-Identified-record value construction excludes the parent's identity fields, but
-the retained history operation still supplies the complete parent record and
-its identity origins to the older mapping-inheritance replay machinery.
-
-Reproduction:
-
-```yaml
-configkit:
-  sequences: [{destination: [items], key: id}]
-items:
-  - id: base
-    value: parent
-  - id: child
-    extends: {id: base}
-```
-
-The effective child ID and origin are correct. However,
-`Explain(Keys("items").Index(1).Key("id"))` reports `child` → `base` → `child`,
-attributing the intermediate value to an inheritance operation and the parent's
-ID location. `Metadata.History` likewise claims an inherited parent-ID
-contribution. That identity is expressly excluded from inherited payload; it was
-never an applied value at the child path.
-
-Required remediation: make history replay and transferred-origin projection use
-the same identity exclusion as value construction. Preserve the child's authored
-identity and presence, ordinary inherited payload history, and the parent record's
-own source history at its original path. Cover compound identities and identity
-fields alongside inherited payload at both record and ancestor queries; keep the
-existing inspection API and sensitivity behavior.
-
-## 16. Avoid Deep-Copying Accumulated Item Policies Per Record — P2
-
-Status: [x] Complete.
-
-Completion: item projection now takes one owned policy copy per destination,
-updates that local state incrementally, and stores it once after projection.
-Incoming scopes still cross detached ownership boundaries. Inspection confirms
-that accumulated record scopes are no longer deep-copied per item; no timing
-threshold or benchmark campaign was needed. Existing stable-index, sparse-range,
-fallback-offset, and ownership coverage passes, with a new regression for
-interleaved destinations, distinct record owners, and sibling/cross-load mutation
-independence. Verified focused item-policy tests, `./scripts/check.sh`, and
-`go test -race ./...`.
-
-Owners: [`projectNestedCollectionPolicies`](../../internal/engine/imports.go)
-and its use of copying `collectionPolicyScope.at` / `set` helpers in
-[`collection_policy_state.go`](../../internal/engine/collection_policy_state.go).
-
-The prefix-scan correction in finding 11 is present, but a separate quadratic
-cost remains on the same independently authored-record path. For every imported
-item with local policy, `at` deep-clones the destination's accumulated record
-order and every retained record's nested scope. After adding one owner, `set`
-deep-clones the growing policy again. With N fixed-size item-local policies at
-one destination, this copies approximately `1 + 2 + ... + N` retained scopes,
-even when every assigned sequence range is available.
-
-This follows directly from the loop and `cloneCollectionPolicy`; no busy-host
-timing comparison is required. It is an integration cost from reusing detached
-read/write helpers within private incremental construction, rather than from
-the required final ownership copies.
-
-Required remediation: construct each destination's owned record-policy state
-without repeatedly cloning all earlier records, retaining necessary copies at
-ownership boundaries. Preserve record ordering, distinct owners, sparse assigned
-positions, fallback offsets, and cross-load detachment. Reuse the existing item
-policy and ownership tests; add allocation/scaling evidence only where useful.
-Do not introduce public caches, pooling, new caps, or performance thresholds.
-
-## 17. Reconcile Legacy Sequence-History Contract Text — P2
-
-Status: [x] Complete.
-
-Completion: metadata and explanation prose now describe history through the
-selected collection operation, assigned item positions, and displaced payload.
-The text distinguishes sequence snapshots and atomic diffs from append/upsert
-combination and links to the canonical operation tables. Whole-record upsert,
-existing queries, and sensitivity guarantees are unchanged. Verified against
-replay/combination code, local documentation-link tests, and `git diff --check`;
-the unchanged runtime is covered by the preceding everyday and full race checks.
-
-Owner: the metadata/provenance and explanation sections of
-[`docs/config.md`](../config.md#layered-metadata).
-
-The metadata section still says that a later tier supplying a sequence records
-replacement at the sequence path and that history does not imply identity-based
-merging across tiers. That describes the former unconditional replacement model,
-not the implemented append/upsert history or the collection operation tables in
-the same document. The explanation paragraph about atomic sequences also needs
-review so readers can distinguish snapshot/diff granularity from collection
-combination semantics.
-
-Required remediation: describe history for the selected collection operation and
-link to the canonical operation tables. Preserve the existing atomic sequence
-snapshot/diff behavior and whole-record upsert semantics; correcting the prose
-does not authorize recursive record-field merging, a new query API, or broader
-history guarantees.
-
-## Algorithm Audit Baseline And Scope
-
-Reviewed commit: `c73daf891d0f29c96f7026bdabcccb571f44a751`.
-Feature baseline: `4f941f8ba4c9a9ed0afa87b07fe6ba29f131d1c6`.
-
-This audit examines the sequence identity/operation, scoped-policy, inheritance,
-inspection, and post-decode CLI algorithms, including the completed corrections
-above. The intervening discovery roadmap commits add no implementation to audit.
-The accepted scheduler, deterministic ordering, exact numeric normalization,
-selector trie, and deliberately pairwise registration-overlap validation remain
-selected design choices; they are not findings here.
-
-Evidence and validation:
-
-- Code inspection establishes the complexity of findings 19–23 on bounded-depth
-  inputs with linear-sized results. Required result/history copies and genuinely
-  growing inheritance output are excluded from the alleged excess work. Bounds
-  describe internal operations, not new public performance guarantees.
-- An isolated public-API probe reproduces finding 18 using an in-memory
-  filesystem, without external services or real credentials.
-- Temporary tests supplied through a Go overlay, outside the repository, checked
-  all 768 combinations of append/upsert/replace over the 16 unique-ID lists drawn
-  from three IDs. Stable ordering, duplicate rejection, and whole-record payload
-  replacement agreed with an independent reference calculation.
-- A structural overlay test confirmed that 4, 8, and 16 one-item contributions
-  retain 10, 36, and 136 reverse-index cells respectively in `indexSequenceRanges`.
-  These are representation counts, not timing or real-world benchmark results.
-- `go test ./internal/engine -run 'Sequence|Collection|Inheritance|Override|SourceVersion|Metadata|History' -count=1`
-  passed with Go 1.26.10 on Linux. Passing existing tests does not cover the
-  incorrect selector scope or establish efficient growth.
-
-The following are focused corrections within the selected model. This review
-changes only the remediation record.
-
-## 18. Match Application Registrations In Effective Item Coordinates — P2
-
-Status: [ ] Open.
-
-Owner: `composePendingItems` in
-[`imports.go`](../../internal/engine/imports.go), especially its local
-`composeDocumentsWithCollectionPolicy` call; the current
-`TestPendingItemCompositionAppliesApplicationIdentityConstraints` in
-[`item_policy_test.go`](../../internal/engine/item_policy_test.go) also needs review.
-
-Pending item/helper documents are composed in item-local coordinates while the
-unchanged global application selector policy is supplied to their composer.
-A registration for root `[prompts]` therefore also constrains an unrelated
-`[items, 0, prompts]` collection before placement. This contradicts complete,
-effective-destination matching and makes an otherwise unused registration reject
-valid configuration. Application selectors do not traverse sequence indexes.
-
-Reproduction: main imports `item.yml` with destination `[items]` and placement
-`sequence-item`. The item contains `id: outer` and
-`prompts: [{text: payload}]`. Loading succeeds without a registration; adding
-only `SequenceIdentityAt(Keys("prompts"), "id")` fails with `compose.invalid`
-at `/items/0/prompts/0/id`, although no root `prompts` collection exists.
-
-Required remediation: separate item-local YAML policy from application matching
-in effective coordinates, including nested helper scopes. Preserve applicable
-outer collection constraints without reinterpreting global selectors as local
-ones. Add ordinary/layered regressions for unused root and dynamic selectors
-alongside imported item-local collections. Correct tests that pin the accidental
-scope; do not change selector meaning or require consumers to remove valid rules.
-
-## 19. Avoid Dense Global-Position Storage For Sparse Item Projections — P2
-
-Status: [ ] Open.
-
-Owners: `newSequenceItemProjectionIndex` in
-[`sequence_projection.go`](../../internal/engine/sequence_projection.go),
-`indexSequenceRange` / `projectedCompositionOrigins` in
-[`compose.go`](../../internal/engine/compose.go), and contribution indexing in
-[`history.go`](../../internal/engine/history.go).
-
-The reverse projection slice is sized to `maxOutput + 1`, even for one incoming
-item assigned to a late assembled position. `indexSequenceRange` materializes
-these indexes for ordinary contiguous append ranges too. With N one-item sources
-contributing to one sequence, reverse slices retain `1 + 2 + ... + N` cells.
-Initialization time and simultaneously retained auxiliary storage are therefore
-quadratic despite only N actual mappings. Origin/history projection constructs
-the same representation through additional callers. Finding 16's policy-copy
-correction does not remove this independent cost.
-
-Required remediation: retain arithmetic start/end lookup for contiguous ranges
-and use storage proportional to actual mappings for scattered positions, such as
-a sparse reverse map. Audit shared callers and avoid rebuilding equivalent indexes
-inside item queries. Preserve bidirectional lookup, absent positions, upsert
-replacement evidence, ordering, and detached results. Verify the representation
-on many one-item sources and noncontiguous upserts; add no timing threshold or
-collection-size limit.
-
-## 20. Remove Full Metadata Scans Per CLI Replacement Leaf — P2
-
-Status: [ ] Open.
-
-Owners: `updateOverridePresence`, `deletePresencePathSubtree`, and
-`updateOverrideOrigins` in
-[`override_decode.go`](../../internal/engine/override_decode.go).
-
-Every replaced scalar or subtree scans all presence keys, then all origin keys,
-to find matching prefixes. A single partial-mapping override updating N scalar
-children in an N-field configuration performs quadratic map-entry visits. This
-does not require N CLI sources or growing historical output: the same effective
-maps remain approximately N entries throughout the update.
-
-Required remediation: delete affected entries by walking the already known
-existing subtree, or use one batch-local prefix index where needed. Retain
-necessary copies at ownership boundaries without rescanning unrelated paths for
-each leaf. Preserve retained explicit zero/false values, removal of replaced
-descendants, null/kind changes, origin attribution, and detached loads. Use the
-existing nested CLI and ownership regressions plus focused broad-mapping coverage;
-do not change override semantics or impose batch limits.
-
-## 21. Localize Inheritance Policy Work To The Relevant Scope — P2
-
-Status: [ ] Open.
-
-Owners: `inheritancePolicyScopeAtPath` in
-[`inheritance.go`](../../internal/engine/inheritance.go), graph preparation in
-[`inheritance_schedule.go`](../../internal/engine/inheritance_schedule.go),
-and policy handoff/retirement in
-[`inheritance_walk.go`](../../internal/engine/inheritance_walk.go).
-
-Scope localization scans every entry in the supplied policy table for each
-definition. Mapping graph preparation also clones the full supplied table, and
-the walker passes its global table to each independent scope and scans that table
-again during replacement. For S scopes or siblings and P unrelated collection
-policies, these paths perform repeated S-by-P work and copies.
-
-For example, N independent mapping groups can each contain a constant-sized
-`base`/`child extends base` pair and one YAML collection declaration beneath its
-base. Input, expanded values, and relationship evidence remain linear in N, but
-each group's graph/localization visits or clones the other groups' policies.
-This is separate from necessary expansion along a genuinely growing parent chain
-and from the selected dependency scheduler.
-
-Required remediation: partition or index policies by semantic scope once and
-carry the relevant state through traversal, including efficient descendant
-retirement as inheritance introduces or removes destinations. Clone owned
-in-scope state rather than unrelated global state. Preserve record owners,
-pending declarations, mode lifetime, destination-local registration matching,
-and independent templates. Verify wide independent groups and nested item scopes;
-do not replace the agreed scheduling algorithm or add global caches.
-
-## 22. Index Fallback Location Lookup During Effective Projection — P2
-
-Status: [ ] Open.
-
-Owner: `documentWithMetadataLocations` in
-[`override_context.go`](../../internal/engine/override_context.go), using
-`nearestDocumentLocation` in
-[`validation.go`](../../internal/engine/validation.go).
-
-The new effective-projection walk calls the unindexed nearest-location helper
-for each field lacking a real origin location, often once for its mapping entry
-and again for its value. Each call starts at the fallback root and linearly scans
-mapping entries. With N authored root fields and N different default-only root
-fields, each default's failed lookup scans all N authored keys: quadratic work
-for linear-sized input and output. The shared lifecycle calls this path even
-without CLI assignments or a resolver.
-
-Required remediation: share an indexed fallback lookup or traverse corresponding
-containers together while carrying the nearest available location. Existing
-`documentLookup` / mapping-index machinery can support bounded-depth lookups
-without repeated wide scans. Preserve nearest-ancestor fallback, source IDs,
-synthetic locations, secret-reference diagnostics, and publication ownership.
-Exercise mixed authored/default-only broad mappings; do not weaken attribution
-or skip needed location attachment to avoid the work.
-
-## 23. Index Repeated Collection-Policy Document Lookups — P2
-
-Status: [ ] Open.
-
-Owners: `mergeOverlayMappingsWithCollectionPolicy` in
-[`overlay.go`](../../internal/engine/overlay.go) and
-`mergeInheritanceCollectionPolicies` in
-[`inheritance_collection.go`](../../internal/engine/inheritance_collection.go).
-
-These routines loop over collection policies/declarations and call plain
-`documentAtPath` for each destination. That helper linearly searches every
-mapping on the path. N independent collections below N sibling mapping keys
-therefore cause quadratic mapping-entry searches when checking which lower
-policies the incoming tier touches. The inheritance parent/child lookups and
-retained-value declaration checks have the same pattern. YAML declarations alone
-can establish these policies; the deliberately pairwise application-registration
-overlap check is not involved.
-
-Required remediation: reuse a document lookup built once for each stable input
-tree, or use the already collected claims where their scope is sufficient.
-Index a mutated result only after its value-changing phase, rather than retaining
-a stale lookup. Preserve absent destinations, non-sequence pending declarations,
-literal key/index distinctions, retirement, and error locations. Verify broad
-independent collections across precedence and inheritance; no new public cache,
-shape requirement, or performance gate is needed.

docs/roadmap/sequences.md

@@ -1,998 +0,0 @@
-# Sequence Identity And Combination Roadmap
-
-Status: settled feature roadmap; implementation is specified in the
-[staged implementation plan](implementation.md). This document defines the
-targeted end state, not an implemented contract. The implementation plan owns
-execution order, exact Go names and signatures, bookkeeping, and validation.
-
-## Purpose And Scope
-
-Allow configuration authors to assemble collections from independently authored
-records, declare identity in configuration, and progressively extend a baseline
-collection through higher layers. Authors can replace the entire collection,
-append distinct records, or replace complete records by identity while retaining
-unmatched records. Sibling inheritance uses the same sequence operations and
-identity-based defaults. Within an identified sequence, a record can explicitly
-inherit settings from another record in that sequence while keeping its own
-identity.
-
-The motivating use case is a prompt catalog whose source-backed baseline supplies
-records and whose application or installation layers add or supersede records.
-Applications can optionally impose identity constraints that configuration must
-respect and that hold in the final loaded configuration. These constraints can
-cover exact destinations or collections beneath dynamically named mapping keys,
-without requiring the application to discover those names before loading.
-
-The feature covers ordinary and layered loading, imported items, inline
-sequences, selected overlays, and sibling inheritance. Typed defaults, CLI
-assignments, and application resolution retain their own combination semantics;
-CLI application moves later in the loading lifecycle.
-
-Automatic recursive merging of matching records remains deferred
-(see [future work](future.md#1-recursive-body-merging-for-identified-records));
-explicit inheritance between distinct records is included. The initial
-declaration surface is `configkit.sequences`; `key` and `mode` shorthand on
-imports is deferred. Application lists remain
-ordinary YAML sequences without control wrappers. Unkeyed sequence concatenation
-is outside this feature.
-
-## Current Baseline And Required Changes
-
-The [import contract](../config.md#explicit-imports) owns existing selectors,
-destinations, placement, and ordering. `placement: sequence-item` already places
-each selected source mapping as one item at its effective destination. Item-local
-helper imports can contribute fields before placement.
-
-The [composition contract](../config.md#strict-document-composition),
-[layered-loading contract](../config.md#layered-loading), and
-[inheritance contract](../config.md#sibling-inheritance) describe the implemented
-baseline:
-
-| Area | Current behavior | Target behavior |
-| --- | --- | --- |
-| Identity declaration | `Definition.SequenceCompositions` registers one literal string field at each registered mapping path. | YAML collection declarations and optional authoritative application registrations support field sets and scalar identity components. |
-| Application registration destinations | Registrations select exact literal mapping paths. | Exact and structural selectors can impose identity beneath arbitrary mapping keys, independently at each matched destination. |
-| Strict composition | Registered sequences append distinct string identities and reject duplicates; repeated unregistered sequences conflict. | Collections with effective identity append distinct records and reject duplicates. Applicable lower-tier identity is available before composing higher-tier peers. Unidentified sequences retain strict-conflict behavior. |
-| Source precedence and selected overlays | A present higher sequence replaces the lower sequence wholesale. | Collection policy selects `replace`, `append`, or `upsert`; overlays can declare policy for inline sequences. |
-| Sibling inheritance | An inheriting mapping recursively overlays its resolved parent; supplied sequences replace wholesale. | Mappings retain recursive combination, while sequences use destination policy and the same three operations. |
-| Sequence-member inheritance | A sequence element has no sibling scope for its own inheritance relationship. | An identified sequence provides a sibling scope: a record can inherit from one record in the same sequence, selected by complete identity. |
-| CLI overrides | Apply before inheritance and typed defaults/decoding. | Apply after inheritance and typed defaults/decoding, before application resolution. |
-| Identity enforcement | Registration governs strict sequence composition. | Structural identity checks cover identified combinations; application registrations additionally constrain final effective sequences. |
-
-Source-backed underlays participate in source assembly. `Definition.Defaults`
-provides typed defaults later; extending source combination does not turn typed
-default lists into source underlays.
-
-## Collection Declarations
-
-Use a source-root `configkit.sequences` block containing collection declarations.
-Each entry identifies a `destination` and may declare `key`, `mode`, or both.
-Identity and mode can be supplied independently. This form serves imported items,
-inline records, ordinary mapping imports, and inline sequences in selected
-overlays.
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      key: [namespace, id]
-      mode: append
-  imports:
-    - dir: prompts
-      pattern: '*.yaml'
-      destination: [prompts]
-      placement: sequence-item
-```
-
-Each selected file contributes a record such as:
-
-```yaml
-namespace: support
-id: welcome
-text: Welcome
-```
-
-Here append retains a lower-layer collection and adds distinct higher-layer
-records. Identity and combination controls belong to the collection declaration,
-so imported and inline contributions share one policy surface.
-
-### Destinations And Scope
-
-`destination` uses the existing sequence of literal mapping keys. Resolve it
-relative to the declaring source's placement: a fragment mounted under `catalog`
-can declare `[prompts]` locally. A sequence-item source can declare a nested
-collection within its own item without referring to an eventual numeric index.
-Preserve source and item-local scope boundaries.
-
-A collection's scope is its resolved destination within its composition scope.
-Resolve relative destinations before reconciling declarations. Different imported
-items retain separate scopes for their nested collections. A declaration governs
-all contributions to its collection in the tier, including imported and inline
-records; it does not govern only records in the declaring file.
-
-Gather declarations before composing the relevant scope so their effect does not
-depend on which contribution is assembled first. Retain applicable controls
-through source precedence and inheritance, then keep them out of application
-decoding.
-
-Sequence declarations are independent of filesystem import enablement. Selected
-overlays may contain `configkit.sequences`, extending the current blanket
-prohibition on overlay `configkit` declarations. This does not enable imports
-in overlays.
-
-### Overlay Authoring
-
-For an existing sequence `[foo, bar]`, an overlay can replace `foo` in full:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      key: id
-      mode: upsert
-prompts:
-  - id: foo
-    text: Revised definition
-```
-
-The result is `[foo*, bar]`. The explicit upsert mode can be omitted when no
-different explicit mode applies from a lower tier: effective identity then
-selects upsert. To add a distinct record instead:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      key: id
-      mode: append
-prompts:
-  - id: baz
-    text: Another prompt
-```
-
-The result is `[foo, bar, baz]`. To replace the whole sequence:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      mode: replace
-prompts:
-  - id: bar
-    text: Bar definition
-  - id: baz
-    text: Baz definition
-```
-
-The result is `[bar, baz]`. Explicitly selecting the default operation is valid.
-With no applicable explicit mode, omitting this declaration selects replace for
-an unidentified collection and upsert for an identified one. A retained explicit
-mode continues to govern.
-
-## Identity Definitions And Constraints
-
-### Field Sets
-
-`key` accepts a string or a nonempty list of strings naming direct, literal
-fields in each record. A name such as `metadata.id` denotes that literal field,
-not a nested lookup. Treat field names as a set: remove repeated names and
-ignore authored order when comparing definitions.
-
-Thus `id`, `[id]`, and `[id, id]` are equivalent; `[namespace, id]`,
-`[id, namespace]`, and `[namespace, id, id]` are also equivalent. An empty list
-is invalid because it supplies no identity fields. These rules apply to both
-configuration declarations and application registrations.
-
-Use a consistent field order internally to construct identity tuples. Preserve
-component types and tuple boundaries so unstructured concatenation cannot cause
-otherwise distinct identities to collide.
-
-### Scalar Components And Equality
-
-Each identified record must be a mapping with every designated identity field
-present. Components may be strings, numbers, booleans, or explicit null. Missing
-fields and mapping or sequence components are invalid. Present null differs
-from absence; two single-field null identities are equal. Compound identities
-are equal when all corresponding components are equal.
-
-Strings compare exactly, including empty strings, without trimming, case folding,
-or normalization. Scalar categories remain distinct: `"1"`, `1`, `true`, and
-null are different identities.
-
-Numbers compare by mathematical value across integer, fractional, and exponent
-spellings: `1`, `1.0`, and `1e0` are equal, as are `0` and `-0`. Use exact numeric
-normalization for identity comparison without rounding through a floating-point
-conversion. This does not rewrite authored values or change existing typed
-decoding and projection semantics. The parser's rejection of non-finite numeric
-forms remains unchanged.
-
-### Application-Owned Constraints
-
-Retain an optional application interface for registering collections and imposing
-their identity definitions. Configuration may define identity where no application
-constraint exists. When a declaration omits `key`, an applicable registration
-supplies identity. An explicit equivalent definition is accepted, and a different
-definition fails under every mode, including replace.
-
-For example, if the application requires `id`, `key: [id, name]` is invalid:
-that compound identity would allow repeated `id` values with different names.
-A registration is an authoritative constraint, not an omission-filling default.
-Copying a collection through inheritance does not create another application
-registration; registrations constrain their registered destinations.
-
-Identity constraints validate actual sequences. They do not independently require
-a path to exist, be non-null, or have sequence shape. The application schema and
-validation own those requirements. Final-result enforcement for application
-registrations is defined under [Validation And Diagnostics](#validation-and-diagnostics).
-
-### Application Registration API
-
-Use one public sequence-identity registration type, collected in one field on
-`Definition`. Each registration contains an explicit destination selector and
-its identity fields. Exact and dynamic registrations share matching, overlap
-validation, identity comparison, and lifecycle behavior.
-
-Provide two constructors returning that same registration type:
-
-- An exact-path convenience constructor accepting a literal `Path` and identity
-  field names. It converts the path into a selector containing literal segments.
-- An explicit-selector constructor accepting a structural selector and identity
-  field names, for destinations that include arbitrary mapping keys.
-
-Both constructors accept identity fields as variadic strings, so one-field and
-compound identities use the same construction form. Apply the settled field-set
-rules, including deduplication. Invalid registrations, including an empty identity
-field set, fail during application-definition validation.
-
-Existing `Path` values retain their literal meaning throughout ConfigKit. The
-exact-path constructor supplies convenience at the registration boundary; dynamic
-matching uses the explicit selector type. Both forms can appear together in the
-same definition field.
-
-Replace the current registration shape with this model. Preserving the existing
-`SequenceComposition` fields and `Definition.SequenceCompositions` spelling is not
-a requirement, and no compatibility facade is needed solely to retain that syntax.
-Choose names centered on sequence identity, reflecting its role in composition,
-inheritance, and final validation. Exact Go type, field, and constructor names
-and signatures belong in the implementation plan.
-
-### Dynamic Application Registration Destinations
-
-Allow applications to register identity for collections whose containing mapping
-keys become known only during loading. For example:
-
-```yaml
-collections:
-  weatherreporter:
-    prompts:
-      - id: foo.bar
-        text: Report the weather.
-  homewatch:
-    prompts:
-      - id: foo.bar
-        text: Report the household status.
-```
-
-One application registration can require `id` for every namespace's `prompts`
-list. The equal IDs above are valid because each list has its own uniqueness
-scope. An optional root `prompts` list can have a separate exact registration.
-
-Use an explicit application-side structural selector composed of literal mapping
-keys and an arbitrary-mapping-key segment. Conceptually, the namespace rule is:
-
-```text
-Literal("collections") -> AnyMappingKey -> Literal("prompts")
-```
-
-This notation explains the selector; it is not a Go signature or YAML syntax.
-Each segment consumes exactly one mapping level. Match the complete destination,
-not an arbitrary prefix or suffix. Literal keys remain exact, including empty
-strings and keys containing punctuation such as `*`, `/`, or `.`. An arbitrary-key
-segment is distinct from any literal spelling and does not match a sequence
-index. Existing `Path` values retain their literal meaning.
-
-This capability belongs to the reusable application definition and applies to
-ordinary and layered loads. YAML collection destinations and import destinations
-retain their existing literal syntax. Recursive search, regular expressions,
-filesystem glob rules, sequence-index selectors, and custom matching callbacks
-are outside this addition.
-
-At each concrete matched destination, apply the same authoritative identity
-constraint as an exact registration. Use the settled field-set and scalar-equality
-rules. Do not pool identities across destinations or require records to repeat
-the containing mapping key as an identity component. Applications retain ownership
-of namespace interpretation, qualified runtime IDs, and domain validation.
-
-Matching supplies effective identity before relevant strict composition and
-precedence operations. Without an applicable explicit mode, this identity selects
-upsert under the existing default rule. It also supplies identity for inheritance
-within the matched sequence. YAML declarations must agree with it under every
-mode, including replace. These are ordinary application constraints with a broader
-destination selector, not a separate final-validation facility.
-
-### Registration Overlap
-
-Equivalent application constraints coalesce at any shared destination. Equivalence
-uses the identity field-set rules: for example, `id` and `[id, id]` agree.
-Incompatible constraints fail; exact registrations do not override broader
-selectors, and registration order grants no precedence.
-
-Detect incompatible overlapping registrations while validating the application
-definition, before source acquisition. For these fixed-depth mapping selectors,
-overlap exists when the selectors have the same number of segments and every
-corresponding pair can match a common key. Unequal literal keys at any position
-make the selectors disjoint; an arbitrary-key segment can overlap a literal or
-another arbitrary-key segment. Compare segment structure rather than rendered
-path strings. No configuration pre-read is needed.
-
-Thus an `id` rule for all namespace prompt lists conflicts with a `[id, name]`
-rule for one named namespace, even if that namespace is absent from a particular
-load. This is an invalid application definition. A valid selector matching no
-collections is harmless: registration does not require a path to exist, create
-collections, or impose shape and nullability requirements.
-
-### Matching Across The Loading Lifecycle
-
-Match application selectors against effective configuration destinations after
-accounting for input placement. Physical filenames and source-local paths do not
-determine applicability. Make constraints available before composing contributions
-at those destinations, including namespaces first introduced by imports, mounted
-inputs, or higher tiers. Apply matching constraints to key-only YAML declarations
-as well, even before their sequence exists. Consumers need neither a preliminary
-load nor source inspection to enumerate namespaces.
-
-During inheritance, apply registrations at the inheriting destination, including
-newly created matching destinations. Copying a collection does not transfer its
-application registrations to a different path. The existing YAML-policy ownership
-and transfer rules remain separate from application selector matching.
-
-During final validation, find matching actual sequences in the final effective
-configuration. This includes namespaces introduced or replaced by typed defaults,
-CLI overrides, and application resolution. Reusing only destinations found during
-source assembly would miss these values. Use the same selector meaning and
-identity validator in both phases; later assignments do not trigger composition
-or inheritance again. YAML-only identities retain their existing enforcement
-boundary.
-
-## Strict Composition And Precedence Operations
-
-### Compose Peers Before Applying Precedence
-
-Resolve applicable identity and mode policy, strictly compose the tier's peer
-contributions, then apply one selected operation per collection against the
-accumulated lower value. The tier operation does not sequentially replace peer
-source contributions. Selected overlay contributions use the same combination
-semantics.
-
-Within a strict tier, equivalent identity definitions coalesce and different
-field sets fail. Agreeing explicit modes coalesce; conflicting explicit modes
-fail. Omission expresses no preference. A root declaration or a later declaration
-has no precedence over conflicting peers. Explicitly selecting a default mode
-is still a real instruction and can conflict with another explicit mode.
-
-Identified peer sequences append distinct records. Duplicate identities among
-peers or within a contributed list fail under all three modes, including replace
-and upsert. Equal or disjoint record payloads do not excuse a duplicate identity.
-Unidentified sequences retain their current strict-composition conflict behavior.
-
-For example, higher-tier fragments `[C]` and `[D]` with an agreed identity assemble
-as `[C, D]`. Against lower `[A, B]`, append or upsert produces `[A, B, C, D]`,
-while replace produces `[C, D]`. Peers `[B]` and `[B*]` conflict even under
-upsert because neither has higher precedence.
-
-### Available Operations
-
-Use one spelling for each mode: `replace`, `append`, and `upsert`.
-
-| Mode | Behavior at source-precedence and inheritance boundaries |
-| --- | --- |
-| `replace` | Replace the complete lower sequence with the assembled higher sequence. |
-| `append` | Retain lower records and append higher records; duplicate identities fail. |
-| `upsert` | Replace matching records in full, retain unmatched lower records, and append new identities. |
-
-Append and upsert require effective identity. ConfigKit does not infer identity
-from an `id` field or record position. Whole-sequence replacement does not require
-identity agreement with lower YAML policy, but application constraints still apply.
-
-Letters denote record identities; `B*` has the same identity as `B` with a different
-payload:
-
-| Lower | Higher | Mode | Result |
-| --- | --- | --- | --- |
-| `[A, B]` | `[C]` | `append` | `[A, B, C]` |
-| `[A, B]` | `[C]` | `replace` | `[C]` |
-| `[A, B]` | `[B*]` | `append` | Duplicate-identity error |
-| `[A, B]` | `[B*]` | `replace` | `[B*]` |
-| `[A, B]` | `[B*]` | `upsert` | `[A, B*]` |
-| `[A, B]` | `[B*, C]` | `upsert` | `[A, B*, C]` |
-
-Upsert replaces the whole matched record. If lower `B` has `id`, `text`, and
-`temperature`, while higher `B` has only `id` and `text`, the result at this
-boundary has only `id` and `text`. Subsequent defaulting or other lifecycle
-operations retain their own semantics. Mappings continue to combine recursively;
-matching sequence records do not recursively merge fields.
-
-### Mode Selection And Persistence
-
-Select the operation in this order:
-
-1. Reconcile explicit modes within the current tier; conflicting peers fail.
-2. Use the current tier's explicit mode if supplied.
-3. Otherwise retain an applicable explicit mode from a lower tier.
-4. With no explicit mode, infer upsert when effective identity is available.
-5. Otherwise infer replace.
-
-Effective identity can come from an application registration, current declarations,
-or applicable lower policy, subject to the identity agreement and lifetime rules
-below. Determine the inferred mode from available identity before applying the
-operation. A plain higher-tier sequence can therefore use upsert without repeating
-a key declaration.
-
-An explicit mode persists at its applicable destination until a higher-priority
-explicit declaration changes it. New sequence values alone do not reset it, and
-an unrelated overlay leaves it intact. A mode-only declaration can change policy
-without supplying records; it neither clears the collection nor retroactively
-recombines earlier tiers. Later source contributions and inheritance use the
-resulting policy.
-
-| Lower explicit mode | Higher declaration | Effective mode |
-| --- | --- | --- |
-| `append` | Mode omitted | `append` |
-| `append` | `upsert` | `upsert` |
-| `upsert` | `replace` | `replace` |
-| None | Mode omitted, effective identity available | Inferred `upsert` |
-| None | Mode omitted, no effective identity | Inferred `replace` |
-
-Only explicit modes persist. Inferred defaults are recomputed from effective
-identity when needed. Explicit replace continues to apply in subsequent tiers
-until another mode is declared; introducing identity alone does not override it.
-Mode persistence does not preserve YAML identity discarded by replacement, nor
-policies owned by discarded records. Source precedence and inheritance use the
-same destination policy, without separate per-phase settings. CLI assignments
-remain outside mode selection.
-
-### Presence, Empty Sequences, Null, And Kind Mismatches
-
-At a collection's precedence boundary:
-
-| Higher contribution | `replace` | `append` / `upsert` |
-| --- | --- | --- |
-| Absent | Leave the lower value unchanged. | Leave the lower value unchanged. |
-| Explicit `[]` | Clear the sequence. | Contribute no records. |
-| Explicit null or another non-sequence value | Ordinary replacement, subject to application schema. | Error: the operation requires a sequence. |
-
-A declaration alone does not clear a collection. An optional import selecting
-nothing contributes nothing and does not erase existing records. An absent lower
-collection can be initialized by an incoming sequence under any mode, subject to
-identity requirements. Explicit lower null or another non-sequence value is not
-absence: append and upsert fail against it, while replacement remains available.
-An empty incoming sequence bypasses neither kind checks nor identity requirements.
-
-With effective identity and no explicit mode, `prompts: []` uses upsert and does
-not clear the collection. Clearing requires replace, selected in the current tier
-or retained from lower policy. Replacing an identified collection with null also
-requires replace and a schema permitting null. Collection null is distinct from
-a present null identity component inside a record.
-
-### Ordering
-
-Retain existing dependency-ready source scheduling within each tier and authored
-item order within lists. Gathering declarations does not change assembly order;
-identity values do not sort records.
-
-- Append keeps lower records in order, followed by higher records in assembled order.
-- Replace keeps only higher records in their assembled order.
-- Upsert keeps a matched record's lower position and appends new identities in
-  higher-tier order.
-
-For example, lower `[A, B, C]` and higher `[C*, A*, D]` produce `[A*, B, C*, D]`
-under upsert. Whole-sequence replacement gives the higher tier control of the
-complete sequence order.
-
-## Policy Agreement, Lifetime, And Ownership
-
-### Identity Agreement And Replacement
-
-Append and upsert use one agreed identity for all participating records:
-
-| Lower identity | Higher declaration | Result when combining records |
-| --- | --- | --- |
-| `id` | Omitted | Use `id`. |
-| `id` | `id`, `[id]`, or `[id, id]` | Accept the equivalent definition. |
-| `id` | `[namespace, id]` | Error: different definitions. |
-| None | `id` | Establish `id` and validate all participating records. |
-
-Introducing identity for an unkeyed lower sequence requires checking both lower
-and incoming records, including uniqueness within each contribution. With no
-identity from the application or either participating collection, append and
-upsert fail.
-
-YAML identity follows the collection that survives an operation:
-
-| Operation | Effective YAML identity |
-| --- | --- |
-| Append or upsert | Retain or establish the agreed identity. |
-| Whole-sequence replacement | Discard lower YAML identity and use the replacing tier's identity declaration, if any. |
-| Replacement by null or another non-sequence value | Discard lower YAML identity and retain any identity explicitly declared by the replacing tier. |
-
-Application constraints remain authoritative under every operation. Without an
-application constraint, replacement can introduce a different identity, such as
-changing from `id` to `name`. Replacing a sequence without a new key declaration
-or application registration produces an unkeyed sequence. No identity-removal
-directive is needed.
-
-The reset occurs only when the collection value is actually replaced. Declaring
-replace without supplying a replacement does not authorize changing the identity
-of retained records. Explicit mode lifetime is separate from YAML identity
-lifetime.
-
-### Ancestor Replacement
-
-Replacing an enclosing mapping can remove a collection without replacing that
-collection directly. Apply the same identity and ownership rules to this removal:
-discard the removed collection's lower YAML identity, and discard policies owned
-by any removed records. Reintroducing the same path later does not restore those
-discarded policies.
-
-Independently declared explicit modes retain their existing persistence rules.
-Removing a descendant collection does not remove a mode whose declaring scope
-survives. Application registrations remain applicable at matching destinations,
-including destinations reintroduced later. Identity newly declared by the
-replacing tier follows the existing declaration and ownership rules.
-
-For example, a root-level declaration establishes `key: id` and `mode: append`
-for `[catalog, prompts]`. A higher tier replaces `catalog` with null, removing
-the prompts collection and its YAML identity. The independently declared append
-mode survives. If a later tier recreates `catalog.prompts`, append requires
-identity supplied by a new declaration or an application registration. Without
-that identity, loading fails; the later tier can instead explicitly choose
-replace. The replacement of `catalog` itself follows ordinary ancestor-value
-precedence and does not attempt to append null to the removed prompts list.
-
-### Declarations Without Sequence Values
-
-A key declaration without accompanying records applies immediately to an existing
-retained sequence: check agreement and validate its records, without recombining
-earlier tiers. For example, a higher tier that only declares
-`destination: [prompts], key: id` identifies and validates the retained lower
-`prompts` sequence.
-
-If no collection exists, retain the declaration in its owning scope for later
-contributions. An otherwise empty underlay can establish identity for main-tier
-fragments. An explicit identity declaration can also remain pending at a
-non-sequence destination; it does not independently impose a shape requirement.
-The presence and kind rules still determine whether an attempted operation fails.
-Item-local declarations retain their ownership boundaries.
-
-### Lower-Tier Identity During Strict Composition
-
-Resolve applicable identity before composing a higher tier's peers, using
-application registrations, available lower-tier policy, and declarations gathered
-within the higher tier's scope. Authors need not repeat a baseline key solely
-because the higher tier has multiple fragments.
-
-For example, an underlay establishes `key: id` for `prompts`, and two main-tier
-fragments contribute `[foo]` and `[bar]` without a key declaration. They strictly
-compose as `[foo, bar]` before the main tier combines with the accumulated lower
-collection. If both contribute `foo`, composition fails; upsert does not give
-precedence to peers.
-
-Lower YAML identity remains subject to agreement, replacement, and ownership.
-A replacing collection must not be constrained by obsolete lower identity just
-because policy resolution precedes composition. Likewise, a replacement record
-cannot borrow nested identity owned by the discarded lower record, even when
-outer identity or numeric position matches. Application constraints still apply.
-Higher-tier declarations do not retroactively repair invalid lower-tier composition.
-
-### Nested Record Ownership
-
-Attach item-local policies to their owning contributions so they follow surviving
-records independently of numeric indexes. Whole-record upsert discards the old
-record's local policies along with its fields. Whole-sequence replacement discards
-policies owned by removed records. A replacement record supplies its own local
-declarations; equal outer identity or final index does not transfer discarded
-policies.
-
-For example, a source imported as one record into `prompts` can declare:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [steps]
-      key: id
-      mode: append
-id: foo
-steps:
-  - id: prepare
-```
-
-The declaration governs `steps` inside that `foo` record. Retaining `foo` retains
-its local policy, including the explicit mode. Upserting a new `foo` discards the
-old `steps` and its declaration even though the outer record keeps its position.
-
-Policies independently applicable at a destination are not discarded merely
-because an individual record is replaced. Ownership determines which declarations
-are removed. Application registrations remain constraints at their registered
-destinations. Inheritance copies transfer identity and choose operations as
-specified next.
-
-## Sibling Inheritance
-
-Complete source-tier assembly before resolving inheritance. Retain existing
-mapping-sibling scopes and add identified-sequence scopes as described below.
-Both use the existing dependency-ready algorithm. At each inheritance combination,
-the resolved parent's collection is the lower value and the inheriting mapping's
-local collection is the higher value. Mappings combine recursively; upsert within
-an identified sequence remains whole-record replacement. Explicit record
-inheritance is a separate construction step after those replacements.
-
-Use the inheriting destination collection's policy: an applicable explicit mode,
-including one retained through source tiers, wins. Otherwise infer upsert with
-effective identity and replace without it. There is no separate default-append
-rule for inheritance. The same destination policy governs source-precedence and
-inheritance combinations.
-
-YAML identity follows a collection copied through inheritance, subject to agreement
-and replacement rules. A source collection's mode does not automatically become an
-instruction at a different inheriting destination. For example, source `steps`
-has `key: id` and `mode: append`; an inheriting mapping supplies local `steps`
-without an applicable explicit mode. The copied identity selects upsert there.
-An explicit destination append instead selects append. A matching destination key
-agrees; a different key conflicts when combining records, while explicit replace
-can establish new YAML identity subject to application constraints.
-
-For example:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [parent, items]
-      key: id
-child:
-  items:
-    - id: foo
-      value: A
-    - id: bar
-      value: B
-parent:
-  extends: child
-  items:
-    - id: baz
-      value: C
-```
-
-Despite the names, `parent` inherits from `child`. Identity at `[parent, items]`
-selects upsert, producing `[foo, bar, baz]`. If the local sequence instead supplies
-a revised `foo`, the result is `[foo*, bar]`. Append rejects that repeated identity;
-replace retains only the local sequence.
-
-The same ordering, presence, and kind rules apply at inheritance boundaries.
-An omitted local sequence retains the inherited sequence. Explicit `[]` clears it
-under replace and contributes no records under append or upsert. Retain controls
-until inheritance uses them, then exclude them from application decoding.
-
-### Inheritance Between Identified Sequence Records
-
-An identified sequence supplies a sibling scope for its records. Identity can
-come from an application registration or an applicable YAML declaration. A record
-may inherit from one other record in that same sequence using an `extends`
-mapping that selects the parent's complete identity:
-
-```yaml
-configkit:
-  sequences:
-    - destination: [prompts]
-      key: id
-prompts:
-  - id: base
-    model: standard
-    temperature: 0.5
-  - id: concise
-    extends: {id: base}
-    temperature: 0.2
-```
-
-The final `concise` record is:
-
-```yaml
-id: concise
-model: standard
-temperature: 0.2
-```
-
-The `base` record remains in the collection. Parents are ordinary records subject
-to the same decoding and validation as other records; there is no abstract
-template category.
-
-Use mapping-form references for both single and compound keys. With
-`key: [namespace, id]`, a reference can be
-`extends: {namespace: shared, id: base}`. Match components using the collection's
-field-set and scalar-equality rules, including mathematical numeric equality.
-`extends: {id: null}` selects a null-valued identity; `extends: null` cancels a
-relationship.
-
-Parent-reference mappings use ordinary strict mapping composition. Item-local
-helper imports may supply disjoint fields of a compound reference; overlapping
-fields conflict, even when equal. For example, a helper's
-`extends: {namespace: shared}` and the importing record's `extends: {id: base}`
-compose into `extends: {namespace: shared, id: base}`. This assembles one record;
-it does not permit strict peers to contribute duplicate record identities.
-
-Before parent lookup, require the surviving effective reference mapping to contain
-exactly the collection's identity fields, with supported scalar values. Missing
-or extra fields fail. Validate the completed reference during inheritance, rather
-than requiring each helper contribution to be independently complete. Superseded
-references remain historical evidence; earlier strict-composition conflicts
-remain errors.
-
-Every record must supply its complete own identity before inheritance. Preserve
-the child's identity fields and inherit only the parent's nonidentity payload;
-inheritance cannot supply a missing child identity. Child payload fields take
-precedence, mappings combine recursively, and nested sequences use the inheriting
-destination's collection policy. Consume the record's directive before decoding.
-
-Support one parent within the same identified sequence, including forward
-references and chains. Missing parents and cycles, including self-reference,
-fail the load. Cross-collection references, position-based references, multiple
-parents, and inheritance between unidentified sequence members are outside this
-scope. Existing string-form references between mapping siblings retain their
-meaning; they do not become shorthand for sequence identity references.
-
-Resolve relationships against records surviving source-tier assembly and selected
-overlays. A higher layer can replace a parent before inheritance runs; removing
-that parent makes a surviving reference fail. Whole-record upsert discards the
-old record's directive along with its payload. Omitting `extends` in the
-replacement record does not retain the displaced record's relationship. An
-explicit reference to the record's own identity is a cycle, not a way to access
-its lower-tier predecessor.
-
-Reuse Kahn's algorithm with the existing stable canonical semantic-pointer key
-for dependency-ready ordering. Build parent lookup from the sequence's effective
-identity, and preserve the assembled list order independently of resolution
-order. Enclosing relationships resolve before nested relationships. Parent
-templates retain nested instructions for evaluation at each inheriting
-destination, following the existing destination-local inheritance rule. Copied
-collection identity, destination operation selection, and record-owned policy
-transfer follow the rules above.
-
-Record inheritance runs in the existing structural phase, before typed defaults,
-decoding, CLI overrides, and application resolution. Those later stages do not
-trigger another inheritance pass. Preserve inherited presence, origin and
-history evidence, sensitivity, declaring resource contexts, and existing
-expansion budgets just as for mapping-sibling inheritance. Identity lookup must
-not expose identity values in diagnostics. Existing index-based inspection paths
-remain sufficient; no new identity-selector path API is needed.
-
-## Loading Lifecycle
-
-Use the following order:
-
-1. Assemble source tiers and selected overlays, resolving policy before each
-   tier's strict composition and applying collection operations at precedence
-   boundaries.
-2. Resolve sibling inheritance in mappings and identified sequences, then apply
-   typed defaults and decode the assembled configuration.
-3. Apply explicit CLI overrides to that configuration and check supplied values
-   against the application schema.
-4. Run application resolution against the overridden configuration.
-5. Resolve effective secret references, validate the final configuration including
-   application-registered identities, and publish it.
-
-### Typed Defaults And Application Resolution
-
-Source-backed underlays, including application-supplied mapping sources,
-participate in sequence combination. Typed `Definition.Defaults` remains the
-initial decoded state: supplied sequences replace default sequences. Source
-append does not append to a typed default list.
-
-Application resolution retains assignment and recorded-transformation semantics.
-ConfigKit does not automatically combine a callback's assigned sequence with its
-previous value. Application-registered identities receive final validation
-independently of how defaults, CLI overrides, or callbacks produce their values.
-
-### CLI Precedence
-
-CLI overrides are the highest-precedence input to application resolution.
-Supplied sequences replace whole sequences, including sequences inside supplied
-mappings. Mappings retain recursive override behavior; empty sequences and
-permitted nulls retain explicit replacement meaning. Configuration append or
-upsert does not reinterpret CLI assignments. Inheritance and defaulting do not
-run again afterward: overriding assembled `[A, B]` with `[C]` presents `[C]` to
-application resolution without inheritance reintroducing prior records.
-
-Application resolution sees CLI values and their explicit-presence and source
-metadata. `Resolution.Default` preserves explicit CLI values; normalization and
-recorded application transformations may still change them. For example, a derived
-URL can use a CLI hostname, while an explicit CLI URL can be preserved by an
-absence-based default. Retain recording and provenance requirements for those
-transformations.
-
-Reject structural directives such as `extends` and `configkit` in CLI override
-paths or supplied values. CLI input supplies application values after structural
-processing; it cannot supply or cancel inheritance directives, change completed
-relationships, or prevent earlier inheritance failures.
-
-Typed decoding also precedes CLI application, so a CLI replacement cannot repair
-an earlier decoding failure. A source string in an integer field fails before a
-valid CLI integer can apply. This changes current override-before-decoding
-behavior. Semantic validation remains after overrides and application resolution.
-
-Secret lookup uses effective references after overrides and resolution. Neither
-stage bypasses final application identity constraints or semantic validation.
-
-## Validation And Diagnostics
-
-### Structural Identity Checks
-
-Allow item-local helper imports to complete and consume applicable non-main
-source-version headers before checking payload identity for collection assembly.
-A helper can supply an identity component; a consumed header cannot supply one
-after removal.
-
-Structural controls are likewise unavailable as source identity components.
-Check identity against application payload fields, excluding fields acting as
-structural directives even when the engine retains them for later processing.
-For example, `key: extends` with an item containing `extends: null` does not
-supply a null identity: that field cancels inheritance and is consumed. Report
-the ordinary missing-identity-component error at the collection/item path using
-available origins, without exposing values. Do not defer this failure until
-directive consumption or final application validation.
-
-This rule follows a field's structural role; it does not introduce a broader
-restriction on unrelated literal field names. Final application identity checks
-continue to examine the effective typed payload under their existing rules.
-
-Validate each identified contribution before combination and check uniqueness in
-the resulting identified collection, including inheritance operations. Upsert
-matches across precedence are replacements; duplicates within a contribution
-remain errors. Later inheritance, typed defaults, or callbacks cannot repair an
-identity missing at an earlier assembly boundary.
-
-YAML-only identities are enforced during structural composition, precedence
-assembly, and identified inheritance operations. They have no final-result
-guarantee after typed defaulting, CLI overrides, or application resolution. Those
-stages can produce duplicate identities or otherwise violate a YAML-only identity.
-Applications needing a final guarantee register the constraint through the
-application-owned interface.
-
-Adding record inheritance does not expand this guarantee: a YAML declaration
-can enable parent lookup without imposing final validation on values later
-produced by defaults, CLI overrides, or application resolution.
-
-### Final Application Constraints
-
-On successful loading, final effective collections covered by application
-registrations satisfy their declared identity constraints. Retain assembly-time
-checks and validate final effective sequences before publication, after defaults,
-inheritance, overrides, application resolution, and secret lookup.
-
-This covers both exact and dynamic application registrations. Evaluate dynamic
-selectors against the final effective configuration, with independent validation
-for each concrete matched sequence as described under
-[Matching Across The Loading Lifecycle](#matching-across-the-loading-lifecycle).
-
-ConfigKit performs these identity checks; consumers register the constraint
-without needing to repeat duplicate detection in `Definition.Validate`.
-Application-owned semantic validation continues to cover domain-specific rules.
-
-This guarantee covers actual sequences, not path existence or allowed shape.
-It does not require every intermediate callback state to satisfy the constraint
-and does not cover caller mutations after loading returns. Final checks examine
-effective typed values; assembly checks validate authored contributions. A final
-scalar alone does not prove that a source explicitly supplied that field.
-
-Use the existing canonical final-value representation and shared publication
-path as the foundation. The guarantee requires neither a new mutation-history
-system nor exact causal reconstruction for records supplied by defaults or
-callbacks.
-
-### Diagnostics And Inspection
-
-Identity diagnostics identify collection and item paths using available origin
-information without exposing identity values. Conflicting declarations identify
-the collection destination and declaration locations.
-
-For dynamic registrations, use concrete matched collection and item paths for
-value diagnostics, with available origins and existing sensitivity rules.
-Definition-time overlap errors identify the conflicting registrations; they do
-not require or invent a concrete destination from a loaded document.
-
-Correctness includes final values, projected item paths, source destinations,
-provenance, history, sensitivity, and passive resource contexts. Appending can
-change final indexes; replacing changes which contributions survive. Preserve
-the existing loading and inspection contracts through both operations, including
-policy ownership and inheritance transfer.
-
-## Work Needed From The Baseline
-
-The baseline needs the following connected capabilities to reach the target
-state. Execution order and test sequencing belong to the
-[implementation plan](implementation.md):
-
-- **Declarations and public API:** extend application registration to the settled
-  identity model with one public registration type and definition field, an
-  exact-path convenience constructor, and an explicit-selector constructor.
-  Preserve literal `Path` semantics. Parse and consume `configkit.sequences` across
-  supported source forms and selected overlays; preserve placement-relative and
-  item-local scopes. Keep sequence declarations independent of filesystem import
-  enablement.
-- **Identity and policy representation:** implement field-set comparison, typed
-  scalar identity tuples with exact numeric equality, declaration agreement,
-  pending declarations, explicit-mode persistence, replacement lifetime, and
-  record-owned policy transfer. Choose bookkeeping proportional to these semantics.
-  Handle descendant collection removal through ancestor replacement, preserving
-  independently declared modes and application constraints while discarding
-  removed collection identity and record-owned policies.
-- **Registration matching:** support literal and arbitrary-mapping-key segments,
-  detect incompatible selector overlap during definition validation, and resolve
-  applicable constraints at concrete placed destinations. Reuse the identity
-  rules and validator for exact and dynamic registrations. Keep matching separate
-  from source discovery and from YAML declaration ownership.
-- **Assembly handoffs:** make accumulated lower policy available before higher-tier
-  strict composition. Separate source acquisition from policy-aware composition
-  or otherwise revise assembly sequencing; current main-before-underlay composition
-  is not a design constraint. Preserve distinctions among acquisition order,
-  dependency-ready scheduling within tiers, and precedence order. Account for
-  source records, callbacks, main-version checks, and failure diagnostics. Retain
-  version ownership and validation semantics; higher replacement cannot hide
-  lower-tier structural failures.
-- **Combination and inheritance:** implement the three operations with strict peer
-  handling, whole-record replacement, presence rules, stable ordering, and the
-  destination policy rules for inheritance. Extend sibling-scope discovery and
-  parent lookup to identified sequences, validate mapping-form references, and
-  preserve child identities while copying parent payloads. Reuse the shared
-  dependency-ready scheduler and destination-local nested resolution. Preserve
-  provenance, history, sensitivity, paths, and resource contexts as records survive,
-  move, or disappear.
-- **Lifecycle and validation:** move CLI application after inheritance and typed
-  defaults/decoding, preserve explicit-presence and source metadata for application
-  resolution, reject CLI structural directives, and add final application identity
-  validation through the shared publication path. Keep YAML-only enforcement
-  within structural operations. Match dynamic registrations during assembly and
-  inheritance, and against the final effective configuration rather than a saved
-  list of source-backed destinations.
-  Exclude structural controls from source identity checks even while retaining
-  those controls for later inheritance processing.
-- **Documentation and verification:** exercise ordinary and layered loading,
-  imported and inline collections, item-local scopes, version-header consumption,
-  selected overlays, inheritance, typed defaults, CLI, callbacks, and inspection.
-  Cover single and compound parent references, scalar equality, forward references
-  and chains, missing parents and cycles, parent replacement/removal, child identity
-  preservation, nested destination policy, cancellation, and stable output order.
-  Verify helper-composed references, overlapping reference fields, missing or extra
-  fields, and replacement of references together with their containing records.
-  Verify independent namespace uniqueness, root and namespaced registrations,
-  literal punctuation keys, equivalent and incompatible overlaps, and conflicts
-  detected even without a matching input destination. Cover namespaces introduced
-  by imports, placement, higher tiers, inheritance, defaults, CLI, and callbacks;
-  preserve ordinary strict-peer and upsert behavior. Check YAML disagreement under
-  replace, harmless absent or empty collections, schema-owned shape requirements,
-  unrelated sequences remaining unselected, and concrete-path diagnostics.
-  Verify both constructors feed the same registration model, including mixed
-  exact and dynamic registrations, compound identities, and definition-time
-  rejection of empty identity field sets.
-  Cover ancestor replacement followed by collection reintroduction, including a
-  surviving append mode with and without renewed identity, explicit replacement,
-  and application-registered identity. Verify that `extends: null` cannot supply
-  an identity component and that structural-control exclusion uses ordinary
-  value-free missing-identity diagnostics.
-  Update the public contract and consumer examples when behavior lands.
-
-The collection-policy and CLI-ordering decisions are accepted in
-[ADR 0017](../adr/0017-collection-sequence-policy.md) for collection policy and
-[ADR 0018](../adr/0018-cli-after-structural-resolution.md) for CLI ordering.
-Their scoped supersession is recorded in the affected earlier ADRs, and the
-[architecture policy](../policy/architecture.md) reflects the accepted decisions.
-These govern the target architecture; the configuration contract continues to
-describe implemented behavior until the feature lands. This roadmap does not
-select migration machinery.
-
-ADR 0017 now includes inheritance within identified sequences;
-[ADR 0009](../adr/0009-sibling-inheritance.md) records that scope extension in its
-status while preserving its decision content. The architecture policy is aligned.
-Dynamic selectors extend the destinations covered by application registration;
-they retain the accepted application authority and final-validation boundary.
-
-No open feature decisions remain. The implementation plan translates this end
-state into reviewable stages without adding guarantees, restrictions, or
-combination semantics.

examples/consumers/discovery_test.go

@@ -0,0 +1,45 @@
+package consumers_test
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type workingDirectoryPresetConfig struct {
+	Name string `yaml:"name"`
+}
+
+func Example_workingDirectoryPreset() {
+	original, err := os.Getwd()
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer func() { _ = os.Chdir(original) }()
+	root, err := os.MkdirTemp("", "configkit-working-search-")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer os.RemoveAll(root)
+	if err := os.Chdir(root); err != nil {
+		fmt.Println(err)
+		return
+	}
+	if err := os.WriteFile(filepath.Join(root, "service.yml"), []byte("name: example\n"), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	result, err := configkit.Load[workingDirectoryPresetConfig](
+		configkit.SearchConfig("service", configkit.WorkingDirectory()),
+	)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(result.Config.Name, filepath.Base(result.Sources()[0].Path))
+	// Output: example service.yml
+}

examples/consumers/sequence_catalog.go

@@ -31,13 +31,23 @@ type SequenceCatalogEntry struct {
 	Credential  configkit.Secret `yaml:"credential"`
 }
 
-// LoadSequenceCatalog assembles imported main fragments over a file-backed
-// underlay, applies one selected collection overlay, then resolves and validates
-// the resulting application configuration.
+// LoadSequenceCatalog searches its caller-selected main path before a custom
+// fallback directory, assembles imported fragments over a file-backed underlay,
+// applies one selected collection overlay, then resolves and validates once.
 func LoadSequenceCatalog(mainPath, overlayName, runLabel string) (*configkit.Result[SequenceCatalogConfig], error) {
-	root := filepath.Dir(mainPath)
+	absoluteMain, err := filepath.Abs(mainPath)
+	if err != nil {
+		return nil, err
+	}
+	root := filepath.Dir(absoluteMain)
 	filesystem := configkit.NewFilesystem("sequence catalog", os.DirFS(root))
 	filesystemContext := &configkit.FilesystemContext{BaseDir: ".", Root: ".", FS: &filesystem}
+	mainSearch := configkit.SearchConfig("", configkit.Searches(
+		configkit.Paths(absoluteMain),
+		configkit.SearchDirectory(filepath.Join(root, "fallback"), configkit.DirectorySearchOptions{
+			Filenames: []configkit.FilenameGroup{configkit.CustomFilenames(filepath.Base(absoluteMain))},
+		}),
+	), configkit.SearchOptions{Required: true})
 	definition := configkit.Definition[SequenceCatalogConfig]{
 		SequenceIdentities: []configkit.SequenceIdentity{
 			configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id"),
@@ -51,8 +61,9 @@ func LoadSequenceCatalog(mainPath, overlayName, runLabel string) (*configkit.Res
 		Validate: validateSequenceCatalog,
 	}
 	return configkit.LoadLayered(configkit.Input{
-		Source:  filesystem.File(filepath.Base(mainPath)),
-		Imports: true,
+		Source:     mainSearch,
+		Imports:    true,
+		Filesystem: &configkit.FilesystemContext{Root: root},
 	}, configkit.LayeredOptions[SequenceCatalogConfig]{
 		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
 			Source: filesystem.File("underlay.yml"),

facade_discovery_test.go

@@ -0,0 +1,316 @@
+package configkit
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+type facadeDiscoveryConfig struct {
+	Value    string            `yaml:"value"`
+	Settings map[string]string `yaml:"settings"`
+}
+
+type facadeDiscoveryLifecycleConfig struct {
+	Value    string `yaml:"value"`
+	Resolved string `yaml:"resolved"`
+}
+
+type facadeDiscoveryPlacementConfig struct {
+	Items []facadeDiscoveryPlacementItem `yaml:"items"`
+}
+
+type facadeDiscoveryPlacementItem struct {
+	Name string `yaml:"name"`
+}
+
+func TestFacadeSearchPresetUsesCurrentWorkingDirectory(t *testing.T) {
+	original, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	root := t.TempDir()
+	if err := os.Chdir(root); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := os.Chdir(original); err != nil {
+			t.Errorf("restore working directory: %v", err)
+		}
+	})
+	writeFacadeImportFile(t, root, "myapp.yml", "value: working\n")
+
+	result, err := Load[facadeDiscoveryConfig](SearchConfig("myapp", WorkingDirectory()))
+	if err != nil || result.Config.Value != "working" {
+		t.Fatalf("working-directory search = %#v, %v", result, err)
+	}
+	if sources := result.Sources(); len(sources) != 1 || sources[0].Path != canonicalFacadePath(t, filepath.Join(root, "myapp.yml")) {
+		t.Fatalf("working-directory sources = %#v", sources)
+	}
+}
+
+func TestFacadeSearchCustomGenericNamesAndEmptyFilenameSelection(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	filename := writeFacadeImportFile(t, root, "config.yml", "value: found\n")
+	custom := SearchDirectory(root, DirectorySearchOptions{
+		Filenames: []FilenameGroup{CustomFilenames("config.yml")},
+	})
+	result, err := Load[facadeDiscoveryConfig](SearchConfig("", custom))
+	if err != nil || result.Config.Value != "found" {
+		t.Fatalf("custom generic-only search with empty slug = %#v, %v", result, err)
+	}
+	if sources := result.Sources(); len(sources) != 1 || sources[0].Path != canonicalFacadePath(t, filename) {
+		t.Fatalf("custom generic-only sources = %#v", sources)
+	}
+
+	defaults := func() facadeDiscoveryConfig { return facadeDiscoveryConfig{Value: "default"} }
+	empty := SearchDirectory(root, DirectorySearchOptions{Filenames: []FilenameGroup{}})
+	absent, err := Load(SearchConfig("myapp", empty), Options[facadeDiscoveryConfig]{
+		Definition: Definition[facadeDiscoveryConfig]{Defaults: defaults},
+	})
+	if err != nil || absent.Config.Value != "default" || len(absent.Sources()) != 0 {
+		t.Fatalf("explicit empty filename selection = %#v, %v", absent, err)
+	}
+	omitted, err := Load[facadeDiscoveryConfig](SearchConfig("myapp", SearchDirectory(root)))
+	if err != nil || omitted.Config.Value != "found" {
+		t.Fatalf("omitted filename selection did not use defaults = %#v, %v", omitted, err)
+	}
+}
+
+func TestFacadeSearchOrderingRequirednessAndSnapshots(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	selectedPath := writeFacadeImportFile(t, root, "config.yml", "value: selected\n")
+	laterPath := writeFacadeImportFile(t, root, "later.yml", "value: later\n")
+	paths := []string{filepath.Join(root, "missing.yml")}
+	names := []string{"config.yml"}
+	groups := []FilenameGroup{CustomFilenames(names...)}
+	directoryOptions := []DirectorySearchOptions{{Filenames: groups}}
+	search := Searches(Paths(paths...), SearchDirectory(root, directoryOptions...), Paths(laterPath))
+	source := SearchConfig("", search, SearchOptions{Required: true})
+
+	paths[0] = laterPath
+	names[0] = "later.yml"
+	groups[0] = CustomFilenames("later.yml")
+	directoryOptions[0].Filenames[0] = CustomFilenames("later.yml")
+	search = Paths(laterPath)
+
+	result, err := Load[facadeDiscoveryConfig](source)
+	if err != nil || result.Config.Value != "selected" {
+		t.Fatalf("ordered required search after caller mutation = %#v, %v", result, err)
+	}
+	if sources := result.Sources(); len(sources) != 1 || sources[0].Path != canonicalFacadePath(t, selectedPath) {
+		t.Fatalf("ordered search sources = %#v", sources)
+	}
+
+	_, err = Load[facadeDiscoveryConfig](SearchConfig("", Paths(filepath.Join(root, "missing.yml")), SearchOptions{Required: true}))
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("required exhausted search error = %v, want *LoadError", err)
+	}
+	foundMissing := false
+	for _, diagnostic := range loadErr.Diagnostics() {
+		foundMissing = foundMissing || diagnostic.Code == CodeSourceMissing
+	}
+	if !foundMissing {
+		t.Fatalf("required exhausted search diagnostics = %#v", loadErr.Diagnostics())
+	}
+}
+
+func TestFacadeSearchWorksForLayeredMainAndUnderlay(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	mainPath := writeFacadeImportFile(t, root, "main.yml", "settings: {main: value}\n")
+	underlayPath := writeFacadeImportFile(t, root, "underlay/base.yml", "settings: {underlay: value}\n")
+	result, err := LoadLayered(
+		Input{Source: SearchConfig("", Paths(mainPath))},
+		LayeredOptions[facadeDiscoveryConfig]{Underlays: []Underlay{{Inputs: []Input{{
+			Source: SearchConfig("", SearchDirectory(filepath.Dir(underlayPath), DirectorySearchOptions{
+				Filenames: []FilenameGroup{CustomFilenames("base.yml")},
+			})),
+		}}}}},
+	)
+	if err != nil || !reflect.DeepEqual(result.Config.Settings, map[string]string{"main": "value", "underlay": "value"}) {
+		t.Fatalf("layered search = %#v, %v", result, err)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Path != canonicalFacadePath(t, mainPath) || sources[0].Layer != LayerBase ||
+		sources[1].Path != canonicalFacadePath(t, underlayPath) || sources[1].Layer != LayerUnderlay {
+		t.Fatalf("layered search sources = %#v", sources)
+	}
+}
+
+func TestFacadeSearchSelectedAndAbsentUseRootOnlyContext(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	mainPath := writeFacadeImportFile(t, root, "main.yml", "value: main\n")
+	writeFacadeImportFile(t, root, "overlays/testing.yml", "value: overlay\n")
+	filesystem := &FilesystemContext{Root: root}
+	options := Options[facadeDiscoveryConfig]{Overlay: "testing", Filesystem: filesystem}
+
+	selected, err := Load(SearchConfig("", Paths(mainPath)), options)
+	if err != nil || selected.Config.Value != "overlay" {
+		t.Fatalf("selected search with root-only context = %#v, %v", selected, err)
+	}
+	sources := selected.Sources()
+	if len(sources) != 2 || sources[0].Path != canonicalFacadePath(t, mainPath) || sources[0].ID != "source-1" ||
+		sources[1].ID != "source-2" || sources[1].Layer != LayerOverlay {
+		t.Fatalf("selected search source evidence = %#v", sources)
+	}
+	mainContext, ok := selected.SourceContext("source-1")
+	if !ok || mainContext.BaseDir != "." {
+		t.Fatalf("selected file base context = %#v, %v", mainContext, ok)
+	}
+
+	missing := SearchConfig("", Paths(filepath.Join(root, "missing.yml")))
+	searchedAbsent, err := Load(missing, options)
+	if err != nil {
+		t.Fatalf("exhausted search with root-only overlay context: %v", err)
+	}
+	explicitAbsent, err := Load(NoSource(), options)
+	if err != nil {
+		t.Fatalf("NoSource with root-only overlay context: %v", err)
+	}
+	if !reflect.DeepEqual(searchedAbsent.Config, explicitAbsent.Config) ||
+		!reflect.DeepEqual(searchedAbsent.Sources(), explicitAbsent.Sources()) {
+		t.Fatalf("searched absence differs from NoSource: %#v / %#v", searchedAbsent, explicitAbsent)
+	}
+	left, leftOK := searchedAbsent.Metadata().Lookup(Keys("value"))
+	right, rightOK := explicitAbsent.Metadata().Lookup(Keys("value"))
+	if leftOK != rightOK || !reflect.DeepEqual(left, right) || !leftOK {
+		t.Fatalf("searched absence metadata = %#v, %v; NoSource metadata = %#v, %v", left, leftOK, right, rightOK)
+	}
+	if sources := searchedAbsent.Sources(); len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Layer != LayerOverlay {
+		t.Fatalf("absent search created unexpected source evidence = %#v", sources)
+	}
+}
+
+func TestFacadeSearchRejectsSelectedContextWithoutFallback(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	inside := writeFacadeImportFile(t, root, "inside.yml", "value: fallback\n")
+	outsideRoot := t.TempDir()
+	outside := writeFacadeImportFile(t, outsideRoot, "outside.yml", "value: selected\n")
+	search := SearchConfig("", Paths(outside, inside))
+
+	_, err := Load(search, Options[facadeDiscoveryConfig]{Filesystem: &FilesystemContext{Root: root}})
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) || !hasFacadeDiagnostic(loadErr, CodeImportOutsideRoot) {
+		t.Fatalf("selected outside-root file error = %v, want containment failure", err)
+	}
+
+	_, err = Load(search, Options[facadeDiscoveryConfig]{Filesystem: &FilesystemContext{BaseDir: root, Root: root}})
+	if !errors.As(err, &loadErr) || !hasFacadeDiagnostic(loadErr, CodeOptionsInvalid) {
+		t.Fatalf("selected file with explicit base error = %v, want option failure", err)
+	}
+
+	native := NewFilesystem("native", fstest.MapFS{})
+	_, err = Load(search, Options[facadeDiscoveryConfig]{Filesystem: &FilesystemContext{FS: &native, BaseDir: ".", Root: "."}})
+	if !errors.As(err, &loadErr) || !hasFacadeDiagnostic(loadErr, CodeOptionsInvalid) {
+		t.Fatalf("selected file with native context error = %v, want option failure", err)
+	}
+}
+
+func TestFacadeSearchRunsCallbacksOnceAndKeepsInspectionWithSelectedFile(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	selectedPath := writeFacadeImportFile(t, root, "selected.yml", "value: selected\n")
+	writeFacadeImportFile(t, root, "later.yml", "value: later\n")
+	defaultsCalls, resolveCalls, validateCalls := 0, 0, 0
+	result, err := Load(SearchConfig("", Paths(selectedPath, filepath.Join(root, "later.yml"))), Options[facadeDiscoveryLifecycleConfig]{
+		Definition: Definition[facadeDiscoveryLifecycleConfig]{
+			Defaults: func() facadeDiscoveryLifecycleConfig {
+				defaultsCalls++
+				return facadeDiscoveryLifecycleConfig{}
+			},
+			Resolve: func(configuration *facadeDiscoveryLifecycleConfig, resolution *Resolution) error {
+				resolveCalls++
+				configuration.Resolved = "from:" + configuration.Value
+				resolution.Record(Keys("resolved"), ChangeNormalization, "derive from selected value", Keys("value"))
+				return nil
+			},
+			Validate: func(_ *facadeDiscoveryLifecycleConfig, _ *Validation) { validateCalls++ },
+		},
+	})
+	if err != nil || result.Config.Value != "selected" || result.Config.Resolved != "from:selected" {
+		t.Fatalf("search lifecycle result = %#v, %v", result, err)
+	}
+	if defaultsCalls != 1 || resolveCalls != 1 || validateCalls != 1 {
+		t.Fatalf("search callbacks ran defaults=%d resolve=%d validate=%d times", defaultsCalls, resolveCalls, validateCalls)
+	}
+	sources := result.Sources()
+	if len(sources) != 1 || sources[0].Path != canonicalFacadePath(t, selectedPath) || sources[0].ID != "source-1" {
+		t.Fatalf("selected source evidence = %#v", sources)
+	}
+	valueInfo, found := result.Metadata().Lookup(Keys("value"))
+	if !found || len(valueInfo.Origin.Locations) != 1 || valueInfo.Origin.Locations[0].SourceID != sources[0].ID {
+		t.Fatalf("selected value metadata = %#v, %v", valueInfo, found)
+	}
+	var rendered strings.Builder
+	if err := result.WriteEffective(&rendered); err != nil || !strings.Contains(rendered.String(), "value: selected") || strings.Contains(rendered.String(), "later") {
+		t.Fatalf("effective output = %q, %v", rendered.String(), err)
+	}
+}
+
+func TestFacadeSearchSequenceItemAndAbsentInputPlacement(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	selectedPath := writeFacadeImportFile(t, root, "item.yml", "name: selected\n")
+	options := Options[facadeDiscoveryPlacementConfig]{
+		Destination:   Keys("items"),
+		PlacementMode: PlacementModeSequenceItem,
+	}
+	selected, err := Load(SearchConfig("", Paths(selectedPath)), options)
+	if err != nil || len(selected.Config.Items) != 1 || selected.Config.Items[0].Name != "selected" {
+		t.Fatalf("selected sequence item = %#v, %v", selected, err)
+	}
+	if sources := selected.Sources(); len(sources) != 1 || sources[0].Destination.String() != "/items" || sources[0].PlacementMode != PlacementModeSequenceItem {
+		t.Fatalf("selected sequence-item source = %#v", sources)
+	}
+
+	defaults := func() facadeDiscoveryPlacementConfig {
+		return facadeDiscoveryPlacementConfig{Items: []facadeDiscoveryPlacementItem{}}
+	}
+	absent, err := Load(SearchConfig("", Paths(filepath.Join(root, "missing.yml"))), Options[facadeDiscoveryPlacementConfig]{
+		Definition:    Definition[facadeDiscoveryPlacementConfig]{Defaults: defaults},
+		Destination:   options.Destination,
+		PlacementMode: options.PlacementMode,
+	})
+	if err != nil || len(absent.Config.Items) != 0 || len(absent.Sources()) != 0 {
+		t.Fatalf("absent search created a sequence item: %#v, %v", absent, err)
+	}
+}
+
+func hasFacadeDiagnostic(err *LoadError, code Code) bool {
+	for _, diagnostic := range err.Diagnostics() {
+		if diagnostic.Code == code {
+			return true
+		}
+	}
+	return false
+}
+
+func canonicalFacadePath(t *testing.T, path string) string {
+	t.Helper()
+	absolute, err := filepath.Abs(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	canonical, err := filepath.EvalSymlinks(absolute)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return canonical
+}

facade_item_registration_scope_test.go

@@ -0,0 +1,69 @@
+package configkit_test
+
+import (
+	"fmt"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestFacadeItemScopesDoNotReinterpretGlobalRegistrations(t *testing.T) {
+	t.Parallel()
+	for _, layered := range []bool{false, true} {
+		for _, dynamic := range []bool{false, true} {
+			for _, missingOuterID := range []bool{false, true} {
+				t.Run(fmt.Sprintf("layered=%t/dynamic=%t/missing-outer-id=%t", layered, dynamic, missingOuterID), func(t *testing.T) {
+					payload := "prompts: [{text: helper}]\n"
+					unused := configkit.SequenceIdentityAt(configkit.Keys("prompts"), "id")
+					if dynamic {
+						payload = "catalog: {local: {prompts: [{text: helper}]}}\n"
+						unused = configkit.SequenceIdentityMatching(configkit.SequenceSelector{
+							configkit.SequenceLiteralKey("catalog"), configkit.SequenceAnyMappingKey(), configkit.SequenceLiteralKey("prompts"),
+						}, "id")
+					}
+					item := "configkit: {imports: [{path: helper.yml}, {path: child.yml, destination: [children], placement: sequence-item}]}\n"
+					if !missingOuterID {
+						item += "id: outer\n"
+					}
+					files := configkit.NewFilesystem("items", fstest.MapFS{
+						"main.yml": {Data: []byte("configkit: {imports: [{path: item.yml, destination: [items], placement: sequence-item}]}\n")},
+						"item.yml": {Data: []byte(item)}, "helper.yml": {Data: []byte(payload)},
+						"child.yml":        {Data: []byte("id: child\nconfigkit: {imports: [{path: child-helper.yml}]}\n")},
+						"child-helper.yml": {Data: []byte(payload)},
+					})
+					definition := configkit.Definition[map[string]any]{SequenceIdentities: []configkit.SequenceIdentity{
+						unused, configkit.SequenceIdentityAt(configkit.Keys("items"), "id"),
+					}}
+					var result *configkit.Result[map[string]any]
+					var err error
+					if layered {
+						result, err = configkit.LoadLayered(configkit.Input{Source: files.File("main.yml"), Imports: true}, configkit.LayeredOptions[map[string]any]{Definition: definition})
+					} else {
+						result, err = configkit.Load(files.File("main.yml"), configkit.Options[map[string]any]{Imports: true, Definition: definition})
+					}
+					if missingOuterID {
+						failure := requireFacadeLoadError(t, err)
+						if result != nil || len(failure.Diagnostics()) != 1 || failure.Diagnostics()[0].Code != configkit.CodeComposeInvalid || failure.Diagnostics()[0].Path.String() != "/items/0/id" {
+							t.Fatalf("outer constraint diagnostics = %#v", failure.Diagnostics())
+						}
+						return
+					}
+					if err != nil {
+						t.Fatal(err)
+					}
+					for _, record := range []configkit.Path{configkit.Keys("items").Index(0), configkit.Keys("items").Index(0).Key("children").Index(0)} {
+						path := record
+						if dynamic {
+							path = path.Key("catalog").Key("local")
+						}
+						info, found := result.Metadata().Lookup(path.Key("prompts").Index(0).Key("text"))
+						if !found || !info.Explicit {
+							t.Fatalf("nested helper payload lost at %s", path)
+						}
+					}
+				})
+			}
+		}
+	}
+}

internal/engine/collection_lookup_test.go

@@ -0,0 +1,98 @@
+package engine
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestOverlayBroadCollectionPoliciesAndRetainedDeclarations(t *testing.T) {
+	var baseBody, overlayBody strings.Builder
+	basePolicies := collectionPolicyScope{}
+	var declarations []sequenceDeclarationSpec
+	for index := range 64 {
+		name := fmt.Sprintf("group/%03d~", index)
+		fmt.Fprintf(&baseBody, "%s: {items: [{id: base}], retained: [{id: retained}], scalar: pending}\n", name)
+		fmt.Fprintf(&overlayBody, "%s: {items: [{id: overlay}], added: true}\n", name)
+		basePolicies.set(Keys(name, "items"), collectionPolicy{
+			identityFields: []string{"id"}, identityPresent: true,
+			explicitMode: sequenceModeAppend, explicitModePresent: true,
+		})
+		for _, key := range []string{"retained", "missing", "scalar"} {
+			declarations = append(declarations, sequenceDeclarationSpec{
+				destination: Keys(name, key), identityFields: []string{"id"}, identityPresent: true,
+				mode: sequenceModeAppend, modePresent: true,
+			})
+		}
+	}
+	base, diagnostics := composeDocuments(parseCompositionDocument(t, "base", baseBody.String()))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	base.collectionPolicies = basePolicies
+	assembly := newAssemblyAccumulator(base)
+	overlay := parseCompositionDocument(t, "overlay", overlayBody.String())
+	diagnostics = assembly.applyOverlayWithCollectionPolicy(overlay, nil, collectionPolicyScope{}, nil, LayerOverlay, sequencePolicy{}, declarations)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	lookup := buildDocumentLookup(assembly.current.root)
+	for index := range 64 {
+		name := fmt.Sprintf("group/%03d~", index)
+		items, found := documentAtPathWithLookup(assembly.current.root, Keys(name, "items"), lookup)
+		if !found || items.Kind != document.Sequence || len(items.Sequence) != 2 || items.Sequence[0].Mapping[0].Value.Text != "base" || items.Sequence[1].Mapping[0].Value.Text != "overlay" {
+			t.Fatalf("collection %s did not append in order", name)
+		}
+		for _, key := range []string{"retained", "missing", "scalar"} {
+			policy, found := assembly.current.collectionPolicies.at(Keys(name, key))
+			if !found || !policy.identityPresent || !policy.explicitModePresent || policy.explicitMode != sequenceModeAppend {
+				t.Fatalf("retained/pending declaration lost at %s/%s", name, key)
+			}
+		}
+		if _, found := documentAtPathWithLookup(assembly.current.root, Keys(name, "missing"), lookup); found {
+			t.Fatal("pending declaration created a value")
+		}
+		if origin := assembly.current.origins[Keys(name, "items").Index(1).Key("id").String()]; origin.Layer != LayerOverlay {
+			t.Fatal("appended item lost overlay attribution")
+		}
+	}
+}
+
+func TestInheritanceBroadCollectionsInOneDefinition(t *testing.T) {
+	var body strings.Builder
+	policies := collectionPolicyScope{}
+	for _, definition := range []string{"base", "child"} {
+		fmt.Fprintf(&body, "%s:\n", definition)
+		if definition == "child" {
+			body.WriteString("  extends: base\n")
+		}
+		for index := range 64 {
+			name := fmt.Sprintf("group/%03d~", index)
+			fmt.Fprintf(&body, "  %s: {items: [{id: %s}]}\n", name, definition)
+			policy := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+			if definition == "child" {
+				policy.explicitMode, policy.explicitModePresent = sequenceModeAppend, true
+			}
+			policies.set(Keys(definition, name, "items"), policy)
+			policies.set(Keys(definition, name, "pending"), collectionPolicy{identityFields: []string{"id"}, identityPresent: true})
+		}
+	}
+	root := parseCompositionDocument(t, "wide-collections", body.String())
+	result, diagnostics := expandInheritanceWithPolicies(root, nil, policies, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	lookup := buildDocumentLookup(result.root)
+	for index := range 64 {
+		name := fmt.Sprintf("group/%03d~", index)
+		items, found := documentAtPathWithLookup(result.root, Keys("child", name, "items"), lookup)
+		if !found || len(items.Sequence) != 2 || items.Sequence[0].Mapping[0].Value.Text != "base" || items.Sequence[1].Mapping[0].Value.Text != "child" {
+			t.Fatalf("inherited collection %s did not append in order", name)
+		}
+		if policy, found := result.collectionPolicies.at(Keys("child", name, "pending")); !found || !policy.identityPresent {
+			t.Fatalf("pending inherited policy lost at %s", name)
+		}
+	}
+}

internal/engine/compose.go

@@ -1,6 +1,10 @@
 package engine
 
-import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+import (
+	"maps"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
 
 // composedDocument is the private handoff between source assembly and later
 // precedence layers. Claims remain ordered by source and declaration traversal;
@@ -89,19 +93,18 @@ type sequenceRange struct {
 	projection  sequenceItemProjectionIndex
 }
 
-func (assigned sequenceRange) projections() []sequenceItemProjection {
-	if assigned.items != nil {
-		return append([]sequenceItemProjection{}, assigned.items...)
-	}
-	return sequenceItemProjections(assigned.end-assigned.start, assigned.start)
-}
-
 func (assigned sequenceRange) outputForInput(inputIndex int) (int, bool) {
 	if assigned.projection.outputByInput != nil {
 		return assigned.projection.outputForInput(inputIndex)
 	}
 	if assigned.items != nil {
-		return newSequenceItemProjectionIndex(assigned.items).outputForInput(inputIndex)
+		output, found := 0, false
+		for _, item := range assigned.items {
+			if item.inputIndex == inputIndex && inputIndex >= 0 && item.outputIndex >= 0 {
+				output, found = item.outputIndex, true
+			}
+		}
+		return output, found
 	}
 	if inputIndex < 0 || inputIndex >= assigned.end-assigned.start {
 		return 0, false
@@ -114,7 +117,13 @@ func (assigned sequenceRange) inputForOutput(outputIndex int) (int, bool) {
 		return assigned.projection.inputForOutput(outputIndex)
 	}
 	if assigned.items != nil {
-		return newSequenceItemProjectionIndex(assigned.items).inputForOutput(outputIndex)
+		input, found := 0, false
+		for _, item := range assigned.items {
+			if item.outputIndex == outputIndex && outputIndex >= 0 && item.inputIndex >= 0 {
+				input, found = item.inputIndex, true
+			}
+		}
+		return input, found
 	}
 	if outputIndex < assigned.start || outputIndex >= assigned.end {
 		return 0, false
@@ -123,8 +132,8 @@ func (assigned sequenceRange) inputForOutput(outputIndex int) (int, bool) {
 }
 
 func indexSequenceRange(assigned sequenceRange) sequenceRange {
-	if assigned.projection.outputByInput == nil && assigned.projection.inputByOutput == nil {
-		assigned.projection = newSequenceItemProjectionIndex(assigned.projections())
+	if assigned.items != nil && assigned.projection.outputByInput == nil && assigned.projection.inputByOutput == nil {
+		assigned.projection = newSequenceItemProjectionIndex(assigned.items)
 	}
 	return assigned
 }
@@ -459,8 +468,8 @@ func cloneSequenceRanges(ranges []sequenceRange) []sequenceRange {
 		if assigned.items != nil {
 			cloned[i].items = append([]sequenceItemProjection{}, assigned.items...)
 		}
-		cloned[i].projection.outputByInput = append([]int(nil), assigned.projection.outputByInput...)
-		cloned[i].projection.inputByOutput = append([]int(nil), assigned.projection.inputByOutput...)
+		cloned[i].projection.outputByInput = maps.Clone(assigned.projection.outputByInput)
+		cloned[i].projection.inputByOutput = maps.Clone(assigned.projection.inputByOutput)
 	}
 	return cloned
 }
@@ -502,9 +511,15 @@ func projectedCompositionOrigins(inputs []*document.Node, ranges []sequenceRange
 	origins := make(map[string]Origin)
 	sequencesBySource := make([][]sequenceHistoryContribution, len(inputs))
 	for _, assigned := range ranges {
-		sequencesBySource[assigned.sourceIndex] = append(sequencesBySource[assigned.sourceIndex], sequenceHistoryContribution{
-			path: assigned.path.clone(), mode: sequenceHistoryMapped, items: assigned.projections(),
-		})
+		contribution := sequenceHistoryContribution{
+			path: assigned.path.clone(), mode: sequenceHistoryAppend,
+			offset: assigned.start, length: assigned.end - assigned.start,
+		}
+		if assigned.items != nil {
+			contribution.mode = sequenceHistoryMapped
+			contribution.items = assigned.items
+		}
+		sequencesBySource[assigned.sourceIndex] = append(sequencesBySource[assigned.sourceIndex], contribution)
 	}
 	for sourceIndex, input := range inputs {
 		application := historyApplication{value: input, sequences: sequencesBySource[sourceIndex]}

internal/engine/discovery.go

@@ -0,0 +1,540 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// Search is an immutable ordered description of possible host-file inputs.
+// It does not access the filesystem until a later loading integration consumes it.
+type Search struct {
+	entries []searchEntry
+	issues  []string
+}
+
+// FilenameGroup is an immutable ordered group used by a directory search.
+type FilenameGroup struct {
+	kind   filenameGroupKind
+	names  []string
+	issues []string
+}
+
+// DirectorySearchOptions controls the filename groups used by SearchDirectory.
+// A nil Filenames slice selects the default groups; a non-nil empty slice selects none.
+type DirectorySearchOptions struct {
+	Filenames []FilenameGroup
+}
+
+// SearchOptions controls whether a later source consumption requires a match.
+type SearchOptions struct {
+	Required bool
+}
+
+// SearchConfig creates a source that selects the first matching host file when
+// consumed. Description and option errors are reported during loading.
+func SearchConfig(application string, search Search, supplied ...SearchOptions) Source {
+	captured := Search{entries: cloneSearchEntries(search.entries), issues: append([]string(nil), search.issues...)}
+	source := Source{kind: sourceSearch, search: &captured, searchApp: application}
+	if len(supplied) > 1 {
+		source.searchIssues = []string{"SearchConfig accepts at most one options value"}
+	} else if len(supplied) == 1 {
+		source.searchRequired = supplied[0].Required
+	}
+	return source
+}
+
+type filenameGroupKind uint8
+
+const (
+	filenameGroupEmpty filenameGroupKind = iota
+	filenameGroupNamed
+	filenameGroupGeneric
+	filenameGroupCustom
+)
+
+type searchEntryKind uint8
+
+const (
+	searchEntryPath searchEntryKind = iota + 1
+	searchEntryDirectory
+)
+
+type directoryBaseKind uint8
+
+const (
+	directoryBaseLiteral directoryBaseKind = iota
+	directoryBaseUnixLocal
+	directoryBaseUnixEtc
+	directoryBaseUserConfig
+	directoryBaseHome
+	directoryBaseWorking
+)
+
+type searchEntry struct {
+	kind       searchEntryKind
+	path       string
+	baseKind   directoryBaseKind
+	directory  string
+	appendSlug bool
+	slugPrefix string
+	filenames  []FilenameGroup
+}
+
+// NamedFilenames selects <application>.yml followed by <application>.yaml.
+func NamedFilenames() FilenameGroup {
+	return FilenameGroup{kind: filenameGroupNamed}
+}
+
+// GenericFilenames selects config.yml, config.yaml, main.yml, and main.yaml.
+func GenericFilenames() FilenameGroup {
+	return FilenameGroup{kind: filenameGroupGeneric}
+}
+
+// CustomFilenames selects literal filename components in the supplied order.
+func CustomFilenames(names ...string) FilenameGroup {
+	group := FilenameGroup{kind: filenameGroupCustom, names: append([]string(nil), names...)}
+	for _, name := range group.names {
+		if !validFilenameComponent(name) {
+			group.issues = append(group.issues, "custom filename must be a usable host filename component")
+		}
+	}
+	return group
+}
+
+// SearchDirectory searches the supplied directory using the selected filename groups.
+// With omitted options or nil Filenames, it selects Named then Generic filenames.
+func SearchDirectory(directory string, supplied ...DirectorySearchOptions) Search {
+	search := Search{}
+	if len(supplied) > 1 {
+		search.issues = append(search.issues, "directory search accepts at most one options value")
+	}
+	var groups []FilenameGroup
+	if len(supplied) == 0 || supplied[0].Filenames == nil {
+		groups = []FilenameGroup{NamedFilenames(), GenericFilenames()}
+	} else {
+		groups = cloneFilenameGroups(supplied[0].Filenames)
+	}
+	search.entries = append(search.entries, searchEntry{
+		kind: searchEntryDirectory, baseKind: directoryBaseLiteral,
+		directory: directory, filenames: groups,
+	})
+	return search
+}
+
+// Paths describes literal host-file candidates in the supplied order.
+func Paths(paths ...string) Search {
+	search := Search{entries: make([]searchEntry, 0, len(paths))}
+	for _, path := range append([]string(nil), paths...) {
+		search.entries = append(search.entries, searchEntry{kind: searchEntryPath, path: path})
+	}
+	return search
+}
+
+// Searches concatenates descriptions in order and snapshots their entries.
+func Searches(searches ...Search) Search {
+	result := Search{}
+	for _, search := range append([]Search(nil), searches...) {
+		result.entries = append(result.entries, cloneSearchEntries(search.entries)...)
+		result.issues = append(result.issues, search.issues...)
+	}
+	return result
+}
+
+// UnixSystemDirectories describes the conventional Unix system directories.
+func UnixSystemDirectories() Search {
+	namedGeneric := []FilenameGroup{NamedFilenames(), GenericFilenames()}
+	return Search{entries: []searchEntry{
+		{kind: searchEntryDirectory, baseKind: directoryBaseUnixLocal, appendSlug: true, filenames: cloneFilenameGroups(namedGeneric)},
+		{kind: searchEntryDirectory, baseKind: directoryBaseUnixEtc, appendSlug: true, filenames: cloneFilenameGroups(namedGeneric)},
+		{kind: searchEntryDirectory, baseKind: directoryBaseUnixEtc, filenames: []FilenameGroup{NamedFilenames()}},
+	}}
+}
+
+// UserConfigDirectory describes the Go user-configuration directory searches.
+func UserConfigDirectory() Search {
+	return Search{entries: []searchEntry{
+		{kind: searchEntryDirectory, baseKind: directoryBaseUserConfig, appendSlug: true, filenames: defaultFilenameGroups()},
+		{kind: searchEntryDirectory, baseKind: directoryBaseUserConfig, filenames: []FilenameGroup{NamedFilenames()}},
+	}}
+}
+
+// HomeDirectory describes the Go home-directory searches.
+func HomeDirectory() Search {
+	return Search{entries: []searchEntry{
+		{kind: searchEntryDirectory, baseKind: directoryBaseHome, appendSlug: true, slugPrefix: ".", filenames: defaultFilenameGroups()},
+		{kind: searchEntryDirectory, baseKind: directoryBaseHome, filenames: []FilenameGroup{NamedFilenames()}},
+	}}
+}
+
+// WorkingDirectory describes application-named files in the current directory.
+func WorkingDirectory() Search {
+	return Search{entries: []searchEntry{{
+		kind: searchEntryDirectory, baseKind: directoryBaseWorking,
+		filenames: []FilenameGroup{NamedFilenames()},
+	}}}
+}
+
+func defaultFilenameGroups() []FilenameGroup {
+	return []FilenameGroup{NamedFilenames(), GenericFilenames()}
+}
+
+func cloneFilenameGroups(groups []FilenameGroup) []FilenameGroup {
+	if groups == nil {
+		return nil
+	}
+	cloned := make([]FilenameGroup, len(groups))
+	for i, group := range groups {
+		cloned[i] = FilenameGroup{
+			kind: group.kind, names: append([]string(nil), group.names...),
+			issues: append([]string(nil), group.issues...),
+		}
+	}
+	return cloned
+}
+
+func cloneSearchEntries(entries []searchEntry) []searchEntry {
+	if entries == nil {
+		return nil
+	}
+	cloned := make([]searchEntry, len(entries))
+	for i, entry := range entries {
+		cloned[i] = entry
+		cloned[i].filenames = cloneFilenameGroups(entry.filenames)
+	}
+	return cloned
+}
+
+func validateSearch(search Search, application string) []Diagnostic {
+	var diagnostics []Diagnostic
+	for _, issue := range search.issues {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, issue))
+	}
+	slugNeeded := false
+	for _, entry := range search.entries {
+		switch entry.kind {
+		case searchEntryPath:
+			if entry.path == "" {
+				diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "search path must not be empty"))
+			}
+		case searchEntryDirectory:
+			if entry.baseKind == directoryBaseLiteral && entry.directory == "" {
+				diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "directory search path must not be empty"))
+			}
+			if entry.appendSlug {
+				slugNeeded = true
+			}
+			for _, group := range entry.filenames {
+				switch group.kind {
+				case filenameGroupEmpty, filenameGroupGeneric:
+				case filenameGroupNamed:
+					slugNeeded = true
+				case filenameGroupCustom:
+				default:
+					diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "filename group is invalid"))
+				}
+				for _, issue := range group.issues {
+					diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, issue))
+				}
+			}
+		default:
+			diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "search description is invalid"))
+		}
+	}
+	if slugNeeded && !validFilenameComponent(application) {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "application name must be a usable host filename component"))
+	}
+	return canonicalDiagnostics(diagnostics)
+}
+
+func validFilenameComponent(value string) bool {
+	if value == "" || value == "." || value == ".." || strings.IndexByte(value, 0) >= 0 {
+		return false
+	}
+	if filepath.IsAbs(value) || filepath.VolumeName(value) != "" || filepath.Base(value) != value {
+		return false
+	}
+	if runtime.GOOS == "windows" {
+		if !filepath.IsLocal(value) || strings.ContainsAny(value, `<>:"|?*`) || strings.HasSuffix(value, ".") || strings.HasSuffix(value, " ") {
+			return false
+		}
+		for _, character := range value {
+			if character < 32 {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+var errUnixSystemPresetUnavailable = errors.New("Unix system directories are unavailable on this platform")
+
+type discoveryDirectoryHelpers struct {
+	userConfigDir func() (string, error)
+	userHomeDir   func() (string, error)
+	workingDir    func() (string, error)
+	unixSupported func() bool
+}
+
+type searchCandidateIterator struct {
+	search      Search
+	application string
+	helpers     discoveryDirectoryHelpers
+	entryIndex  int
+	groupIndex  int
+	nameIndex   int
+	directory   string
+	dirReady    bool
+}
+
+func newSearchCandidateIterator(search Search, application string, helpers discoveryDirectoryHelpers) (*searchCandidateIterator, []Diagnostic) {
+	if diagnostics := validateSearch(search, application); len(diagnostics) != 0 {
+		return nil, diagnostics
+	}
+	defaults := defaultDiscoveryDirectoryHelpers()
+	if helpers.userConfigDir == nil {
+		helpers.userConfigDir = defaults.userConfigDir
+	}
+	if helpers.userHomeDir == nil {
+		helpers.userHomeDir = defaults.userHomeDir
+	}
+	if helpers.workingDir == nil {
+		helpers.workingDir = defaults.workingDir
+	}
+	if helpers.unixSupported == nil {
+		helpers.unixSupported = defaults.unixSupported
+	}
+	return &searchCandidateIterator{search: search, application: application, helpers: helpers}, nil
+}
+
+func defaultDiscoveryDirectoryHelpers() discoveryDirectoryHelpers {
+	return discoveryDirectoryHelpers{
+		userConfigDir: os.UserConfigDir,
+		userHomeDir:   os.UserHomeDir,
+		workingDir:    os.Getwd,
+		unixSupported: unixSystemPresetSupported,
+	}
+}
+
+// next expands one candidate at a time, resolving only the directory currently
+// reached. Directory filenames are visited before moving to the next directory.
+func (iterator *searchCandidateIterator) next() (string, bool, error) {
+	for {
+		if iterator.dirReady {
+			entry := iterator.search.entries[iterator.entryIndex]
+			for iterator.groupIndex < len(entry.filenames) {
+				group := entry.filenames[iterator.groupIndex]
+				count := filenameGroupLength(group)
+				if iterator.nameIndex < count {
+					name := filenameGroupAt(group, iterator.nameIndex, iterator.application)
+					iterator.nameIndex++
+					return filepath.Join(iterator.directory, name), true, nil
+				}
+				iterator.groupIndex++
+				iterator.nameIndex = 0
+			}
+			iterator.dirReady = false
+			iterator.entryIndex++
+		}
+		if iterator.entryIndex >= len(iterator.search.entries) {
+			return "", false, nil
+		}
+		entry := iterator.search.entries[iterator.entryIndex]
+		if entry.kind == searchEntryPath {
+			iterator.entryIndex++
+			path, err := iterator.absoluteCandidate(entry.path)
+			if err != nil {
+				return "", false, err
+			}
+			return path, true, nil
+		}
+		if !directoryHasCandidates(entry.filenames) {
+			iterator.entryIndex++
+			continue
+		}
+		directory, err := iterator.resolveDirectory(entry)
+		if err != nil {
+			return "", false, err
+		}
+		iterator.directory = directory
+		iterator.groupIndex = 0
+		iterator.nameIndex = 0
+		iterator.dirReady = true
+	}
+}
+
+func (iterator *searchCandidateIterator) absoluteCandidate(path string) (string, error) {
+	if filepath.IsAbs(path) {
+		return filepath.Clean(path), nil
+	}
+	// Drive-relative and drive-root-relative Windows paths need native
+	// resolution; joining a rooted path to the whole working directory changes it.
+	if filepath.VolumeName(path) != "" || runtime.GOOS == "windows" && len(path) > 0 && os.IsPathSeparator(path[0]) {
+		return filepath.Abs(path)
+	}
+	working, err := iterator.helpers.workingDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Clean(filepath.Join(working, path)), nil
+}
+
+func (iterator *searchCandidateIterator) resolveDirectory(entry searchEntry) (string, error) {
+	var base string
+	switch entry.baseKind {
+	case directoryBaseLiteral:
+		base = entry.directory
+	case directoryBaseUnixLocal:
+		if !iterator.helpers.unixSupported() {
+			return "", errUnixSystemPresetUnavailable
+		}
+		base = "/usr/local/etc"
+	case directoryBaseUnixEtc:
+		if !iterator.helpers.unixSupported() {
+			return "", errUnixSystemPresetUnavailable
+		}
+		base = "/etc"
+	case directoryBaseUserConfig:
+		var err error
+		base, err = iterator.helpers.userConfigDir()
+		if err != nil {
+			return "", err
+		}
+	case directoryBaseHome:
+		var err error
+		base, err = iterator.helpers.userHomeDir()
+		if err != nil {
+			return "", err
+		}
+	case directoryBaseWorking:
+		var err error
+		base, err = iterator.helpers.workingDir()
+		if err != nil {
+			return "", err
+		}
+	default:
+		return "", errors.New("directory search base is invalid")
+	}
+	if entry.appendSlug {
+		base = filepath.Join(base, entry.slugPrefix+iterator.application)
+	}
+	if entry.baseKind == directoryBaseLiteral && !filepath.IsAbs(base) {
+		return iterator.absoluteCandidate(base)
+	}
+	return filepath.Clean(base), nil
+}
+
+func directoryHasCandidates(groups []FilenameGroup) bool {
+	for _, group := range groups {
+		if filenameGroupLength(group) != 0 {
+			return true
+		}
+	}
+	return false
+}
+
+func filenameGroupLength(group FilenameGroup) int {
+	switch group.kind {
+	case filenameGroupNamed:
+		return 2
+	case filenameGroupGeneric:
+		return 4
+	case filenameGroupCustom:
+		return len(group.names)
+	default:
+		return 0
+	}
+}
+
+func filenameGroupAt(group FilenameGroup, index int, application string) string {
+	switch group.kind {
+	case filenameGroupNamed:
+		if index == 0 {
+			return application + ".yml"
+		}
+		return application + ".yaml"
+	case filenameGroupGeneric:
+		return [...]string{"config.yml", "config.yaml", "main.yml", "main.yaml"}[index]
+	case filenameGroupCustom:
+		return group.names[index]
+	default:
+		return ""
+	}
+}
+
+type discoveryHostProbe struct {
+	evalSymlinks func(string) (string, error)
+	stat         func(string) (fs.FileInfo, error)
+}
+
+func defaultDiscoveryHostProbe() discoveryHostProbe {
+	return discoveryHostProbe{evalSymlinks: filepath.EvalSymlinks, stat: os.Stat}
+}
+
+// resolveSearchInput associates a terminal candidate failure with the same file
+// evidence as ordinary acquisition. Exhaustion and base failures have no file.
+func resolveSearchInput(source Source, sourceID string, helpers discoveryDirectoryHelpers, probe discoveryHostProbe) (Source, *SourceInfo, []Diagnostic) {
+	selected, diagnostics := resolveSearchSource(source, source.search, source.searchApp, source.searchRequired, helpers, probe)
+	if len(diagnostics) == 0 || selected.kind != sourceFile {
+		return selected, nil, diagnostics
+	}
+	info := &SourceInfo{ID: sourceID, Label: selected.label, Path: selected.path, Layer: LayerBase, Kind: SourceKindFile}
+	for index := range diagnostics {
+		diagnostics[index].Primary = &Location{SourceID: sourceID}
+	}
+	return selected, info, diagnostics
+}
+
+// resolveSearchSource selects one ordinary file source. A nil description leaves
+// an existing source untouched; the selected file is read later by readSource.
+// A terminal candidate failure returns its file descriptor for error evidence.
+func resolveSearchSource(source Source, description *Search, application string, required bool, helpers discoveryDirectoryHelpers, probe discoveryHostProbe) (Source, []Diagnostic) {
+	if description == nil {
+		return source, nil
+	}
+	iterator, diagnostics := newSearchCandidateIterator(*description, application, helpers)
+	if len(diagnostics) != 0 {
+		return Source{}, diagnostics
+	}
+	defaults := defaultDiscoveryHostProbe()
+	if probe.evalSymlinks == nil {
+		probe.evalSymlinks = defaults.evalSymlinks
+	}
+	if probe.stat == nil {
+		probe.stat = defaults.stat
+	}
+	for {
+		candidate, ok, err := iterator.next()
+		if err != nil {
+			if errors.Is(err, errUnixSystemPresetUnavailable) {
+				return Source{}, []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "Unix system directory search is unavailable on this platform")}
+			}
+			return Source{}, []Diagnostic{classifySourceError(err, "")}
+		}
+		if !ok {
+			if required {
+				return Source{}, []Diagnostic{sourceDiagnostic(CodeSourceMissing, "", "required configuration search found no file")}
+			}
+			return NoSource(), nil
+		}
+		canonical, err := probe.evalSymlinks(candidate)
+		if err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				continue
+			}
+			return File(candidate), []Diagnostic{classifySourceError(err, "")}
+		}
+		if _, err := probe.stat(canonical); err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				continue
+			}
+			failed := File(candidate)
+			failed.path = canonical
+			return failed, []Diagnostic{classifySourceError(err, "")}
+		}
+		return File(candidate), nil
+	}
+}

internal/engine/discovery_load_test.go

@@ -0,0 +1,430 @@
+package engine
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"testing"
+)
+
+type discoveryLoadConfig struct {
+	Service struct {
+		Settings map[string]string `yaml:"settings"`
+	} `yaml:"service"`
+	Settings map[string]string `yaml:"settings"`
+	Value    string            `yaml:"value"`
+}
+
+func TestSearchLoadMatchesExplicitFileThroughImportsAndContext(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	mainPath := writeImportFile(t, root, "app.yml", `configkit:
+  imports: [{path: fragments/extra.yml}]
+settings: {main: selected}
+`)
+	importPath := writeImportFile(t, root, "fragments/extra.yml", "settings: {imported: value}\n")
+	search := SearchConfig("app", SearchDirectory(root))
+	options := Options[discoveryLoadConfig]{
+		Imports: true, Filesystem: &FilesystemContext{Root: root}, destination: Keys("service"),
+	}
+	searched, err := testLoad(search, options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicit, err := testLoad(File(mainPath), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertEquivalentDiscoveryLoads(t, searched.Config, searched.Inspection, explicit.Config, explicit.Inspection,
+		Keys("service", "settings", "main"), Keys("service", "settings", "imported"))
+
+	sources := searched.Sources()
+	if len(sources) != 2 || sources[0].Kind != SourceKindFile || sources[0].Path != canonicalTestPath(t, mainPath) || sources[1].Path != canonicalTestPath(t, importPath) {
+		t.Fatalf("searched source evidence = %#v", sources)
+	}
+}
+
+func TestSearchLayeredMainAndUnderlayMatchExplicitFiles(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	mainPath := writeImportFile(t, root, "main.yml", "settings: {main: value}\n")
+	underlayPath := writeImportFile(t, root, "underlay/base.yml", `configkit:
+  imports: [{path: extra.yml}]
+settings: {underlay: value}
+`)
+	importPath := writeImportFile(t, root, "underlay/extra.yml", "settings: {imported: value}\n")
+	mainSearch := SearchConfig("", SearchDirectory(root, DirectorySearchOptions{Filenames: []FilenameGroup{CustomFilenames("main.yml")}}))
+	underlaySearch := SearchConfig("", SearchDirectory(filepath.Join(root, "underlay"), DirectorySearchOptions{Filenames: []FilenameGroup{CustomFilenames("base.yml")}}))
+	load := func(main, underlay Source) (*Loaded[discoveryLoadConfig], error) {
+		return LoadLayered(
+			Input{Source: main, Imports: true}, testSecretBinding(),
+			LayeredOptions[discoveryLoadConfig]{Underlays: []Underlay{{Inputs: []Input{{Source: underlay, Imports: true}}}}},
+		)
+	}
+	searched, err := load(mainSearch, underlaySearch)
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicit, err := load(File(mainPath), File(underlayPath))
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertEquivalentDiscoveryLoads(t, searched.Config, searched.Inspection, explicit.Config, explicit.Inspection,
+		Keys("settings", "main"), Keys("settings", "underlay"), Keys("settings", "imported"))
+
+	sources := searched.Inspection.Sources()
+	if len(sources) != 3 || sources[0].ID != "source-1" || sources[0].Layer != LayerBase || sources[1].ID != "source-2" || sources[1].Layer != LayerUnderlay || sources[2].Path != canonicalTestPath(t, importPath) || sources[2].UnderlayIndex != 1 {
+		t.Fatalf("layered search source evidence = %#v", sources)
+	}
+}
+
+func TestOptionalSearchExhaustionMatchesNoSource(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	missing := filepath.Join(root, "missing.yml")
+	defaults := func() discoveryLoadConfig {
+		return discoveryLoadConfig{Value: "default"}
+	}
+	options := Options[discoveryLoadConfig]{
+		Definition: Definition[discoveryLoadConfig]{Defaults: defaults},
+		Imports:    true, Filesystem: &FilesystemContext{Root: root},
+	}
+	searched, err := testLoad(SearchConfig("", Paths(missing)), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicit, err := testLoad(NoSource(), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	assertEquivalentDiscoveryLoads(t, searched.Config, searched.Inspection, explicit.Config, explicit.Inspection, Keys("value"))
+	if len(searched.Sources()) != 0 {
+		t.Fatalf("optional exhausted search created sources: %#v", searched.Sources())
+	}
+}
+
+func TestSearchSelectedFileFailuresDoNotFallBack(t *testing.T) {
+	t.Parallel()
+
+	t.Run("malformed YAML", func(t *testing.T) {
+		first := writeImportFile(t, t.TempDir(), "first.yml", ": malformed\n")
+		writeImportFile(t, filepath.Dir(first), "later.yml", "value: valid\n")
+		loaded, err := testLoad[discoveryLoadConfig](SearchConfig("", Paths(first, filepath.Join(filepath.Dir(first), "later.yml"))), Options[discoveryLoadConfig]{})
+		if loaded != nil || err == nil {
+			t.Fatalf("malformed selected source = %#v, %v", loaded, err)
+		}
+	})
+
+	t.Run("version check", func(t *testing.T) {
+		root := t.TempDir()
+		first := writeImportFile(t, root, "first.yml", "version: rejected\n")
+		writeImportFile(t, root, "later.yml", "version: accepted\n")
+		var checked []string
+		loaded, err := testLoad[discoveryLoadConfig](SearchConfig("", Paths(first, filepath.Join(root, "later.yml"))), Options[discoveryLoadConfig]{
+			SourceVersionMode: sourceVersionModeIfPresent,
+			Definition: Definition[discoveryLoadConfig]{Version: &VersionRule{
+				Path: Keys("version"),
+				Check: func(version Version, _ *Validation) error {
+					checked = append(checked, version.Text)
+					if version.Text == "rejected" {
+						return errors.New("version rejected")
+					}
+					return nil
+				},
+			}},
+		})
+		if loaded != nil || err == nil || !reflect.DeepEqual(checked, []string{"rejected"}) {
+			t.Fatalf("version failure search = %#v, %v; checked %#v", loaded, err, checked)
+		}
+	})
+
+	t.Run("decode", func(t *testing.T) {
+		root := t.TempDir()
+		first := writeImportFile(t, root, "first.yml", "value: not-an-integer\n")
+		writeImportFile(t, root, "later.yml", "value: 3\n")
+		type config struct {
+			Value int `yaml:"value"`
+		}
+		loaded, err := testLoad[config](SearchConfig("", Paths(first, filepath.Join(root, "later.yml"))), Options[config]{})
+		if loaded != nil || err == nil {
+			t.Fatalf("decode failure search = %#v, %v", loaded, err)
+		}
+	})
+}
+
+func TestSearchProbeFailureRetainsFileEvidence(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	main := writeImportFile(t, root, "main.yml", "value: main\n")
+	underlay := writeImportFile(t, root, "underlay.yml", "settings: {first: underlay}\n")
+	bad := filepath.Join(main, "child.yml")
+	later := writeImportFile(t, root, "later.yml", "value: later\n")
+	missing := filepath.Join(root, "missing.yml")
+	for _, entry := range []struct {
+		name string
+		id   string
+		load func(Source) (*Loaded[discoveryLoadConfig], error)
+	}{
+		{"ordinary", "source-1", func(source Source) (*Loaded[discoveryLoadConfig], error) {
+			return Load(source, testSecretBinding(), Options[discoveryLoadConfig]{destination: Keys("settings")})
+		}},
+		{"underlay after acquired sources", "source-3", func(source Source) (*Loaded[discoveryLoadConfig], error) {
+			return LoadLayered(Input{Source: File(main)}, testSecretBinding(), LayeredOptions[discoveryLoadConfig]{
+				Underlays: []Underlay{{Inputs: []Input{
+					{Source: File(underlay)}, {Source: source, Destination: Keys("service", "settings")},
+				}}},
+			})
+		}},
+	} {
+		t.Run(entry.name, func(t *testing.T) {
+			_, err := entry.load(File(bad))
+			var explicit *LoadError
+			if !errors.As(err, &explicit) || len(explicit.Diagnostics()) != 1 {
+				t.Fatalf("explicit failed file = %v", err)
+			}
+			if explicit.Diagnostics()[0].Code == CodeSourceMissing {
+				t.Skip("host classifies a non-directory ancestor as absence; controlled probe tests cover access failures")
+			}
+			result, err := entry.load(SearchConfig("", Paths(missing, bad, later)))
+			var searched *LoadError
+			if result != nil || !errors.As(err, &searched) {
+				t.Fatalf("failed search = %#v, %v", result, err)
+			}
+			if !reflect.DeepEqual(searched.Sources(), explicit.Sources()) || !reflect.DeepEqual(searched.Diagnostics(), explicit.Diagnostics()) {
+				t.Fatalf("search failure differs from explicit file:\nsources %#v / %#v\ndiagnostics %#v / %#v", searched.Sources(), explicit.Sources(), searched.Diagnostics(), explicit.Diagnostics())
+			}
+			primary := searched.Diagnostics()[0].Primary
+			if primary == nil || primary.SourceID != entry.id {
+				t.Fatalf("failed candidate location = %#v, want %s", primary, entry.id)
+			}
+		})
+	}
+}
+
+func TestSearchValidationPrecedesCandidateProbing(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	first := writeImportFile(t, root, "first.yml", "value: selected\n")
+	_, err := testLoad[discoveryLoadConfig](SearchConfig("", Searches(Paths(first), Paths(""))), Options[discoveryLoadConfig]{})
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("invalid later search error = %v, want *LoadError", err)
+	}
+	if len(loadErr.Diagnostics()) == 0 || loadErr.Diagnostics()[0].Code != CodeOptionsInvalid {
+		t.Fatalf("invalid later search component diagnostics = %#v", loadErr.Diagnostics())
+	}
+
+	malformed := writeImportFile(t, t.TempDir(), "main.yml", ": malformed\n")
+	loaded, err := LoadLayered(
+		Input{Source: SearchConfig("", Paths(malformed))}, testSecretBinding(),
+		LayeredOptions[discoveryLoadConfig]{Underlays: []Underlay{{Inputs: []Input{{Source: SearchConfig("", Paths(""))}}}}},
+	)
+	if loaded != nil || !errors.As(err, &loadErr) {
+		t.Fatalf("invalid later underlay search = %#v, %v", loaded, err)
+	}
+	for _, diagnostic := range loadErr.Diagnostics() {
+		if diagnostic.Code != CodeOptionsInvalid {
+			t.Fatalf("layered validation acquired main before rejecting underlay: %#v", loadErr.Diagnostics())
+		}
+	}
+}
+
+func TestSearchContextCompatibilityUsesSelectedOutcome(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	first := writeImportFile(t, root, "first.yml", "value: first\n")
+	writeImportFile(t, root, "later.yml", "value: later\n")
+	search := SearchConfig("", Paths(first, filepath.Join(root, "later.yml")))
+	options := Options[discoveryLoadConfig]{Filesystem: &FilesystemContext{BaseDir: root, Root: root}}
+	_, err := testLoad[discoveryLoadConfig](search, options)
+	var loadErr *LoadError
+	if !errors.As(err, &loadErr) {
+		t.Fatalf("selected file with explicit base error = %v, want *LoadError", err)
+	}
+	if len(loadErr.Diagnostics()) == 0 || loadErr.Diagnostics()[0].Code != CodeOptionsInvalid {
+		t.Fatalf("selected file context diagnostics = %#v", loadErr.Diagnostics())
+	}
+}
+
+func TestSearchEmptyContextUsesSelectedOutcome(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	selected := writeImportFile(t, root, "selected.yml", "value: selected\n")
+	missing := filepath.Join(root, "missing.yml")
+	context := &FilesystemContext{}
+	for _, entry := range []struct {
+		name string
+		load func(Source) (*Loaded[discoveryLoadConfig], error)
+	}{
+		{"ordinary", func(source Source) (*Loaded[discoveryLoadConfig], error) {
+			return Load(source, testSecretBinding(), Options[discoveryLoadConfig]{Imports: true, Filesystem: context})
+		}},
+		{"layered main", func(source Source) (*Loaded[discoveryLoadConfig], error) {
+			return LoadLayered(Input{Source: source, Imports: true, Filesystem: context}, testSecretBinding(), LayeredOptions[discoveryLoadConfig]{})
+		}},
+		{"underlay", func(source Source) (*Loaded[discoveryLoadConfig], error) {
+			return LoadLayered(Input{Source: NoSource()}, testSecretBinding(), LayeredOptions[discoveryLoadConfig]{
+				Underlays: []Underlay{{Inputs: []Input{{Source: source, Imports: true, Filesystem: context}}}},
+			})
+		}},
+	} {
+		t.Run(entry.name, func(t *testing.T) {
+			explicit, err := entry.load(File(selected))
+			if err != nil {
+				t.Fatal(err)
+			}
+			searched, err := entry.load(SearchConfig("", Paths(missing, selected)))
+			if err != nil {
+				t.Fatal(err)
+			}
+			assertEquivalentDiscoveryLoads(t, searched.Config, searched.Inspection, explicit.Config, explicit.Inspection, Keys("value"))
+			if searched.Config.Value != "selected" {
+				t.Fatalf("selected value = %q", searched.Config.Value)
+			}
+			for _, source := range []Source{NoSource(), SearchConfig("", Paths(missing))} {
+				result, err := entry.load(source)
+				var failure *LoadError
+				if result != nil || !errors.As(err, &failure) || len(failure.Diagnostics()) != 1 || failure.Diagnostics()[0].Code != CodeOptionsInvalid {
+					t.Fatalf("absent input with empty context = %#v, %v", result, err)
+				}
+			}
+		})
+	}
+}
+
+func TestSearchSourceObservesFileChangesBetweenLoads(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	file := writeImportFile(t, root, "config.yml", "value: first\n")
+	source := SearchConfig("", SearchDirectory(root, DirectorySearchOptions{Filenames: []FilenameGroup{CustomFilenames("config.yml")}}))
+	load := func() string {
+		loaded, err := testLoad[discoveryLoadConfig](source, Options[discoveryLoadConfig]{})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return loaded.Config.Value
+	}
+	if value := load(); value != "first" {
+		t.Fatalf("first search value = %q", value)
+	}
+	if err := os.WriteFile(file, []byte("value: second\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if value := load(); value != "second" {
+		t.Fatalf("updated search value = %q", value)
+	}
+}
+
+func TestSearchSourceObservesUserConfigEnvironmentChanges(t *testing.T) {
+	setBase := func(directory string) string {
+		t.Helper()
+		switch runtime.GOOS {
+		case "windows":
+			t.Setenv("AppData", directory)
+		case "darwin", "ios":
+			t.Setenv("HOME", directory)
+		case "plan9":
+			t.Setenv("home", directory)
+		default:
+			t.Setenv("XDG_CONFIG_HOME", directory)
+		}
+		base, err := os.UserConfigDir()
+		if err != nil {
+			t.Fatal(err)
+		}
+		return base
+	}
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	firstBase := setBase(filepath.Join(t.TempDir(), "first"))
+	first := writeImportFile(t, firstBase, "app/app.yml", "value: first\n")
+	source := SearchConfig("app", UserConfigDirectory())
+	load := func() string {
+		loaded, err := testLoad[config](source, Options[config]{})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return loaded.Config.Value
+	}
+	if value := load(); value != "first" {
+		t.Fatalf("first user-config search value = %q (%s)", value, first)
+	}
+	secondBase := setBase(filepath.Join(t.TempDir(), "second"))
+	second := writeImportFile(t, secondBase, "app/app.yml", "value: second\n")
+	if value := load(); value != "second" {
+		t.Fatalf("updated user-config search value = %q (%s)", value, second)
+	}
+}
+
+func TestSearchSourceObservesWorkingDirectoryChanges(t *testing.T) {
+	original, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := os.Chdir(original); err != nil {
+			t.Errorf("restore working directory: %v", err)
+		}
+	})
+	firstDir := t.TempDir()
+	secondDir := t.TempDir()
+	writeImportFile(t, firstDir, "app.yml", "value: first\n")
+	writeImportFile(t, secondDir, "app.yml", "value: second\n")
+	source := SearchConfig("app", WorkingDirectory())
+	load := func() string {
+		loaded, err := testLoad[discoveryLoadConfig](source, Options[discoveryLoadConfig]{})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return loaded.Config.Value
+	}
+	if err := os.Chdir(firstDir); err != nil {
+		t.Fatal(err)
+	}
+	if value := load(); value != "first" {
+		t.Fatalf("first working-directory search value = %q", value)
+	}
+	if err := os.Chdir(secondDir); err != nil {
+		t.Fatal(err)
+	}
+	if value := load(); value != "second" {
+		t.Fatalf("updated working-directory search value = %q", value)
+	}
+}
+
+func assertEquivalentDiscoveryLoads[T any](t *testing.T, leftConfig T, left *Inspection, rightConfig T, right *Inspection, presencePaths ...Path) {
+	t.Helper()
+	if !reflect.DeepEqual(leftConfig, rightConfig) {
+		t.Fatalf("search config %#v differs from explicit file config %#v", leftConfig, rightConfig)
+	}
+	leftSources, rightSources := left.Sources(), right.Sources()
+	if !reflect.DeepEqual(leftSources, rightSources) {
+		t.Fatalf("search sources %#v differ from explicit file sources %#v", leftSources, rightSources)
+	}
+	leftMetadata, rightMetadata := left.Metadata(), right.Metadata()
+	for _, path := range presencePaths {
+		leftInfo, leftPresent := leftMetadata.Lookup(path)
+		rightInfo, rightPresent := rightMetadata.Lookup(path)
+		if leftPresent != rightPresent || !reflect.DeepEqual(leftInfo, rightInfo) {
+			t.Fatalf("search metadata at %s = %#v, %v; explicit metadata = %#v, %v", path.String(), leftInfo, leftPresent, rightInfo, rightPresent)
+		}
+	}
+	for _, source := range leftSources {
+		leftContext, leftOK := left.SourceContext(source.ID)
+		rightContext, rightOK := right.SourceContext(source.ID)
+		if leftOK != rightOK || leftContext.BaseDir != rightContext.BaseDir || (leftContext.FS != nil) != (rightContext.FS != nil) {
+			t.Fatalf("search context for %s = %#v, %v; explicit context = %#v, %v", source.ID, leftContext, leftOK, rightContext, rightOK)
+		}
+	}
+}

internal/engine/discovery_platform_other.go

@@ -0,0 +1,5 @@
+//go:build !aix && !android && !darwin && !dragonfly && !freebsd && !hurd && !illumos && !ios && !linux && !netbsd && !openbsd && !solaris
+
+package engine
+
+func unixSystemPresetSupported() bool { return false }

internal/engine/discovery_platform_unix.go

@@ -0,0 +1,5 @@
+//go:build aix || android || darwin || dragonfly || freebsd || hurd || illumos || ios || linux || netbsd || openbsd || solaris
+
+package engine
+
+func unixSystemPresetSupported() bool { return true }

internal/engine/discovery_test.go

@@ -0,0 +1,519 @@
+package engine
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestSearchDescriptionEmptyAndSlugRequirements(t *testing.T) {
+	t.Parallel()
+
+	if diagnostics := validateSearch(Search{}, ""); len(diagnostics) != 0 {
+		t.Fatalf("zero search diagnostics = %#v", diagnostics)
+	}
+	if diagnostics := validateSearch(Searches(), ""); len(diagnostics) != 0 {
+		t.Fatalf("empty concatenation diagnostics = %#v", diagnostics)
+	}
+	if diagnostics := validateSearch(Paths(), ""); len(diagnostics) != 0 {
+		t.Fatalf("empty path list diagnostics = %#v", diagnostics)
+	}
+	if diagnostics := validateSearch(Paths("relative/config.yml"), ""); len(diagnostics) != 0 {
+		t.Fatalf("literal path unexpectedly requires a slug: %#v", diagnostics)
+	}
+	generic := SearchDirectory("configs", DirectorySearchOptions{Filenames: []FilenameGroup{GenericFilenames()}})
+	if diagnostics := validateSearch(generic, ""); len(diagnostics) != 0 {
+		t.Fatalf("generic-only directory unexpectedly requires a slug: %#v", diagnostics)
+	}
+	if diagnostics := validateSearch(SearchDirectory("configs"), ""); len(diagnostics) == 0 || diagnostics[0].Code != CodeOptionsInvalid {
+		t.Fatalf("default named directory accepted an empty slug: %#v", diagnostics)
+	}
+	if diagnostics := validateSearch(UnixSystemDirectories(), "myapp"); len(diagnostics) != 0 {
+		t.Fatalf("valid preset slug diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestSearchDescriptionSnapshotsInputs(t *testing.T) {
+	t.Parallel()
+
+	paths := []string{"first.yml", "second.yml"}
+	pathSearch := Paths(paths...)
+	paths[0] = "changed.yml"
+
+	customNames := []string{"one.yml", "two.yml"}
+	group := CustomFilenames(customNames...)
+	customNames[0] = "changed.yml"
+	groups := []FilenameGroup{group}
+	options := DirectorySearchOptions{Filenames: groups}
+	directorySearch := SearchDirectory("configs", options)
+	groups[0] = GenericFilenames()
+	options.Filenames[0] = NamedFilenames()
+
+	searches := []Search{pathSearch, directorySearch}
+	combined := Searches(searches...)
+	searches[0] = Search{}
+
+	if got := combined.entries[0].path; got != "first.yml" {
+		t.Fatalf("snapshotted path = %q", got)
+	}
+	gotNames := combined.entries[2].filenames[0].names
+	if len(gotNames) != 2 || gotNames[0] != "one.yml" || gotNames[1] != "two.yml" {
+		t.Fatalf("snapshotted custom names = %#v", gotNames)
+	}
+	if diagnostics := validateSearch(combined, ""); len(diagnostics) != 0 {
+		t.Fatalf("snapshot description diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestDirectoryFilenameDefaultsAndExplicitEmpty(t *testing.T) {
+	t.Parallel()
+
+	omitted := SearchDirectory("configs")
+	if len(omitted.entries) != 1 || len(omitted.entries[0].filenames) != 2 || omitted.entries[0].filenames[0].kind != filenameGroupNamed || omitted.entries[0].filenames[1].kind != filenameGroupGeneric {
+		t.Fatalf("omitted groups = %#v", omitted.entries)
+	}
+	explicitNil := SearchDirectory("configs", DirectorySearchOptions{})
+	if len(explicitNil.entries[0].filenames) != 2 {
+		t.Fatalf("nil filename selection did not use defaults: %#v", explicitNil.entries[0].filenames)
+	}
+	explicitEmpty := SearchDirectory("configs", DirectorySearchOptions{Filenames: []FilenameGroup{}})
+	if explicitEmpty.entries[0].filenames == nil || len(explicitEmpty.entries[0].filenames) != 0 {
+		t.Fatalf("explicit empty selection lost its distinction: %#v", explicitEmpty.entries[0].filenames)
+	}
+	if len(validateSearch(explicitEmpty, "")) != 0 {
+		t.Fatal("explicit empty filename selection should need no slug")
+	}
+}
+
+func TestSearchDescriptionArityAndErrorConcatenation(t *testing.T) {
+	t.Parallel()
+
+	tooMany := SearchDirectory("configs", DirectorySearchOptions{}, DirectorySearchOptions{})
+	if diagnostics := validateSearch(tooMany, "app"); len(diagnostics) == 0 {
+		t.Fatal("multiple option values should be invalid")
+	}
+	invalid := Paths("")
+	combined := Searches(Paths("good.yml"), invalid)
+	if diagnostics := validateSearch(combined, ""); len(diagnostics) == 0 {
+		t.Fatal("concatenation lost a constructor error")
+	}
+	if diagnostics := validateSearch(Paths("good.yml", ""), ""); len(diagnostics) == 0 {
+		t.Fatal("later invalid path was hidden by an earlier component")
+	}
+}
+
+func TestSearchDescriptionPreservesFilenameGroupOrder(t *testing.T) {
+	t.Parallel()
+
+	groups := []FilenameGroup{GenericFilenames(), NamedFilenames(), CustomFilenames("override.yml")}
+	search := SearchDirectory("configs", DirectorySearchOptions{Filenames: groups})
+	got := search.entries[0].filenames
+	if len(got) != 3 || got[0].kind != filenameGroupGeneric || got[1].kind != filenameGroupNamed || got[2].names[0] != "override.yml" {
+		t.Fatalf("filename group order = %#v", got)
+	}
+	if got := SearchDirectory("configs", DirectorySearchOptions{Filenames: []FilenameGroup{CustomFilenames()}}).entries[0].filenames[0].names; len(got) != 0 {
+		t.Fatalf("empty custom group = %#v", got)
+	}
+}
+
+func TestFilenameComponentValidation(t *testing.T) {
+	t.Parallel()
+
+	for _, value := range []string{"", ".", "..", "nested/name", "bad\x00name"} {
+		if validFilenameComponent(value) {
+			t.Errorf("validFilenameComponent(%q) = true", value)
+		}
+	}
+	if runtime.GOOS == "windows" {
+		for _, value := range []string{"CON", "CONIN$", "CONOUT$", "name.", "bad:name", "bad\x01name"} {
+			if validFilenameComponent(value) {
+				t.Errorf("Windows component %q unexpectedly accepted", value)
+			}
+		}
+		// Device names with extensions depend on the Windows version. Match
+		// Go's platform-aware decision instead of imposing a blanket ban.
+		for _, value := range []string{"nul.txt", "COM¹.log", "LPT³.txt", "CON.extra.yml"} {
+			if got, want := validFilenameComponent(value), filepath.IsLocal(value); got != want {
+				t.Errorf("Windows component %q validity = %v, want Go's %v", value, got, want)
+			}
+		}
+	} else {
+		for _, value := range []string{"CONIN$", "CONOUT$", "nul.txt"} {
+			if !validFilenameComponent(value) {
+				t.Errorf("Windows component restriction applied to this host: %q", value)
+			}
+		}
+	}
+	for _, value := range []string{"name with spaces", "naïve"} {
+		if !validFilenameComponent(value) {
+			t.Errorf("host-valid component %q rejected", value)
+		}
+	}
+	if diagnostics := validateSearch(SearchDirectory("configs", DirectorySearchOptions{Filenames: []FilenameGroup{CustomFilenames("bad/name")}}), ""); len(diagnostics) == 0 {
+		t.Fatal("invalid custom filename was not retained for validation")
+	}
+}
+
+func TestSearchCandidatesPreserveDirectoryAndGroupOrder(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	helpers := discoveryDirectoryHelpers{workingDir: func() (string, error) { return root, nil }}
+	search := Searches(
+		Paths("first.yml"),
+		SearchDirectory("settings", DirectorySearchOptions{Filenames: []FilenameGroup{
+			GenericFilenames(), CustomFilenames("special.yml"),
+		}}),
+		Paths("last.yml"),
+	)
+	candidates := collectSearchCandidates(t, search, "app", helpers)
+	want := []string{
+		filepath.Join(root, "first.yml"),
+		filepath.Join(root, "settings", "config.yml"),
+		filepath.Join(root, "settings", "config.yaml"),
+		filepath.Join(root, "settings", "main.yml"),
+		filepath.Join(root, "settings", "main.yaml"),
+		filepath.Join(root, "settings", "special.yml"),
+		filepath.Join(root, "last.yml"),
+	}
+	assertCandidatePaths(t, candidates, want)
+
+	reversed := SearchDirectory("settings", DirectorySearchOptions{Filenames: []FilenameGroup{NamedFilenames(), GenericFilenames()}})
+	reversedCandidates := collectSearchCandidates(t, reversed, "app", helpers)
+	assertCandidatePaths(t, reversedCandidates[:2], []string{
+		filepath.Join(root, "settings", "app.yml"),
+		filepath.Join(root, "settings", "app.yaml"),
+	})
+}
+
+func TestSearchPresetCandidateLists(t *testing.T) {
+	t.Parallel()
+
+	helpers := discoveryDirectoryHelpers{
+		userConfigDir: func() (string, error) { return "/controlled/config", nil },
+		userHomeDir:   func() (string, error) { return "/controlled/home", nil },
+		workingDir:    func() (string, error) { return "/controlled/work", nil },
+		unixSupported: func() bool { return true },
+	}
+
+	unix := collectSearchCandidates(t, UnixSystemDirectories(), "app", helpers)
+	if len(unix) != 14 {
+		t.Fatalf("Unix preset candidate count = %d: %#v", len(unix), unix)
+	}
+	assertCandidatePaths(t, unix, []string{
+		"/usr/local/etc/app/app.yml", "/usr/local/etc/app/app.yaml", "/usr/local/etc/app/config.yml", "/usr/local/etc/app/config.yaml", "/usr/local/etc/app/main.yml", "/usr/local/etc/app/main.yaml",
+		"/etc/app/app.yml", "/etc/app/app.yaml", "/etc/app/config.yml", "/etc/app/config.yaml", "/etc/app/main.yml", "/etc/app/main.yaml",
+		"/etc/app.yml", "/etc/app.yaml",
+	})
+
+	user := collectSearchCandidates(t, UserConfigDirectory(), "app", helpers)
+	assertCandidatePaths(t, user, []string{
+		"/controlled/config/app/app.yml", "/controlled/config/app/app.yaml", "/controlled/config/app/config.yml", "/controlled/config/app/config.yaml", "/controlled/config/app/main.yml", "/controlled/config/app/main.yaml",
+		"/controlled/config/app.yml", "/controlled/config/app.yaml",
+	})
+	home := collectSearchCandidates(t, HomeDirectory(), "app", helpers)
+	assertCandidatePaths(t, home, []string{
+		"/controlled/home/.app/app.yml", "/controlled/home/.app/app.yaml", "/controlled/home/.app/config.yml", "/controlled/home/.app/config.yaml", "/controlled/home/.app/main.yml", "/controlled/home/.app/main.yaml",
+		"/controlled/home/app.yml", "/controlled/home/app.yaml",
+	})
+	working := collectSearchCandidates(t, WorkingDirectory(), "app", helpers)
+	assertCandidatePaths(t, working, []string{"/controlled/work/app.yml", "/controlled/work/app.yaml"})
+}
+
+func TestSearchCandidateExpansionIsLazyAndSkipsEmptyDirectories(t *testing.T) {
+	t.Parallel()
+
+	configCalls := 0
+	helpers := discoveryDirectoryHelpers{
+		userConfigDir: func() (string, error) {
+			configCalls++
+			return "/controlled/config", nil
+		},
+		workingDir:    func() (string, error) { return "/controlled/work", nil },
+		unixSupported: func() bool { return true },
+	}
+	search := Searches(Paths("/selected.yml"), UserConfigDirectory())
+	iterator, diagnostics := newSearchCandidateIterator(search, "app", helpers)
+	if len(diagnostics) != 0 {
+		t.Fatalf("iterator diagnostics = %#v", diagnostics)
+	}
+	if candidate, ok, err := iterator.next(); err != nil || !ok || candidate != "/selected.yml" {
+		t.Fatalf("first candidate = %q, %v, %v", candidate, ok, err)
+	}
+	if configCalls != 0 {
+		t.Fatalf("later user directory resolved before consumption: %d calls", configCalls)
+	}
+
+	emptyOnly := SearchDirectory("should-not-resolve", DirectorySearchOptions{Filenames: []FilenameGroup{}})
+	emptyCandidates := collectSearchCandidates(t, emptyOnly, "", helpers)
+	if len(emptyCandidates) != 0 || configCalls != 0 {
+		t.Fatalf("empty directory candidates = %#v, helper calls = %d", emptyCandidates, configCalls)
+	}
+	continued := collectSearchCandidates(t, Searches(emptyOnly, Paths("later.yml")), "", helpers)
+	assertCandidatePaths(t, continued, []string{filepath.Join("/controlled/work", "later.yml")})
+	if configCalls != 0 {
+		t.Fatalf("empty directory base was resolved: %d calls", configCalls)
+	}
+}
+
+func TestSearchCandidateBaseFailuresAndPlatformApplicability(t *testing.T) {
+	t.Parallel()
+
+	baseFailure := errors.New("controlled base failure")
+	helpers := discoveryDirectoryHelpers{
+		userConfigDir: func() (string, error) { return "", baseFailure },
+		workingDir:    func() (string, error) { return "/controlled/work", nil },
+		unixSupported: func() bool { return false },
+	}
+	iterator, diagnostics := newSearchCandidateIterator(UserConfigDirectory(), "app", helpers)
+	if len(diagnostics) != 0 {
+		t.Fatalf("user preset validation = %#v", diagnostics)
+	}
+	if _, ok, err := iterator.next(); ok || !errors.Is(err, baseFailure) {
+		t.Fatalf("unavailable user base = ok %v, err %v", ok, err)
+	}
+
+	iterator, diagnostics = newSearchCandidateIterator(UnixSystemDirectories(), "app", helpers)
+	if len(diagnostics) != 0 {
+		t.Fatalf("Unix preset validation = %#v", diagnostics)
+	}
+	if _, ok, err := iterator.next(); ok || !errors.Is(err, errUnixSystemPresetUnavailable) {
+		t.Fatalf("unsupported Unix preset = ok %v, err %v", ok, err)
+	}
+
+	iterator, diagnostics = newSearchCandidateIterator(Searches(Paths("/earlier.yml"), UnixSystemDirectories()), "app", helpers)
+	if len(diagnostics) != 0 {
+		t.Fatalf("composed Unix preset validation = %#v", diagnostics)
+	}
+	if candidate, ok, err := iterator.next(); err != nil || !ok || candidate != "/earlier.yml" {
+		t.Fatalf("earlier candidate = %q, %v, %v", candidate, ok, err)
+	}
+}
+
+func TestRelativeAndShellLikeSearchPathsRemainLiteral(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	helpers := discoveryDirectoryHelpers{workingDir: func() (string, error) { return root, nil }}
+	search := Searches(Paths("~/literal.yml", "$CONFIG/*.yaml"), SearchDirectory("./settings", DirectorySearchOptions{Filenames: []FilenameGroup{CustomFilenames("name with spaces.yml")}}))
+	candidates := collectSearchCandidates(t, search, "", helpers)
+	assertCandidatePaths(t, candidates, []string{
+		filepath.Join(root, "~/literal.yml"),
+		filepath.Join(root, "$CONFIG/*.yaml"),
+		filepath.Join(root, "settings", "name with spaces.yml"),
+	})
+}
+
+func collectSearchCandidates(t *testing.T, search Search, application string, helpers discoveryDirectoryHelpers) []string {
+	t.Helper()
+	iterator, diagnostics := newSearchCandidateIterator(search, application, helpers)
+	if len(diagnostics) != 0 {
+		t.Fatalf("search validation = %#v", diagnostics)
+	}
+	var candidates []string
+	for {
+		candidate, ok, err := iterator.next()
+		if err != nil {
+			t.Fatalf("expand candidate: %v", err)
+		}
+		if !ok {
+			return candidates
+		}
+		candidates = append(candidates, candidate)
+	}
+}
+
+func assertCandidatePaths(t *testing.T, got, want []string) {
+	t.Helper()
+	if len(got) != len(want) {
+		t.Fatalf("candidate count = %d, want %d: %#v", len(got), len(want), got)
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("candidate %d = %q, want %q", i, got[i], want[i])
+		}
+	}
+}
+
+func TestResolveSearchSelectsFirstExistingCandidate(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	first := filepath.Join(root, "first.yml")
+	second := filepath.Join(root, "second.yml")
+	writeDiscoveryFile(t, first, "value: first\n")
+	writeDiscoveryFile(t, second, "value: second\n")
+	selected, diagnostics := resolveSearchSource(Source{}, ptrSearch(Paths(filepath.Join(root, "missing.yml"), first, second)), "", false, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 0 || selected.kind != sourceFile || selected.path != first {
+		t.Fatalf("selection = %#v, diagnostics %#v", selected, diagnostics)
+	}
+	read, diagnostics := readSource(selected, "source-1", defaultSourceLimits())
+	if len(diagnostics) != 0 || string(read.data) != "value: first\n" || read.info.Path != canonicalTestPath(t, first) {
+		t.Fatalf("selected source read = %#v, diagnostics %#v", read, diagnostics)
+	}
+}
+
+func TestResolveSearchNamedExtensionsAndNonregularSelection(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	writeDiscoveryFile(t, filepath.Join(root, "app.yml"), "value: yml\n")
+	writeDiscoveryFile(t, filepath.Join(root, "app.yaml"), "value: yaml\n")
+	selected, diagnostics := resolveSearchSource(Source{}, ptrSearch(SearchDirectory(root)), "app", false, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 0 || selected.path != filepath.Join(root, "app.yml") {
+		t.Fatalf("named extension selection = %#v, %#v", selected, diagnostics)
+	}
+
+	directory := filepath.Join(root, "subdirectory")
+	if err := os.Mkdir(directory, 0o700); err != nil {
+		t.Fatal(err)
+	}
+	good := filepath.Join(root, "good.yml")
+	writeDiscoveryFile(t, good, "value: later\n")
+	selected, diagnostics = resolveSearchSource(Source{}, ptrSearch(Paths(directory, good)), "", false, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 0 || selected.path != directory {
+		t.Fatalf("directory candidate selection = %#v, %#v", selected, diagnostics)
+	}
+	_, diagnostics = readSource(selected, "source-1", defaultSourceLimits())
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSourceNotRegular {
+		t.Fatalf("selected directory read diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestResolveSearchContainedSymlinkAndNoFallbackAfterReadFailure(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	target := filepath.Join(root, "actual.yml")
+	link := filepath.Join(root, "linked.yml")
+	writeDiscoveryFile(t, target, "value: linked\n")
+	if err := os.Symlink(target, link); err != nil {
+		t.Skipf("symlinks unavailable: %v", err)
+	}
+	writeDiscoveryFile(t, filepath.Join(root, "later.yml"), "value: later\n")
+	selected, diagnostics := resolveSearchSource(Source{}, ptrSearch(Paths(link, filepath.Join(root, "later.yml"))), "", false, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 0 || selected.path != link {
+		t.Fatalf("symlink selection = %#v, %#v", selected, diagnostics)
+	}
+	read, diagnostics := readSource(selected, "source-1", defaultSourceLimits())
+	if len(diagnostics) != 0 || string(read.data) != "value: linked\n" || read.info.Path != canonicalTestPath(t, target) {
+		t.Fatalf("symlink read = %#v, %#v", read, diagnostics)
+	}
+	if err := os.Remove(target); err != nil {
+		t.Fatal(err)
+	}
+	_, diagnostics = readSource(selected, "source-1", defaultSourceLimits())
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSourceMissing {
+		t.Fatalf("selected source read failure = %#v", diagnostics)
+	}
+}
+
+func TestResolveSearchRequiredAndOptionalExhaustion(t *testing.T) {
+	t.Parallel()
+
+	optional, diagnostics := resolveSearchSource(Source{}, ptrSearch(Search{}), "", false, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 0 || optional.kind != sourceNone {
+		t.Fatalf("optional empty search = %#v, %#v", optional, diagnostics)
+	}
+	_, diagnostics = resolveSearchSource(Source{}, ptrSearch(Search{}), "", true, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSourceMissing || diagnostics[0].Primary != nil || !strings.Contains(diagnostics[0].Message, "search") {
+		t.Fatalf("required empty search = %#v", diagnostics)
+	}
+
+	root := t.TempDir()
+	missing := filepath.Join(root, "missing.yml")
+	optional, diagnostics = resolveSearchSource(Source{}, ptrSearch(Paths(missing)), "", false, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 0 || optional.kind != sourceNone {
+		t.Fatalf("optional missing search = %#v, %#v", optional, diagnostics)
+	}
+	_, diagnostics = resolveSearchSource(Source{}, ptrSearch(Paths(missing)), "", true, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSourceMissing {
+		t.Fatalf("required missing search = %#v", diagnostics)
+	}
+}
+
+func TestResolveSearchLeavesNonSearchSourcesUntouched(t *testing.T) {
+	t.Parallel()
+
+	source := Bytes("memory", []byte("value: unchanged\n"))
+	resolved, diagnostics := resolveSearchSource(source, nil, "", false, discoveryDirectoryHelpers{}, discoveryHostProbe{})
+	if len(diagnostics) != 0 || resolved.kind != sourceBytes || string(resolved.data) != "value: unchanged\n" {
+		t.Fatalf("non-search source = %#v, diagnostics = %#v", resolved, diagnostics)
+	}
+}
+
+func TestResolveSearchStopsOnReachedProbeError(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	first := filepath.Join(root, "first.yml")
+	second := filepath.Join(root, "second.yml")
+	writeDiscoveryFile(t, first, "value: first\n")
+	writeDiscoveryFile(t, second, "value: second\n")
+	statCalls := 0
+	probe := discoveryHostProbe{stat: func(path string) (fs.FileInfo, error) {
+		statCalls++
+		return nil, fs.ErrPermission
+	}}
+	selected, info, diagnostics := resolveSearchInput(SearchConfig("", Paths(first, second)), "source-3", discoveryDirectoryHelpers{}, probe)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeSourcePermission || statCalls != 1 {
+		t.Fatalf("probe failure = %#v, stat calls = %d", diagnostics, statCalls)
+	}
+	if selected.kind != sourceFile || info == nil || info.Path != canonicalTestPath(t, first) || info.Label != first || info.Kind != SourceKindFile || info.ID != "source-3" {
+		t.Fatalf("failed candidate evidence = %#v, %#v", selected, info)
+	}
+	if diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != info.ID {
+		t.Fatalf("failed candidate diagnostic does not identify its file: %#v", diagnostics)
+	}
+}
+
+func TestResolveSearchLeavesLaterDirectoryUnresolvedAfterSelection(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	first := filepath.Join(root, "selected.yml")
+	writeDiscoveryFile(t, first, "value: selected\n")
+	baseCalls := 0
+	helpers := discoveryDirectoryHelpers{userConfigDir: func() (string, error) {
+		baseCalls++
+		return "", errors.New("later base should not be reached")
+	}}
+	search := ptrSearch(Searches(Paths(first), UserConfigDirectory()))
+	selected, diagnostics := resolveSearchSource(Source{}, search, "app", false, helpers, discoveryHostProbe{})
+	if len(diagnostics) != 0 || selected.path != first || baseCalls != 0 {
+		t.Fatalf("selected = %#v, diagnostics = %#v, later base calls = %d", selected, diagnostics, baseCalls)
+	}
+}
+
+func TestResolveSearchTreatsShellLikeNamesLiterally(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	paths := []string{"~/literal.yml", "$CONFIG/literal.yml", "*.yaml"}
+	for _, name := range paths {
+		writeDiscoveryFile(t, filepath.Join(root, name), "value: literal\n")
+	}
+	helper := discoveryDirectoryHelpers{workingDir: func() (string, error) { return root, nil }}
+	for _, name := range paths {
+		selected, diagnostics := resolveSearchSource(Source{}, ptrSearch(Paths(name)), "", false, helper, discoveryHostProbe{})
+		if len(diagnostics) != 0 || selected.path != filepath.Join(root, name) {
+			t.Fatalf("literal %q selection = %#v, %#v", name, selected, diagnostics)
+		}
+	}
+}
+
+func ptrSearch(search Search) *Search { return &search }
+
+func writeDiscoveryFile(t *testing.T, path, contents string) {
+	t.Helper()
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+}

internal/engine/discovery_windows_test.go

@@ -0,0 +1,91 @@
+//go:build windows
+
+package engine
+
+import (
+	"errors"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestWindowsSearchRejectsReservedComponentsBeforeAcquisition(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	first := writeImportFile(t, root, "first.yml", "value: first\n")
+	for _, name := range []string{"CONIN$", "CONOUT$"} {
+		for _, source := range []Source{
+			SearchConfig(name, Searches(Paths(first), WorkingDirectory())),
+			SearchConfig("", Searches(Paths(first), SearchDirectory(root, DirectorySearchOptions{
+				Filenames: []FilenameGroup{CustomFilenames(name)},
+			}))),
+		} {
+			result, err := testLoad[discoveryLoadConfig](source, Options[discoveryLoadConfig]{})
+			var failure *LoadError
+			if result != nil || !errors.As(err, &failure) || len(failure.Diagnostics()) != 1 || failure.Diagnostics()[0].Code != CodeOptionsInvalid {
+				t.Fatalf("reserved component %q accepted: %#v, %v", name, result, err)
+			}
+			if len(failure.Sources()) != 0 {
+				t.Fatalf("reserved component %q was checked after acquisition: %#v", name, failure.Sources())
+			}
+		}
+	}
+}
+
+func TestWindowsSearchPathsMatchHostAbsoluteResolution(t *testing.T) {
+	root := t.TempDir()
+	file := writeImportFile(t, root, "settings/config.yml", "value: selected\n")
+	working := t.TempDir()
+	t.Chdir(working)
+	relative, err := filepath.Rel(working, file)
+	if err != nil {
+		t.Fatal(err)
+	}
+	volume := filepath.VolumeName(file)
+	rooted := strings.TrimPrefix(file, volume)
+	for _, entry := range []struct {
+		name string
+		path string
+	}{
+		{"relative", relative},
+		{"drive root relative", rooted},
+		{"slash root relative", filepath.ToSlash(rooted)},
+		{"drive relative", volume + relative},
+		{"absolute", file},
+	} {
+		t.Run(entry.name, func(t *testing.T) {
+			want, err := filepath.Abs(entry.path)
+			if err != nil {
+				t.Fatal(err)
+			}
+			explicit, err := testLoad[discoveryLoadConfig](File(entry.path))
+			if err != nil {
+				t.Fatal(err)
+			}
+			for _, search := range []Search{
+				Paths(entry.path),
+				SearchDirectory(filepath.Dir(entry.path), DirectorySearchOptions{
+					Filenames: []FilenameGroup{CustomFilenames(filepath.Base(entry.path))},
+				}),
+			} {
+				iterator, diagnostics := newSearchCandidateIterator(search, "", discoveryDirectoryHelpers{})
+				if len(diagnostics) != 0 {
+					t.Fatal(diagnostics)
+				}
+				candidate, ok, err := iterator.next()
+				if err != nil || !ok || candidate != want {
+					t.Fatalf("candidate = %q, %v, %v; want %q", candidate, ok, err, want)
+				}
+				result, err := testLoad[discoveryLoadConfig](SearchConfig("", search))
+				if err != nil || result.Config.Value != explicit.Config.Value {
+					t.Fatalf("searched path = %#v, %v; explicit value %q", result, err, explicit.Config.Value)
+				}
+				sources := result.Sources()
+				if len(sources) != 1 || sources[0].Path != explicit.Sources()[0].Path {
+					t.Fatalf("searched source = %#v; explicit source %#v", sources, explicit.Sources())
+				}
+			}
+		})
+	}
+}

internal/engine/filesystem.go

@@ -9,6 +9,9 @@ import (
 )
 
 func validateFilesystemContext(source Source, context *FilesystemContext, main parsedSource) *Diagnostic {
+	if source.kind == sourceNone && context.FS == nil {
+		return validateAbsentHostFilesystemContext(context)
+	}
 	if nativeContext(source, context) {
 		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
 		if diagnostic != nil {
@@ -33,10 +36,57 @@ func validateFilesystemContext(source Source, context *FilesystemContext, main p
 	return nil
 }
 
+func validateAbsentHostFilesystemContext(context *FilesystemContext) *Diagnostic {
+	rootPath, err := canonicalExistingPath(context.Root)
+	if err != nil {
+		value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
+		return &value
+	}
+	basePath := rootPath
+	if context.BaseDir != "" {
+		basePath, err = canonicalExistingPath(context.BaseDir)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "import base directory could not be canonicalized", nil)
+			return &value
+		}
+	}
+	if !containedPath(rootPath, basePath) {
+		value := importDiagnostic(CodeImportOutsideRoot, "import base must stay inside the configuration root", nil)
+		return &value
+	}
+	for _, candidate := range []string{rootPath, basePath} {
+		info, err := os.Stat(candidate)
+		if err != nil || !info.IsDir() {
+			value := importDiagnostic(CodeImportInvalid, "configuration root and import base must be directories", nil)
+			return &value
+		}
+	}
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		value := globFilesystemDiagnostic(err, "configuration root could not be opened", Location{})
+		value.Primary = nil
+		return &value
+	}
+	if err := root.Close(); err != nil {
+		value := importDiagnostic(CodeImportRead, "configuration root could not be closed", nil)
+		return &value
+	}
+	return nil
+}
+
 func filesystemOptionDiagnostics(source Source, context *FilesystemContext) []Diagnostic {
 	switch source.kind {
 	case sourceNone:
-		return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "NoSource cannot use filesystem context")}
+		return absentFilesystemOptionDiagnostics(context)
+	case sourceSearch:
+		if context.FS != nil {
+			return absentFilesystemOptionDiagnostics(context)
+		}
+		// A selected host file can derive its root; absence requires an
+		// explicit root. Check that compatibility only after selection.
+		if context.BaseDir != "" && !filepath.IsAbs(context.BaseDir) {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "filesystem base directory must be an absolute host path")}
+		}
 	case sourceFile:
 		if context.FS != nil {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "host file sources cannot use a native filesystem context")}
@@ -69,6 +119,17 @@ func filesystemOptionDiagnostics(source Source, context *FilesystemContext) []Di
 	return nil
 }
 
+func absentFilesystemOptionDiagnostics(context *FilesystemContext) []Diagnostic {
+	if context.FS != nil {
+		if context.FS.namespace == nil || nilFilesystem(context.FS.namespace.filesystem) || context.BaseDir == "" || context.Root == "" || !fs.ValidPath(context.BaseDir) || !fs.ValidPath(context.Root) {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "native filesystem context requires a namespace, base, and root directory")}
+		}
+	} else if context.Root == "" || !filepath.IsAbs(context.Root) || context.BaseDir != "" && !filepath.IsAbs(context.BaseDir) {
+		return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "absent-input filesystem root and base directories must be absolute host paths")}
+	}
+	return nil
+}
+
 func nativeContext(source Source, context *FilesystemContext) bool {
 	return source.kind == sourceFilesystem || context != nil && context.FS != nil
 }

internal/engine/glob_test.go

@@ -268,6 +268,48 @@ func TestFilesystemContextIsValidatedWithoutImportsOrActiveOverlay(t *testing.T)
 	}
 }
 
+func TestNoSourceImportsAndFilesystemContext(t *testing.T) {
+	t.Parallel()
+
+	directory := t.TempDir()
+	for _, imports := range []bool{false, true} {
+		name := "imports disabled"
+		if imports {
+			name = "imports enabled"
+		}
+		t.Run(name, func(t *testing.T) {
+			result, err := testLoad[importTestConfig](NoSource(), Options[importTestConfig]{
+				Imports: imports, Filesystem: &FilesystemContext{Root: directory},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if sources := result.Sources(); len(sources) != 0 {
+				t.Fatalf("NoSource sources = %#v", sources)
+			}
+			if _, ok := result.Metadata().Lookup(Keys("items", "root")); ok {
+				t.Fatal("NoSource contributed explicit input")
+			}
+		})
+	}
+
+	outside := t.TempDir()
+	for _, test := range []struct {
+		name    string
+		context FilesystemContext
+		code    Code
+	}{
+		{name: "missing root", context: FilesystemContext{BaseDir: directory}, code: CodeOptionsInvalid},
+		{name: "relative root", context: FilesystemContext{Root: "."}, code: CodeOptionsInvalid},
+		{name: "missing root directory", context: FilesystemContext{Root: filepath.Join(directory, "missing")}, code: CodeImportInvalid},
+		{name: "base outside root", context: FilesystemContext{BaseDir: outside, Root: directory}, code: CodeImportOutsideRoot},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			requireLoadCode(t, NoSource(), Options[importTestConfig]{Filesystem: &test.context}, test.code)
+		})
+	}
+}
+
 func writeGlobRoot(t *testing.T, directory, pattern string, required bool) string {
 	t.Helper()
 	requiredLine := ""

internal/engine/imports.go

@@ -319,7 +319,7 @@ func composeAcquiredInputWithPolicies(input assembledInput, lower collectionPoli
 	if len(scope.contributions) == 1 && scope.contributions[0].role == importRoleMain && scope.contributions[0].pendingItem != nil {
 		main := scope.contributions[0]
 		localScope := cloneAcquisitionScope(*main.pendingItem)
-		if diagnostics := traversal.composePendingItems(&localScope, policy); len(diagnostics) != 0 {
+		if diagnostics := traversal.composePendingItems(&localScope); len(diagnostics) != 0 {
 			return nil, prependCompositionDiagnosticPaths(diagnostics, main.destination.Index(0))
 		}
 		documents := make([]*document.Node, 0, len(localScope.contributions))
@@ -405,7 +405,7 @@ func composeAcquiredScopeWithPolicy(
 		accounting: accounting, limits: defaultImportLimits(), rule: rule,
 		versionState: versionState, importGraph: graph.clone(),
 	}
-	if diagnostics := traversal.composePendingItems(scope, policy); len(diagnostics) != 0 {
+	if diagnostics := traversal.composePendingItems(scope); len(diagnostics) != 0 {
 		return nil, diagnostics
 	}
 	documents := make([]*document.Node, 0, len(scope.contributions))
@@ -765,22 +765,25 @@ func (s *importTraversal) scheduleAcquisitionScopes(scope *acquisitionScope) {
 	}
 }
 
-func (s *importTraversal) composePendingItems(scope *acquisitionScope, policy sequencePolicy) []Diagnostic {
+func (s *importTraversal) composePendingItems(scope *acquisitionScope) []Diagnostic {
 	for index := range scope.contributions {
 		contribution := &scope.contributions[index]
 		if contribution.pendingItem == nil {
 			continue
 		}
 		itemScope := contribution.pendingItem
-		if diagnostics := s.composePendingItems(itemScope, policy); len(diagnostics) != 0 {
+		if diagnostics := s.composePendingItems(itemScope); len(diagnostics) != 0 {
 			return projectItemDiagnosticPaths(diagnostics, *scope, contribution.destination, index)
 		}
 		documents := make([]*document.Node, 0, len(itemScope.contributions))
 		for _, nested := range itemScope.contributions {
 			documents = append(documents, nested.document)
 		}
+		// An item scope is below a sequence index. Application selectors match
+		// mapping-only effective paths, so none can apply inside this scope.
+		// Its YAML declarations remain relative to the owning record.
 		completed, diagnostics := composeDocumentsWithCollectionPolicy(
-			policy, itemScope.collectionPolicies,
+			sequencePolicy{}, itemScope.collectionPolicies,
 			acquiredSequenceDeclarations(*itemScope), documents...,
 		)
 		if len(diagnostics) != 0 {

internal/engine/imports_test.go

@@ -398,7 +398,7 @@ label: helper
 	childPath := writeImportFile(t, directory, "records/children/child.yml", "value: nested\n")
 
 	loaded, err := LoadLayered(
-		Input{Source: NoSource()}, testSecretBinding(),
+		Input{Source: NoSource(), Imports: true}, testSecretBinding(),
 		LayeredOptions[config]{Underlays: []Underlay{{Inputs: []Input{{
 			Source: File(itemPath), Imports: true, Destination: Keys("catalog", "entries"), placementMode: placementModeSequenceItem,
 		}}}}},

internal/engine/inheritance.go

@@ -114,26 +114,7 @@ func cloneInheritanceOperations(operations []inheritanceOperation) []inheritance
 }
 
 func inheritancePolicyScopeAtPath(scope collectionPolicyScope, prefix Path) collectionPolicyScope {
-	localized := collectionPolicyScope{}
-	for _, entry := range scope.collections {
-		if relative, inside := relativeHistoryPath(prefix, entry.destination); inside {
-			localized.set(relative, cloneCollectionPolicyForInheritance(entry.policy))
-			continue
-		}
-		relative, inside := relativeHistoryPath(entry.destination, prefix)
-		if !inside || len(relative.segments) == 0 || relative.segments[0].kind != pathSegmentIndex {
-			continue
-		}
-		record, found := entry.policy.recordPolicyAt(relative.segments[0].index)
-		if !found {
-			continue
-		}
-		remainder := Path{segments: append([]pathSegment(nil), relative.segments[1:]...)}
-		for _, nested := range inheritancePolicyScopeAtPath(record.scope, remainder).collections {
-			localized.set(nested.destination, cloneCollectionPolicyForInheritance(nested.policy))
-		}
-	}
-	return localized
+	return newInheritancePolicyIndex(scope).localize(prefix)
 }
 
 func projectInheritancePolicyTemplates(templates []collectionPolicyScope, prefix Path) []projectedCollectionPolicy {

internal/engine/inheritance_collection.go

@@ -103,15 +103,19 @@ func mergeInheritanceCollectionPolicies(
 	for key, entry := range child.collections {
 		paths[key] = entry.destination.clone()
 	}
+	var lookup *documentLookup
+	if len(paths) != 0 {
+		lookup = buildDocumentLookup(parentValue, childValue)
+	}
 	var diagnostics []Diagnostic
 	for _, path := range paths {
 		relative, inside := relativeHistoryPath(parentRootPath, path)
 		var retained, incoming *document.Node
 		if inside {
 			if !replaced.contains(path, false) {
-				retained, _ = documentAtPath(parentValue, relative)
+				retained, _ = documentAtPathWithLookup(parentValue, relative, lookup)
 			}
-			incoming, _ = documentAtPath(childValue, relative)
+			incoming, _ = documentAtPathWithLookup(childValue, relative, lookup)
 		}
 		resolution, lower, _, active, issues := inheritanceCollectionResolution(path, parent, child, applicationPolicy, incoming)
 		if len(issues) != 0 {

internal/engine/inheritance_policy_index.go

@@ -0,0 +1,123 @@
+package engine
+
+// inheritancePolicyIndex borrows immutable policy entries. Semantic path
+// segments keep mapping keys distinct from sequence indexes. Record-owned
+// scopes have their own indexes, just as they have their own policy tables.
+type inheritancePolicyIndex struct {
+	entry    *collectionPolicyEntry
+	children map[pathSegment]*inheritancePolicyIndex
+	records  map[int]*inheritancePolicyIndex
+}
+
+func newInheritancePolicyIndex(scope collectionPolicyScope) *inheritancePolicyIndex {
+	index := &inheritancePolicyIndex{}
+	for _, entry := range scope.collections {
+		index.set(entry)
+	}
+	return index
+}
+
+func (index *inheritancePolicyIndex) set(entry collectionPolicyEntry) {
+	node := index
+	for _, segment := range entry.destination.segments {
+		if node.children == nil {
+			node.children = make(map[pathSegment]*inheritancePolicyIndex)
+		}
+		if node.children[segment] == nil {
+			node.children[segment] = &inheritancePolicyIndex{}
+		}
+		node = node.children[segment]
+	}
+	node.entry = &entry
+	node.records = nil
+	for position, owner := range entry.policy.recordOrder {
+		scope := entry.policy.recordPolicies[owner]
+		if len(scope.collections) == 0 {
+			continue
+		}
+		if node.records == nil {
+			node.records = make(map[int]*inheritancePolicyIndex)
+		}
+		node.records[position] = newInheritancePolicyIndex(scope)
+	}
+}
+
+// localize copies only policies in the requested scope. Earlier record-owned
+// policies are considered before direct entries added by destination expansion.
+func (index *inheritancePolicyIndex) localize(prefix Path) collectionPolicyScope {
+	localized := collectionPolicyScope{}
+	node := index
+	for offset, segment := range prefix.segments {
+		if node == nil {
+			return localized
+		}
+		if segment.kind == pathSegmentIndex {
+			if record := node.records[segment.index]; record != nil {
+				remainder := Path{segments: prefix.segments[offset+1:]}
+				for _, entry := range record.localize(remainder).collections {
+					localized.set(entry.destination, entry.policy)
+				}
+			}
+		}
+		node = node.children[segment]
+	}
+	var collect func(*inheritancePolicyIndex)
+	collect = func(node *inheritancePolicyIndex) {
+		if node == nil {
+			return
+		}
+		if node.entry != nil {
+			relative, _ := relativeHistoryPath(prefix, node.entry.destination)
+			localized.set(relative, cloneCollectionPolicyForInheritance(node.entry.policy))
+		}
+		for _, child := range node.children {
+			collect(child)
+		}
+	}
+	collect(node)
+	return localized
+}
+
+func (index *inheritancePolicyIndex) absoluteScope(prefix Path) collectionPolicyScope {
+	scope := collectionPolicyScope{}
+	for _, entry := range index.localize(prefix).collections {
+		scope.set(appendDestination(prefix, entry.destination), entry.policy)
+	}
+	return scope
+}
+
+// replace updates the walker's owned table and its borrowed index together.
+// Retirement visits only direct entries below the changed destination.
+func (index *inheritancePolicyIndex) replace(scope *collectionPolicyScope, prefix Path, replacement collectionPolicyScope) {
+	node := index
+	var parent *inheritancePolicyIndex
+	var last pathSegment
+	for _, segment := range prefix.segments {
+		if node == nil {
+			break
+		}
+		parent, last, node = node, segment, node.children[segment]
+	}
+	var remove func(*inheritancePolicyIndex)
+	remove = func(node *inheritancePolicyIndex) {
+		if node == nil {
+			return
+		}
+		if node.entry != nil {
+			delete(scope.collections, node.entry.destination.String())
+		}
+		for _, child := range node.children {
+			remove(child)
+		}
+	}
+	remove(node)
+	if parent == nil {
+		*index = inheritancePolicyIndex{}
+	} else {
+		delete(parent.children, last)
+	}
+	for _, entry := range replacement.collections {
+		scope.set(entry.destination, entry.policy)
+		index.set(scope.collections[entry.destination.String()])
+	}
+}

internal/engine/inheritance_policy_index_test.go

@@ -0,0 +1,111 @@
+package engine
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestInheritancePolicyIndexKeepsSemanticScopesAndOwnedTemplates(t *testing.T) {
+	policy := collectionPolicy{identityFields: []string{"id"}, identityPresent: true}
+	nested := collectionPolicyScope{}
+	nested.set(Keys("group", "base", "items"), policy)
+	outer := policy
+	record := newCollectionRecordPolicy()
+	record.scope = nested
+	outer.setRecordPolicyAt(0, record)
+	scope := collectionPolicyScope{}
+	scope.set(Keys("records"), outer)
+	scope.set(Keys("numeric", "0", "items"), policy)
+	index := newInheritancePolicyIndex(scope)
+
+	first := index.localize(Keys("records").Index(0).Key("group"))
+	second := index.localize(Keys("records").Index(0).Key("group"))
+	if len(first.collections) != 1 || len(second.collections) != 1 {
+		t.Fatal("record-owned scope was not localized")
+	}
+	entry := first.collections["/base/items"]
+	entry.policy.identityFields[0] = "changed"
+	if second.collections["/base/items"].policy.identityFields[0] != "id" || nested.collections["/group/base/items"].policy.identityFields[0] != "id" {
+		t.Fatal("localized policy shares mutable template state")
+	}
+	if len(index.localize(Keys("records", "0")).collections) != 0 || len(index.localize(Keys("numeric").Index(0)).collections) != 0 {
+		t.Fatal("mapping keys and sequence indexes were conflated")
+	}
+	if len(index.localize(Keys("numeric", "0")).collections) != 1 {
+		t.Fatal("numeric mapping key did not retain its policy")
+	}
+	left := index.localize(Keys("records"))
+	right := index.localize(Keys("records"))
+	leftOwner := left.collections[""].policy.recordOrder[0]
+	rightOwner := right.collections[""].policy.recordOrder[0]
+	if leftOwner == rightOwner || leftOwner == record.owner || rightOwner == record.owner {
+		t.Fatal("independent templates share a record owner")
+	}
+}
+
+func TestInheritancePolicyIndexReplacesOnlyTheChangedScope(t *testing.T) {
+	scope := collectionPolicyScope{}
+	policy := collectionPolicy{explicitMode: sequenceModeAppend, explicitModePresent: true}
+	for index := range 128 {
+		scope.set(Keys(fmt.Sprintf("group-%03d", index), "pending"), policy)
+	}
+	index := newInheritancePolicyIndex(scope)
+	changed := Keys("group-064")
+	if len(index.absoluteScope(changed).collections) != 1 {
+		t.Fatal("scope includes unrelated groups")
+	}
+	replacement := collectionPolicyScope{}
+	replacement.set(changed.Key("introduced"), policy)
+	index.replace(&scope, changed, replacement)
+	if len(scope.collections) != 128 {
+		t.Fatal("replacement lost unrelated pending declarations")
+	}
+	local := index.localize(changed)
+	if _, found := local.collections["/introduced"]; !found || len(local.collections) != 1 {
+		t.Fatal("retired policy survived or introduced policy was not indexed")
+	}
+	index.replace(&scope, changed, collectionPolicyScope{})
+	if len(index.localize(changed).collections) != 0 || len(scope.collections) != 127 {
+		t.Fatal("descendant retirement did not update the table and index together")
+	}
+	if len(index.localize(Keys("group-065")).collections) != 1 {
+		t.Fatal("retirement changed another scope")
+	}
+}
+
+func TestInheritanceWideIndependentYamlCollectionScopes(t *testing.T) {
+	var body strings.Builder
+	body.WriteString("configkit:\n  sequences:\n")
+	for index := range 64 {
+		fmt.Fprintf(&body, "    - {destination: [groups, group-%03d, base, items], key: id, mode: append}\n", index)
+	}
+	body.WriteString("groups:\n")
+	for index := range 64 {
+		fmt.Fprintf(&body, "  group-%03d:\n    base: {items: [{id: shared, value: parent}]}\n    child: {extends: base, items: [{id: shared, value: child}]}\n", index)
+	}
+	type item struct {
+		ID    string `yaml:"id"`
+		Value string `yaml:"value"`
+	}
+	type definition struct {
+		Items []item `yaml:"items"`
+	}
+	type config struct {
+		Groups map[string]map[string]definition `yaml:"groups"`
+	}
+	loaded, err := testLoad[config](Bytes("wide-inheritance", []byte(body.String())))
+	if err != nil {
+		t.Fatal(err)
+	}
+	metadata := loaded.Metadata()
+	for name, group := range loaded.Config.Groups {
+		if len(group["base"].Items) != 1 || group["base"].Items[0].Value != "parent" || len(group["child"].Items) != 1 || group["child"].Items[0].Value != "child" {
+			t.Fatalf("independent scope %s composed incorrectly: %#v", name, group)
+		}
+		path := Keys("groups", name, "child", "items").Index(0).Key("value")
+		if info, exists := metadata.Lookup(path); !exists || !info.Explicit || info.Origin.Layer != LayerBase {
+			t.Fatalf("inherited destination lost source metadata at %s", path)
+		}
+	}
+}

internal/engine/inheritance_schedule.go

@@ -47,9 +47,10 @@ func analyzeInheritanceGraphWithCollectionRules(scope *document.Node, scopePath
 		}}
 	}
 
+	policyIndex := newInheritancePolicyIndex(policies)
 	graph := &inheritanceGraph{
 		scope: scope, scopePath: scopePath,
-		collectionPolicies: cloneCollectionPolicyScopeForInheritance(policies), applicationPolicy: applicationPolicy,
+		collectionPolicies: policyIndex.absoluteScope(scopePath), applicationPolicy: applicationPolicy,
 		byName: make(map[string]int, len(scope.Mapping)),
 	}
 	entries := make(map[string]document.Entry, len(scope.Mapping))
@@ -69,7 +70,7 @@ func analyzeInheritanceGraphWithCollectionRules(scope *document.Node, scopePath
 		graph.byName[name] = index
 		graph.nodes = append(graph.nodes, inheritanceTemplate{
 			name: name, entry: entries[name], path: scopePath.Key(name),
-			collectionPolicies: inheritancePolicyScopeAtPath(policies, scopePath.Key(name)),
+			collectionPolicies: policyIndex.localize(scopePath.Key(name)),
 		})
 	}
 
@@ -355,6 +356,7 @@ func applyInheritanceScopeTemplates(adapter inheritanceScopeAdapter, order []int
 	root := adapter.rebuild(resolved)
 	result.collectionPolicies = collectionPolicyScope{}
 	result.origins = directInheritanceOrigins(&document.Node{Kind: root.Kind, Location: root.Location}, adapter.scopePath(), inputOrigins)
+	policyIndex := newInheritancePolicyIndex(adapter.collectionPolicies())
 	for _, slot := range adapter.slots() {
 		if slot.hasTemplate {
 			index := slot.template
@@ -364,7 +366,7 @@ func applyInheritanceScopeTemplates(adapter inheritanceScopeAdapter, order []int
 			}
 			continue
 		}
-		localPolicies := inheritancePolicyScopeAtPath(adapter.collectionPolicies(), slot.path)
+		localPolicies := policyIndex.localize(slot.path)
 		projectInheritancePolicyRoots(&result.collectionPolicies, localPolicies, slot.path)
 		for pointer, origin := range directInheritanceOrigins(slot.value, slot.path, inputOrigins) {
 			result.origins[pointer] = origin

internal/engine/inheritance_scope.go

@@ -109,10 +109,11 @@ func (scope identifiedSequenceInheritanceScope) applicationPolicy() sequencePoli
 func (scope identifiedSequenceInheritanceScope) slotOverhead() int { return 0 }
 func (scope identifiedSequenceInheritanceScope) templates() []inheritanceExecutionTemplate {
 	result := make([]inheritanceExecutionTemplate, len(scope.graph.nodes))
+	policyIndex := newInheritancePolicyIndex(scope.policies)
 	for index, node := range scope.graph.nodes {
 		result[index] = inheritanceExecutionTemplate{
 			entry: document.Entry{Value: node.item}, path: node.path,
-			collectionPolicies: inheritancePolicyScopeAtPath(scope.policies, node.path),
+			collectionPolicies: policyIndex.localize(node.path),
 			parent:             node.parent, hasParent: node.hasParent,
 			hasDirective: node.directive != nil, cancelled: node.cancelled,
 			directive: document.Entry{Value: node.directive}, directivePath: node.directivePath,

internal/engine/inheritance_walk.go

@@ -11,6 +11,7 @@ type inheritanceWalker struct {
 	nodes              int // logical non-directive nodes in the entire working tree
 	origins            map[string]Origin
 	collectionPolicies collectionPolicyScope
+	policyIndex        *inheritancePolicyIndex
 	applicationPolicy  sequencePolicy
 	limits             projectionLimits
 	retained           *retainedNodeCounter
@@ -66,6 +67,7 @@ func expandInheritanceWithCollectionRuleBudgets(input *document.Node, inputOrigi
 		applicationPolicy:  applicationPolicy,
 		retained:           newRetainedNodeCounter(historyLimit),
 	}
+	walker.policyIndex = newInheritancePolicyIndex(walker.collectionPolicies)
 	if problem := walker.visit(&root, Path{}, true); problem != nil {
 		return nil, []Diagnostic{*problem}
 	}
@@ -113,20 +115,13 @@ func (w *inheritanceWalker) visit(node **document.Node, path Path, unscoped bool
 				limits.nodes -= w.nodes - before
 			}
 			resolved, diagnostics := expandSiblingDefinitionsWithCollectionRules(
-				*node, path, limits, w.origins, w.collectionPolicies, w.applicationPolicy, w.retained,
+				*node, path, limits, w.origins, w.policyIndex.absoluteScope(path), w.applicationPolicy, w.retained,
 			)
 			if len(diagnostics) != 0 {
 				return &diagnostics[0]
 			}
 			*node = resolved.root
-			for pointer, entry := range w.collectionPolicies.collections {
-				if pathHasPrefix(entry.destination, path) {
-					delete(w.collectionPolicies.collections, pointer)
-				}
-			}
-			for _, entry := range resolved.collectionPolicies.collections {
-				w.collectionPolicies.set(entry.destination, entry.policy)
-			}
+			w.policyIndex.replace(&w.collectionPolicies, path, resolved.collectionPolicies)
 			deleteOriginSubtree(w.origins, path, original)
 			for pointer, origin := range resolved.origins {
 				w.origins[pointer] = origin
@@ -203,20 +198,13 @@ func (w *inheritanceWalker) expandIdentifiedSequence(node **document.Node, path
 		limits.nodes -= w.nodes - before
 	}
 	resolved, diagnostics := applyIdentifiedInheritanceTemplates(
-		graph, limits, w.origins, w.collectionPolicies, w.applicationPolicy, w.retained,
+		graph, limits, w.origins, w.policyIndex.absoluteScope(path), w.applicationPolicy, w.retained,
 	)
 	if len(diagnostics) != 0 {
 		return &diagnostics[0]
 	}
 	*node = resolved.root
-	for pointer, entry := range w.collectionPolicies.collections {
-		if pathHasPrefix(entry.destination, path) {
-			delete(w.collectionPolicies.collections, pointer)
-		}
-	}
-	for _, entry := range resolved.collectionPolicies.collections {
-		w.collectionPolicies.set(entry.destination, entry.policy)
-	}
+	w.policyIndex.replace(&w.collectionPolicies, path, resolved.collectionPolicies)
 	deleteOriginSubtree(w.origins, path, original)
 	for pointer, origin := range resolved.origins {
 		w.origins[pointer] = origin

internal/engine/item_policy_test.go

@@ -93,7 +93,7 @@ func TestPendingItemCompositionProjectsNestedIdentityDiagnostics(t *testing.T) {
 	}}}
 
 	traversal := importTraversal{accounting: &importAccounting{}, limits: defaultImportLimits()}
-	diagnostics := traversal.composePendingItems(&outer, sequencePolicy{})
+	diagnostics := traversal.composePendingItems(&outer)
 	if len(diagnostics) != 1 {
 		t.Fatalf("diagnostics = %#v, want one conflict", diagnostics)
 	}
@@ -105,7 +105,7 @@ func TestPendingItemCompositionProjectsNestedIdentityDiagnostics(t *testing.T) {
 	}
 }
 
-func TestPendingItemCompositionAppliesApplicationIdentityConstraints(t *testing.T) {
+func TestPendingItemCompositionAppliesOuterApplicationIdentityConstraints(t *testing.T) {
 	t.Parallel()
 
 	itemScope := acquisitionScope{contributions: []acquiredContribution{{
@@ -116,15 +116,15 @@ func TestPendingItemCompositionAppliesApplicationIdentityConstraints(t *testing.
 		destination: Keys("items"), pendingItem: &itemScope,
 	}}}
 	policy, diagnostics := compileSequenceIdentityPolicy([]sequenceIdentityRegistration{{
-		selector: sequenceIdentitySelector{literalSequenceSelector("prompts")}, identityFields: []string{"id"},
+		selector: sequenceIdentitySelector{literalSequenceSelector("items")}, identityFields: []string{"id"},
 	}})
 	if len(diagnostics) != 0 {
 		t.Fatalf("compile application identity: %#v", diagnostics)
 	}
 
-	traversal := importTraversal{accounting: &importAccounting{}, limits: defaultImportLimits()}
-	diagnostics = traversal.composePendingItems(&outer, policy)
-	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid || diagnostics[0].Path.String() != "/items/0/prompts/0/id" {
+	_, diagnostics = composeAcquiredScopeWithPolicy(&outer, collectionPolicyScope{}, policy,
+		newImportInclusionGraph(), nil, nil, nil, &importAccounting{})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid || diagnostics[0].Path.String() != "/items/0/id" {
 		t.Fatalf("application identity diagnostics = %#v", diagnostics)
 	}
 }

internal/engine/load.go

@@ -21,8 +21,10 @@ type Definition[T any] struct {
 }
 
 // Options contains the application definition and per-load input selections.
-// Imports enables explicit import declarations; Overlay selects one fixed overlay;
-// Overrides contains only values explicitly supplied by an application CLI.
+// Imports enables traversal of declarations in a selected main document; an
+// absent source has no main declarations to traverse. Overlay selects one fixed
+// overlay and requires filesystem context for NoSource. Overrides contain only
+// values explicitly supplied by an application CLI.
 type Options[T any] struct {
 	Definition              Definition[T]
 	Imports                 bool
@@ -39,7 +41,9 @@ type Options[T any] struct {
 	importDefaultsValidated bool
 }
 
-// FilesystemContext supplies filesystem roots for imports and overlays.
+// FilesystemContext supplies filesystem roots for imports and overlays. For an
+// absent host input, Root is required and BaseDir may be omitted to use Root as
+// the import base. Native contexts still require both BaseDir and Root.
 type FilesystemContext struct {
 	BaseDir string
 	Root    string
@@ -198,6 +202,7 @@ func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lo
 	}
 	accumulator.versionState = versionState
 	input, diagnostics := assembleBaseInputTrackedWithVersion(request.main.source, mainOptions, request.main.destination, accounting, nil, nil, versionState)
+	request.main.source = input.source
 	accumulator.sources = input.sources
 	accumulator.add(versionState.diagnostics...)
 	reportedVersionDiagnostics := len(versionState.diagnostics)

internal/engine/load_input.go

@@ -8,6 +8,7 @@ import (
 
 // assembledInput is the source/document handoff across schema and version checks.
 type assembledInput struct {
+	source       Source
 	parsed       parsedSource
 	document     *document.Node
 	composition  *composedDocument
@@ -101,20 +102,41 @@ func assembleBaseInputTrackedWithVersion[T any](source Source, options Options[T
 }
 
 func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], destination Path, accounting *importAccounting, visited map[resourceIdentity]importRoute, hostToken *namespaceToken, versionState *sourceVersionState, role sourceVersionRole) (assembledInput, []Diagnostic) {
+	sourceID := "source-" + strconv.Itoa(accounting.sources+1)
+	selectedSource := source
+	if source.kind == sourceSearch {
+		resolved, failedSource, diagnostics := resolveSearchInput(source, sourceID, defaultDiscoveryDirectoryHelpers(), defaultDiscoveryHostProbe())
+		if len(diagnostics) != 0 {
+			if failedSource != nil {
+				failedSource.Destination = destination.clone()
+				failedSource.placementMode = options.placementMode
+			}
+			return assembledInput{source: resolved, sources: sourceInfos(failedSource), versionState: versionState, accounting: accounting}, diagnostics
+		}
+		selectedSource = resolved
+		if options.Filesystem != nil {
+			if diagnostics := filesystemOptionDiagnostics(selectedSource, options.Filesystem); len(diagnostics) != 0 {
+				return assembledInput{source: selectedSource, versionState: versionState, accounting: accounting}, diagnostics
+			}
+		}
+		if selectedSource.kind == sourceNone && activeOverlay(options.Overlay) && options.Filesystem == nil {
+			return assembledInput{source: selectedSource, versionState: versionState, accounting: accounting}, []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "NoSource overlays require explicit filesystem context")}
+		}
+	}
 	limits := defaultImportLimits()
 	parseLimits := defaultParseLimits()
 	parseLimits.source.remaining = limits.bytes - accounting.bytes
-	parsed, diagnostics := parseSource(source, "source-"+strconv.Itoa(accounting.sources+1), parseLimits)
+	parsed, diagnostics := parseSource(selectedSource, sourceID, parseLimits)
 	if parsed.source != nil {
 		parsed.source.Destination = destination.clone()
 		parsed.source.placementMode = options.placementMode
 	}
-	input := assembledInput{parsed: parsed, document: parsed.document, sources: sourceInfos(parsed.source), versionState: versionState, accounting: accounting}
+	input := assembledInput{source: selectedSource, parsed: parsed, document: parsed.document, sources: sourceInfos(parsed.source), versionState: versionState, accounting: accounting}
 	if len(diagnostics) != 0 {
 		input.sources = sourceInfos(parsed.source)
 		// Mapping value issues already identify application content. Other early
 		// failures describe source syntax or access, not a placed value.
-		if source.kind == sourceMapping && source.mapIssue != nil && len(destination.segments) != 0 {
+		if selectedSource.kind == sourceMapping && selectedSource.mapIssue != nil && len(destination.segments) != 0 {
 			for index := range diagnostics {
 				relative := diagnostics[index].Path
 				segments := clonePathSegments(destination.segments, len(relative.segments))
@@ -200,11 +222,11 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 			versionState.mainProcessing = &processed
 		}
 	}
-	if options.Imports {
+	if options.Imports && selectedSource.kind != sourceNone {
 		if options.deferAssembly {
-			input.composition, input.sources, diagnostics = loadImportedDocumentsTrackedWithProcessingDeferred(source, options.Filesystem, parsed, destination, options.Definition.Version, limits, accounting, visited, hostToken, versionState, versionProcessing, options.sequencePolicy, true)
+			input.composition, input.sources, diagnostics = loadImportedDocumentsTrackedWithProcessingDeferred(selectedSource, options.Filesystem, parsed, destination, options.Definition.Version, limits, accounting, visited, hostToken, versionState, versionProcessing, options.sequencePolicy, true)
 		} else {
-			input.composition, input.sources, diagnostics = loadImportedDocumentsTrackedWithProcessing(source, options.Filesystem, parsed, destination, options.Definition.Version, limits, accounting, visited, hostToken, versionState, versionProcessing, options.sequencePolicy)
+			input.composition, input.sources, diagnostics = loadImportedDocumentsTrackedWithProcessing(selectedSource, options.Filesystem, parsed, destination, options.Definition.Version, limits, accounting, visited, hostToken, versionState, versionProcessing, options.sequencePolicy)
 		}
 		if len(diagnostics) != 0 {
 			return input, diagnostics
@@ -218,8 +240,8 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		if versionProcessing != nil {
 			input.document = versionProcessing.payload
 		}
-		if options.Filesystem != nil && !activeOverlay(options.Overlay) {
-			if diagnostic := validateFilesystemContext(source, options.Filesystem, parsed); diagnostic != nil {
+		if options.Filesystem != nil && (selectedSource.kind == sourceNone || !activeOverlay(options.Overlay)) {
+			if diagnostic := validateFilesystemContext(selectedSource, options.Filesystem, parsed); diagnostic != nil {
 				return input, []Diagnostic{*diagnostic}
 			}
 		}
@@ -251,7 +273,7 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		if accounting.sources+1 > limits.sources || accounting.bytes+parsed.bytes > limits.bytes || accounting.nodes+rawNodes+placementNodes > limits.nodes {
 			return input, []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, parsed.source.ID, "configuration exceeds the aggregate input limit")}
 		}
-		if diagnostic := registerAcquiredSource(source, options.Filesystem, parsed.source, visited, hostToken); diagnostic != nil {
+		if diagnostic := registerAcquiredSource(selectedSource, options.Filesystem, parsed.source, visited, hostToken); diagnostic != nil {
 			return input, []Diagnostic{*diagnostic}
 		}
 		accounting.sources++
@@ -259,7 +281,7 @@ func assembleBaseInputTrackedForRole[T any](source Source, options Options[T], d
 		accounting.nodes += rawNodes + placementNodes
 	}
 	if !activeOverlay(options.Overlay) || options.Imports {
-		contexts, diagnostic := contextsForAcquiredSources(source, options.Filesystem, parsed.source, input.sources)
+		contexts, diagnostic := contextsForAcquiredSources(selectedSource, options.Filesystem, parsed.source, input.sources)
 		if diagnostic != nil {
 			return input, []Diagnostic{*diagnostic}
 		}

internal/engine/metadata_locations_test.go

@@ -0,0 +1,79 @@
+package engine
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestMetadataLocationsAcrossBroadAuthoredAndDefaultMappings(t *testing.T) {
+	type config map[string]map[string]string
+	defaults := config{}
+	var body strings.Builder
+	for index := range 96 {
+		name := fmt.Sprintf("authored-%03d", index)
+		fmt.Fprintf(&body, "%s: {present: source}\n", name)
+		defaults[name] = map[string]string{"absent": "default"}
+		defaults[fmt.Sprintf("default-%03d", index)] = map[string]string{"absent": "default"}
+	}
+	input := parseDocumentForDecode(t, body.String())
+	decoded, issues := decodeConfiguration(mustSchemaFor[config](t), reflect.ValueOf(defaults), input)
+	if len(issues) != 0 {
+		t.Fatal(issues)
+	}
+	located := documentWithMetadataLocations(decoded.final, metadataFromDecoded(decoded), input)
+	inputIndex := buildDocumentLookup(input)
+	locatedIndex := buildDocumentLookup(located)
+	for index := range 96 {
+		authored := Keys(fmt.Sprintf("authored-%03d", index))
+		mapping, _ := documentAtPathWithLookup(input, authored, inputIndex)
+		present, _ := documentAtPathWithLookup(input, authored.Key("present"), inputIndex)
+		for _, test := range []struct {
+			path Path
+			want document.Location
+		}{
+			{authored.Key("present"), present.Location},
+			{authored.Key("absent"), mapping.Location},
+			{Keys(fmt.Sprintf("default-%03d", index), "absent"), input.Location},
+		} {
+			node, found := documentAtPathWithLookup(located, test.path, locatedIndex)
+			if !found || node.Location != test.want {
+				t.Fatalf("location at %s = %#v, want %#v", test.path, node, test.want)
+			}
+		}
+	}
+	walkDocument(located, Path{}, func(_ Path, node *document.Node) {
+		for _, entry := range node.Mapping {
+			if entry.KeyLocation != entry.Value.Location || entry.ValueLocation != entry.Value.Location {
+				t.Fatal("entry and value locations disagree")
+			}
+		}
+	})
+	before := decoded.final.Mapping[0].Value.Mapping[0].Value.Text
+	located.Mapping[0].Value.Mapping[0].Value.Text = "changed snapshot"
+	if decoded.final.Mapping[0].Value.Mapping[0].Value.Text != before || input.Mapping[0].Value.Mapping[0].Value.Text != "source" {
+		t.Fatal("location projection changed a borrowed input")
+	}
+}
+
+func TestIndexedNearestLocationsPreserveMissingAndSyntheticCoordinates(t *testing.T) {
+	root := parseDocumentForDecode(t, "mapping: {'0': value}\nitems: [{value: source}]\nscalar: stop\n")
+	lookup := buildDocumentLookup(root)
+	for _, path := range []Path{Path{}, Keys("missing"), Keys("mapping", "missing"), Keys("mapping", "0"), Keys("mapping").Index(0), Keys("items").Index(4), Keys("items").Index(0).Key("missing"), Keys("scalar", "child"), Path{}.Index(-1)} {
+		want, wantOK := nearestDocumentLocation(root, path)
+		got, gotOK := nearestDocumentLocationWithLookup(root, path, lookup)
+		if got != want || gotOK != wantOK {
+			t.Fatalf("nearest location changed for %s: %#v/%t, want %#v/%t", path, got, gotOK, want, wantOK)
+		}
+	}
+	final := parseDocumentForDecode(t, "value: cli\n")
+	synthetic := Location{SourceID: "cli-1"}
+	metadata := Metadata{fields: map[string]FieldInfo{"/value": {Origin: Origin{Layer: LayerCLI, Locations: []Location{synthetic}}}}}
+	located := documentWithMetadataLocations(final, metadata, root)
+	if location := located.Mapping[0].Value.Location; location.SourceID != "cli-1" || location.Line != 0 || location.Column != 0 {
+		t.Fatalf("synthetic CLI location was replaced by fallback: %#v", location)
+	}
+}

internal/engine/options.go

@@ -31,11 +31,18 @@ func selectOptions[T any](source Source, supplied []Options[T]) (Options[T], []D
 	if options.Filesystem != nil {
 		diagnostics = append(diagnostics, filesystemOptionDiagnostics(source, options.Filesystem)...)
 	}
-	if source.kind == sourceNone && options.Imports {
-		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "NoSource cannot enable imports"))
+	if source.kind == sourceSearch {
+		for _, issue := range source.searchIssues {
+			diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, issue))
+		}
+		if source.search == nil {
+			diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "configuration search is invalid"))
+		} else {
+			diagnostics = append(diagnostics, validateSearch(*source.search, source.searchApp)...)
+		}
 	}
-	if source.kind == sourceNone && options.Overlay != "" && options.Overlay != "default" {
-		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "NoSource cannot select an active overlay"))
+	if source.kind == sourceNone && activeOverlay(options.Overlay) && options.Filesystem == nil {
+		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "NoSource overlays require explicit filesystem context"))
 	}
 	if options.Filesystem == nil && (source.kind == sourceBytes || source.kind == sourceReader || source.kind == sourceMapping) && (options.Imports || activeOverlay(options.Overlay)) {
 		diagnostics = append(diagnostics, optionDiagnostic(CodeOptionsInvalid, "synthetic-source imports or overlays require filesystem context"))

internal/engine/overlay.go

@@ -138,8 +138,12 @@ func mergeOverlayMappingsWithCollectionPolicy(
 	for key := range declarationsByPath {
 		registered[key] = true
 	}
+	var overlayLookup *documentLookup
+	if len(result.collectionPolicies.collections) != 0 {
+		overlayLookup = buildDocumentLookup(overlay)
+	}
 	for key := range result.collectionPolicies.collections {
-		if node, found := documentAtPath(overlay, result.collectionPolicies.collections[key].destination); found && node != nil {
+		if node, found := documentAtPathWithLookup(overlay, result.collectionPolicies.collections[key].destination, overlayLookup); found && node != nil {
 			registered[key] = true
 			registeredPaths[key] = result.collectionPolicies.collections[key].destination.clone()
 		}
@@ -210,6 +214,9 @@ func mergeOverlayMappingsWithCollectionPolicy(
 		return nil, canonicalDiagnostics(combinationDiagnostics)
 	}
 
+	// Values are now complete. Index lazily for declaration-only checks;
+	// this lookup is never retained across the value-changing merge above.
+	var retainedLookup *documentLookup
 	for key, grouped := range declarationsByPath {
 		if policyValues[key] {
 			continue
@@ -226,7 +233,10 @@ func mergeOverlayMappingsWithCollectionPolicy(
 		if found {
 			lowerPolicy = &lower
 		}
-		retained, _ := documentAtPath(assembly.current.root, path)
+		if retainedLookup == nil {
+			retainedLookup = buildDocumentLookup(assembly.current.root)
+		}
+		retained, _ := documentAtPathWithLookup(assembly.current.root, path, retainedLookup)
 		if retained != nil && retained.Kind == document.Sequence && len(current.effectiveIdentityFields) != 0 {
 			if _, issues := validateSequenceIdentity(retained, path, current.effectiveIdentityFields, sequenceIdentityDiagnosticCodes{invalid: CodeSequenceInvalid, conflict: CodeSequenceConflict}); len(issues) != 0 {
 				combinationDiagnostics = append(combinationDiagnostics, issues...)

internal/engine/override_context.go

@@ -171,16 +171,23 @@ func documentWithMetadataLocations(root *document.Node, metadata Metadata, fallb
 	if len(fallback) != 0 {
 		fallbackRoot = fallback[0]
 	}
+	var lookup *documentLookup
+	fallbackLocation := func(path Path) (document.Location, bool) {
+		if lookup == nil {
+			lookup = buildDocumentLookup(fallbackRoot)
+		}
+		return nearestDocumentLocationWithLookup(fallbackRoot, path, lookup)
+	}
 	var visit func(*document.Node, Path)
 	visit = func(node *document.Node, path Path) {
 		pointer := path.String()
 		if info, exists := metadata.fields[pointer]; exists {
 			if location, ok := metadataFieldLocation(metadata, pointer, info.Origin); ok {
 				node.Location = location
-			} else if location, ok := nearestDocumentLocation(fallbackRoot, path); ok {
+			} else if location, ok := fallbackLocation(path); ok {
 				node.Location = location
 			}
-		} else if location, ok := nearestDocumentLocation(fallbackRoot, path); ok {
+		} else if location, ok := fallbackLocation(path); ok {
 			node.Location = location
 		}
 		switch node.Kind {
@@ -192,11 +199,11 @@ func documentWithMetadataLocations(root *document.Node, metadata Metadata, fallb
 					if location, ok := metadataFieldLocation(metadata, childPath.String(), info.Origin); ok {
 						entry.KeyLocation = location
 						entry.ValueLocation = location
-					} else if location, ok := nearestDocumentLocation(fallbackRoot, childPath); ok {
+					} else if location, ok := fallbackLocation(childPath); ok {
 						entry.KeyLocation = location
 						entry.ValueLocation = location
 					}
-				} else if location, ok := nearestDocumentLocation(fallbackRoot, childPath); ok {
+				} else if location, ok := fallbackLocation(childPath); ok {
 					entry.KeyLocation = location
 					entry.ValueLocation = location
 				}

internal/engine/override_decode.go

@@ -2,7 +2,6 @@ package engine
 
 import (
 	"reflect"
-	"strings"
 
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
 )
@@ -171,17 +170,16 @@ func updateOverridePresence(presence map[string]inputPresence, existing, incomin
 		return
 	}
 
-	deletePresencePathSubtree(presence, path)
+	deletePresencePathSubtree(presence, existing, path)
 	collectPresence(incoming, path, presence)
 }
 
-func deletePresencePathSubtree(presence map[string]inputPresence, path Path) {
-	pointer := path.String()
-	for candidate := range presence {
-		if candidate == pointer || strings.HasPrefix(candidate, pointer+"/") {
-			delete(presence, candidate)
-		}
-	}
+func deletePresencePathSubtree(presence map[string]inputPresence, existing *document.Node, path Path) {
+	// The typed pre-CLI tree contains every effective descendant. Visit only
+	// the replaced subtree rather than searching unrelated metadata entries.
+	walkDocument(existing, path, func(path Path, _ *document.Node) {
+		delete(presence, path.String())
+	})
 }
 
 func updateOverrideOrigins(origins map[string]Origin, existing, incoming *document.Node, path Path, location document.Location) {
@@ -200,12 +198,9 @@ func updateOverrideOrigins(origins map[string]Origin, existing, incoming *docume
 		return
 	}
 
-	pointer := path.String()
-	for candidate := range origins {
-		if candidate == pointer || strings.HasPrefix(candidate, pointer+"/") {
-			delete(origins, candidate)
-		}
-	}
+	walkDocument(existing, path, func(path Path, _ *document.Node) {
+		delete(origins, path.String())
+	})
 	collectOverlayOrigins(origins, incoming, path, LayerCLI)
 }
 

internal/engine/override_lifecycle_test.go

@@ -129,35 +129,42 @@ func TestPostDecodeOverrideLifecycleMatchesOrdinaryAndLayeredLoads(t *testing.T)
 func TestNoSourceOverridesReachResolutionAfterDefaults(t *testing.T) {
 	t.Parallel()
 	type config struct {
-		Count   int `yaml:"count"`
-		Derived int `yaml:"derived"`
+		Count   int  `yaml:"count"`
+		Enabled bool `yaml:"enabled"`
+		Derived int  `yaml:"derived"`
 	}
 	calls := 0
 	result, err := testLoad(NoSource(), Options[config]{
 		Definition: Definition[config]{
 			Defaults: func() config {
 				calls++
-				return config{Count: 2}
+				return config{Count: 2, Enabled: true}
 			},
 			Resolve: func(value *config, resolution *Resolution) error {
-				info, ok := resolution.InputInfo(Keys("count"))
-				if !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
-					t.Errorf("resolution count info = %#v, %v", info, ok)
+				for _, key := range []string{"count", "enabled"} {
+					info, ok := resolution.InputInfo(Keys(key))
+					if !ok || !info.Explicit || info.Origin.Layer != LayerCLI {
+						t.Errorf("resolution %s info = %#v, %v", key, info, ok)
+					}
 				}
 				Default(resolution, Keys("count"), &value.Count, 10, "preserve CLI value")
+				Default(resolution, Keys("enabled"), &value.Enabled, true, "preserve CLI value")
 				Default(resolution, Keys("derived"), &value.Derived, value.Count*3, "derive from CLI count")
 				return nil
 			},
 		},
-		Overrides: []Override{{Path: Keys("count"), Value: 4, Flag: "--count"}},
+		Overrides: []Override{
+			{Path: Keys("count"), Value: 0, Flag: "--count"},
+			{Path: Keys("enabled"), Value: false, Flag: "--enabled"},
+		},
 	})
 	if err != nil {
 		t.Fatal(err)
 	}
-	if calls != 1 || result.Config.Count != 4 || result.Config.Derived != 12 {
+	if calls != 1 || result.Config.Count != 0 || result.Config.Enabled || result.Config.Derived != 0 {
 		t.Fatalf("defaults-only CLI result = %#v, defaults calls %d", result.Config, calls)
 	}
-	if sources := result.Sources(); len(sources) != 1 || sources[0].Layer != LayerCLI {
+	if sources := result.Sources(); len(sources) != 2 || sources[0].Layer != LayerCLI || sources[1].Layer != LayerCLI {
 		t.Fatalf("defaults-only CLI sources = %#v", sources)
 	}
 }

internal/engine/override_metadata_test.go

@@ -0,0 +1,82 @@
+package engine
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestBroadPartialCLIOverrideRetainsUnchangedMetadata(t *testing.T) {
+	type child struct {
+		Name string `yaml:"name"`
+	}
+	type record struct {
+		Count   int     `yaml:"count"`
+		Zero    int     `yaml:"zero"`
+		Enabled bool    `yaml:"enabled"`
+		Items   []child `yaml:"items"`
+		Maybe   *child  `yaml:"maybe"`
+	}
+	type config struct {
+		Records map[string]record `yaml:"records"`
+	}
+	var body strings.Builder
+	body.WriteString("records:\n")
+	updates := make(map[string]any)
+	for index := range 96 {
+		name := fmt.Sprintf("key/%d~", index)
+		fmt.Fprintf(&body, "  %s: {count: 9, zero: 0, enabled: false, items: [{name: old}], maybe: {name: old}}\n", name)
+		if index%2 == 0 {
+			updates[name] = map[string]any{"count": 0, "items": []any{}, "maybe": nil}
+		}
+	}
+	input := parseDocumentForDecode(t, body.String())
+	schema := mustSchemaFor[config](t)
+	decoded, issues := decodeConfiguration(schema, reflect.ValueOf(config{}), input)
+	if len(issues) != 0 {
+		t.Fatal(issues)
+	}
+	batch, diagnostics := preparePostStructuralOverrides(overridePreparationInput{root: input}, nil,
+		[]Override{{Path: Keys("records"), Value: updates, Flag: "--records"}}, nil, defaultImportLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	result, issues := applyDecodedOverrides(schema, decoded, batch, decodeContext{})
+	if len(issues) != 0 {
+		t.Fatal(issues)
+	}
+	for index := range 96 {
+		name := fmt.Sprintf("key/%d~", index)
+		path := Keys("records", name)
+		value := result.value.Interface().(config).Records[name]
+		for _, key := range []string{"zero", "enabled"} {
+			pointer := path.Key(key).String()
+			if _, explicit := result.presence[pointer]; !explicit || result.origins[pointer].Layer != LayerBase {
+				t.Fatalf("retained explicit field lost metadata at %s", pointer)
+			}
+		}
+		if index%2 != 0 {
+			if value.Count != 9 || len(value.Items) != 1 || value.Maybe == nil || result.origins[path.Key("count").String()].Layer != LayerBase {
+				t.Fatalf("omitted record changed at %s", path)
+			}
+			continue
+		}
+		if value.Count != 0 || len(value.Items) != 0 || value.Maybe != nil {
+			t.Fatalf("CLI replacement failed at %s: %#v", path, value)
+		}
+		for _, removed := range []Path{path.Key("items").Index(0), path.Key("items").Index(0).Key("name"), path.Key("maybe").Key("name")} {
+			_, present := result.presence[removed.String()]
+			_, origin := result.origins[removed.String()]
+			if present || origin {
+				t.Fatalf("removed descendant kept metadata at %s", removed)
+			}
+		}
+		for _, key := range []string{"count", "items", "maybe"} {
+			pointer := path.Key(key).String()
+			if presence, ok := result.presence[pointer]; !ok || presence.location.SourceID != batch.overrides[0].source.ID || result.origins[pointer].Layer != LayerCLI {
+				t.Fatalf("CLI field lost metadata at %s", pointer)
+			}
+		}
+	}
+}

internal/engine/selected_overlay.go

@@ -59,9 +59,24 @@ func loadSelectedOverlayIntoWithVersion(source Source, context *FilesystemContex
 		}
 		return loadSelectedOverlayWithAccessAndVersion(access, base, sources, name, rule, limits, versionState, policy)
 	}
-	rootPath, _, _, diagnostic := importFilesystem(source, context, main)
-	if diagnostic != nil {
-		return cloneSourceInfos(sources), []Diagnostic{*diagnostic}
+	var rootPath string
+	if source.kind == sourceNone {
+		if context == nil {
+			value := optionDiagnostic(CodeOptionsInvalid, "NoSource overlays require explicit filesystem context")
+			return cloneSourceInfos(sources), []Diagnostic{value}
+		}
+		var err error
+		rootPath, err = canonicalExistingPath(context.Root)
+		if err != nil {
+			value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
+			return cloneSourceInfos(sources), []Diagnostic{value}
+		}
+	} else {
+		var diagnostic *Diagnostic
+		rootPath, _, _, diagnostic = importFilesystem(source, context, main)
+		if diagnostic != nil {
+			return cloneSourceInfos(sources), []Diagnostic{*diagnostic}
+		}
 	}
 	access, err := openHostAccess(rootPath)
 	if err != nil {

internal/engine/selected_overlay_test.go

@@ -2,11 +2,14 @@ package engine
 
 import (
 	"errors"
+	"io/fs"
 	"os"
+	"path"
 	"path/filepath"
 	"reflect"
 	"runtime"
 	"testing"
+	"testing/fstest"
 )
 
 type overlayTestConfig struct {
@@ -21,6 +24,81 @@ type overlayTestConfig struct {
 	Schema   map[string]string `yaml:"schema"`
 }
 
+func TestNoSourceSelectedOverlayUsesRootedHostContext(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Resource string `yaml:"resource"`
+	}
+
+	root := t.TempDir()
+	overlayPath := writeImportFile(t, root, "overlays/testing.yml", "resource: asset.txt\n")
+	writeImportFile(t, root, "overlays/asset.txt", "host asset\n")
+	loaded, err := testLoad[config](NoSource(), Options[config]{
+		Overlay: "testing", Filesystem: &FilesystemContext{Root: root},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sources := loaded.Sources()
+	if len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Path != canonicalTestPath(t, overlayPath) || sources[0].Layer != LayerOverlay || sources[0].Kind != SourceKindFile {
+		t.Fatalf("NoSource overlay sources = %#v", sources)
+	}
+	context, ok := loaded.Inspection.SourceContext("source-1")
+	if !ok || context.BaseDir != "overlays" {
+		t.Fatalf("overlay resource context = %#v, %v", context, ok)
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, loaded.Config.Resource))
+	if err != nil || string(data) != "host asset\n" {
+		t.Fatalf("overlay resource = %q, %v", data, err)
+	}
+}
+
+func TestNoSourceSelectedOverlayRequiresContextAndSelection(t *testing.T) {
+	t.Parallel()
+
+	requireLoadCode(t, NoSource(), Options[importTestConfig]{Overlay: "testing"}, CodeOptionsInvalid)
+
+	root := t.TempDir()
+	requireLoadCode(t, NoSource(), Options[importTestConfig]{
+		Overlay: "testing", Filesystem: &FilesystemContext{Root: root},
+	}, CodeOverlayMissing)
+
+	outside := t.TempDir()
+	requireLoadCode(t, NoSource(), Options[importTestConfig]{
+		Overlay: "testing", Filesystem: &FilesystemContext{Root: root, BaseDir: outside},
+	}, CodeImportOutsideRoot)
+}
+
+func TestNoSourceSelectedOverlayUsesCompleteNativeContext(t *testing.T) {
+	t.Parallel()
+	type config struct {
+		Resource string `yaml:"resource"`
+	}
+
+	filesystem := NewFilesystem("embedded", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("resource: asset.txt\n")},
+		"overlays/asset.txt":   &fstest.MapFile{Data: []byte("native asset\n")},
+	})
+	loaded, err := testLoad[config](NoSource(), Options[config]{
+		Overlay: "testing", Filesystem: &FilesystemContext{FS: &filesystem, BaseDir: ".", Root: "."},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sources := loaded.Sources()
+	if len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Path != "overlays/testing.yml" || sources[0].Layer != LayerOverlay || sources[0].Kind != SourceKindFilesystem {
+		t.Fatalf("native NoSource overlay sources = %#v", sources)
+	}
+	context, ok := loaded.Inspection.SourceContext("source-1")
+	if !ok || context.BaseDir != "overlays" {
+		t.Fatalf("native overlay resource context = %#v, %v", context, ok)
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, loaded.Config.Resource))
+	if err != nil || string(data) != "native asset\n" {
+		t.Fatalf("native overlay resource = %q, %v", data, err)
+	}
+}
+
 func TestLoadAppliesSelectedOverlayAfterBaseAndBeforeDefaults(t *testing.T) {
 	directory := t.TempDir()
 	mainPath := writeImportFile(t, directory, "main.yml", `configkit:

internal/engine/sequence_projection.go

@@ -1,33 +1,17 @@
 package engine
 
 type sequenceItemProjectionIndex struct {
-	outputByInput []int
-	inputByOutput []int
+	outputByInput map[int]int
+	inputByOutput map[int]int
 }
 
 func newSequenceItemProjectionIndex(projections []sequenceItemProjection) sequenceItemProjectionIndex {
-	maxInput := -1
-	maxOutput := -1
-	for _, projection := range projections {
-		if projection.inputIndex > maxInput {
-			maxInput = projection.inputIndex
-		}
-		if projection.outputIndex > maxOutput {
-			maxOutput = projection.outputIndex
-		}
-	}
 	index := sequenceItemProjectionIndex{
-		outputByInput: make([]int, maxInput+1),
-		inputByOutput: make([]int, maxOutput+1),
-	}
-	for input := range index.outputByInput {
-		index.outputByInput[input] = -1
-	}
-	for output := range index.inputByOutput {
-		index.inputByOutput[output] = -1
+		outputByInput: make(map[int]int, len(projections)),
+		inputByOutput: make(map[int]int, len(projections)),
 	}
 	for _, projection := range projections {
-		if projection.inputIndex >= 0 && projection.inputIndex < len(index.outputByInput) && projection.outputIndex >= 0 && projection.outputIndex < len(index.inputByOutput) {
+		if projection.inputIndex >= 0 && projection.outputIndex >= 0 {
 			index.outputByInput[projection.inputIndex] = projection.outputIndex
 			index.inputByOutput[projection.outputIndex] = projection.inputIndex
 		}
@@ -36,15 +20,11 @@ func newSequenceItemProjectionIndex(projections []sequenceItemProjection) sequen
 }
 
 func (index sequenceItemProjectionIndex) outputForInput(inputIndex int) (int, bool) {
-	if inputIndex < 0 || inputIndex >= len(index.outputByInput) || index.outputByInput[inputIndex] < 0 {
-		return 0, false
-	}
-	return index.outputByInput[inputIndex], true
+	output, found := index.outputByInput[inputIndex]
+	return output, found
 }
 
 func (index sequenceItemProjectionIndex) inputForOutput(outputIndex int) (int, bool) {
-	if outputIndex < 0 || outputIndex >= len(index.inputByOutput) || index.inputByOutput[outputIndex] < 0 {
-		return 0, false
-	}
-	return index.inputByOutput[outputIndex], true
+	input, found := index.inputByOutput[outputIndex]
+	return input, found
 }

internal/engine/sequence_projection_test.go

@@ -0,0 +1,49 @@
+package engine
+
+import "testing"
+
+func TestSparseSequenceProjectionIndexesOnlyAssignedPositions(t *testing.T) {
+	index := newSequenceItemProjectionIndex([]sequenceItemProjection{
+		{inputIndex: 0, outputIndex: 9000}, {inputIndex: 3000, outputIndex: 2},
+	})
+	if len(index.outputByInput) != 2 || len(index.inputByOutput) != 2 {
+		t.Fatal("sparse projection retained unassigned positions")
+	}
+	for input, output := range map[int]int{0: 9000, 3000: 2} {
+		if got, found := index.outputForInput(input); !found || got != output {
+			t.Fatalf("input %d = %d, %t", input, got, found)
+		}
+		if got, found := index.inputForOutput(output); !found || got != input {
+			t.Fatalf("output %d = %d, %t", output, got, found)
+		}
+	}
+	for _, position := range []int{-1, 1, 10000} {
+		if _, found := index.outputForInput(position); found {
+			t.Fatalf("unassigned input %d exists", position)
+		}
+		if _, found := index.inputForOutput(position); found {
+			t.Fatalf("unassigned output %d exists", position)
+		}
+	}
+}
+
+func TestContiguousSequenceRangesKeepArithmeticLookup(t *testing.T) {
+	ranges := make([]sequenceRange, 128)
+	for position := range ranges {
+		ranges[position] = sequenceRange{sourceIndex: position, path: Keys("items"), start: position, end: position + 1}
+	}
+	for _, assigned := range indexSequenceRanges(ranges) {
+		if len(assigned.projection.outputByInput) != 0 || len(assigned.projection.inputByOutput) != 0 {
+			t.Fatal("contiguous range materialized a projection index")
+		}
+		if output, found := assigned.outputForInput(0); !found || output != assigned.start {
+			t.Fatal("arithmetic forward lookup lost its offset")
+		}
+		if input, found := assigned.inputForOutput(assigned.start); !found || input != 0 {
+			t.Fatal("arithmetic reverse lookup lost its offset")
+		}
+		if _, found := assigned.inputForOutput(assigned.end); found {
+			t.Fatal("range includes its exclusive end")
+		}
+	}
+}

internal/engine/source.go

@@ -21,17 +21,21 @@ const (
 // Source is an opaque configuration input. Construct sources with File,
 // Filesystem.File, Bytes, Reader, or NoSource; the zero value is invalid.
 type Source struct {
-	kind      sourceKind
-	path      string
-	label     string
-	namespace *nativeNamespace
-	data      []byte
-	reader    *readerSource
-	mapping   *document.Node
-	mapIssue  *valueIssue
-	mapBytes  int64
-	mapNodes  int
-	createErr sourceCreationError
+	kind           sourceKind
+	path           string
+	label          string
+	namespace      *nativeNamespace
+	data           []byte
+	reader         *readerSource
+	mapping        *document.Node
+	mapIssue       *valueIssue
+	mapBytes       int64
+	mapNodes       int
+	createErr      sourceCreationError
+	search         *Search
+	searchApp      string
+	searchRequired bool
+	searchIssues   []string
 }
 
 type sourceKind uint8
@@ -44,6 +48,7 @@ const (
 	sourceReader
 	sourceMapping
 	sourceNone
+	sourceSearch
 )
 
 type sourceCreationError uint8
@@ -174,7 +179,9 @@ func Mapping(label string, values map[string]any) Source {
 	return source
 }
 
-// NoSource explicitly selects a defaults-only input. It creates no source record.
+// NoSource selects an absent input for defaults and resolution. It creates no
+// source record and has no main declarations to traverse when imports are enabled.
+// An active overlay requires explicit filesystem context.
 func NoSource() Source {
 	return Source{kind: sourceNone}
 }

internal/engine/source_context.go

@@ -51,10 +51,14 @@ func contextsForAcquiredSources(source Source, context *FilesystemContext, baseS
 			value := importDiagnostic(CodeImportInvalid, "configuration root could not be canonicalized", nil)
 			return nil, &value
 		}
-		basePath, err = canonicalExistingPath(context.BaseDir)
-		if err != nil {
-			value := importDiagnostic(CodeImportInvalid, "import base directory could not be canonicalized", nil)
-			return nil, &value
+		if source.kind == sourceNone && context.BaseDir == "" {
+			basePath = rootPath
+		} else {
+			basePath, err = canonicalExistingPath(context.BaseDir)
+			if err != nil {
+				value := importDiagnostic(CodeImportInvalid, "import base directory could not be canonicalized", nil)
+				return nil, &value
+			}
 		}
 	} else {
 		return nil, nil

internal/engine/source_version_test.go

@@ -539,6 +539,41 @@ func TestSourceVersionNoSourceMainRetainsAbsentCallback(t *testing.T) {
 			if !called || len(result.diagnostics) != 0 || result.evidence != nil || result.payload != nil {
 				t.Fatalf("NoSource result = %#v, callback called = %v", result, called)
 			}
+
+			type config struct{}
+			for _, reject := range []bool{false, true} {
+				called = false
+				definition := Definition[config]{Version: &VersionRule{
+					Path: Keys("version"),
+					Check: func(version Version, _ *Validation) error {
+						called = true
+						if version != (Version{}) {
+							t.Errorf("NoSource load version = %#v", version)
+						}
+						if reject {
+							return errors.New("rejected absent main")
+						}
+						return nil
+					},
+				}}
+				loaded, loadErr := testLoad(NoSource(), Options[config]{
+					Definition: definition, Imports: true, SourceVersionMode: test.mode,
+				})
+				if !called {
+					t.Fatal("NoSource load did not call the source-version checker")
+				}
+				if reject {
+					var failure *LoadError
+					if !errors.As(loadErr, &failure) {
+						t.Fatalf("rejected NoSource load = %v, want *LoadError", loadErr)
+					}
+					requireSourceVersionDiagnostic(t, failure.Diagnostics(), CodeVersionHookFailed, "/version")
+					continue
+				}
+				if loadErr != nil || loaded == nil || len(loaded.Sources()) != 0 {
+					t.Fatalf("accepted NoSource load = %#v, %v", loaded, loadErr)
+				}
+			}
 		})
 	}
 }

internal/engine/validation.go

@@ -124,6 +124,10 @@ func validationMutationDiagnostic(path Path) Diagnostic {
 }
 
 func nearestDocumentLocation(root *document.Node, path Path) (document.Location, bool) {
+	return nearestDocumentLocationWithLookup(root, path, nil)
+}
+
+func nearestDocumentLocationWithLookup(root *document.Node, path Path, lookup *documentLookup) (document.Location, bool) {
 	if root == nil || !path.valid() {
 		return document.Location{}, false
 	}
@@ -135,18 +139,22 @@ func nearestDocumentLocation(root *document.Node, path Path) (document.Location,
 			if segment.kind == pathSegmentIndex {
 				return nearest, locationKnown(nearest)
 			}
-			found := false
-			for _, entry := range current.Mapping {
-				if entry.Key == segment.key {
-					current = entry.Value
-					nearest = current.Location
-					found = true
-					break
+			position := -1
+			if lookup != nil {
+				position = lookup.find(current, segment.key)
+			} else {
+				for index, entry := range current.Mapping {
+					if entry.Key == segment.key {
+						position = index
+						break
+					}
 				}
 			}
-			if !found {
+			if position < 0 {
 				return nearest, locationKnown(nearest)
 			}
+			current = current.Mapping[position].Value
+			nearest = current.Location
 		case document.Sequence:
 			index, ok := schemaIndex(segment)
 			if !ok || index >= len(current.Sequence) {

lifecycle.go

@@ -83,8 +83,10 @@ type ImportDefaults struct {
 }
 
 // Options contains the application definition and per-load input selections.
-// Imports enables explicit import declarations; Overlay selects one fixed overlay;
-// Overrides contains only values explicitly supplied by an application CLI.
+// Imports enables traversal of declarations in a selected main document; an
+// absent source has no main declarations to traverse. Overlay selects one fixed
+// overlay and requires filesystem context for NoSource. Overrides contain only
+// values explicitly supplied by an application CLI.
 // Enabled source-version modes interpret Definition.Version.Path in each
 // document's local coordinates. Placement projects checks and main protection
 // through the input destination; item placement adds the source's item
@@ -109,9 +111,11 @@ type Options[T any] struct {
 }
 
 // FilesystemContext supplies roots for imports and overlays. With FS nil,
-// BaseDir and Root use absolute host paths. With FS set, they use slash-relative
-// paths in that namespace ("." denotes its root). File sources derive BaseDir
-// from the selected file; a native file's explicit Root limits its scope.
+// BaseDir and Root use absolute host paths. For NoSource, Root is required and
+// BaseDir may be omitted to use Root as the import base. With FS set, BaseDir
+// and Root use slash-relative paths in that namespace ("." denotes its root).
+// File sources derive BaseDir from the selected file; a native file's explicit
+// Root limits its scope.
 type FilesystemContext struct {
 	BaseDir string
 	Root    string

source.go

@@ -8,7 +8,8 @@ import (
 )
 
 // Source is an opaque configuration input. Construct sources with File,
-// Filesystem.File, Bytes, Reader, Mapping, or NoSource; the zero value is invalid.
+// Filesystem.File, SearchConfig, Bytes, Reader, Mapping, or NoSource; the zero
+// value is invalid. SearchConfig selects a host file when a load consumes it.
 type Source struct{ value engine.Source }
 
 // Filesystem is a reusable opaque namespace for a caller-owned fs.FS. Copies
@@ -47,7 +48,9 @@ func Mapping(label string, values map[string]any) Source {
 	return Source{value: engine.Mapping(label, values)}
 }
 
-// NoSource explicitly selects a defaults-only input. It creates no source record.
+// NoSource selects an absent input for defaults and resolution. It creates no
+// source record and has no main declarations to traverse when imports are enabled.
+// An active overlay requires explicit filesystem context.
 func NoSource() Source { return Source{value: engine.NoSource()} }
 
 func (s Source) toEngine() engine.Source { return s.value }