Integrate collection policies across layered assembly

docs/config.md

@@ -130,16 +130,20 @@ nothing. Underlay groups are listed from lowest to highest priority. Repeating
 a file within one group remains an import/repetition error; the same reusable
 source may be selected again in another tier.
 
-ConfigKit acquires and composes the main input first, constructs the schema,
-and checks the optional application version from the main tier only. Underlay
-roots cannot claim that version path (`underlay.version`); imported fragments
-retain `import.version` protection. It then acquires and composes underlay
-groups in order, recursively combines the groups, and applies the main tier,
-one selected overlay, and explicit CLI overrides. Scalar, sequence, and null
-definitions replace lower values between tiers; mappings combine recursively.
-Syntax and strict-composition errors inside a tier cannot be masked by a
-higher-priority tier. Defaults, typed decoding, resolution, secret lookup,
-validation, and publication run once on the final assembled input.
+ConfigKit acquires the main input first and constructs the schema. It then
+acquires and strictly composes underlay groups from lowest to highest priority,
+passing each group's accumulated lower collection policy into the next group.
+Each group is applied once as a precedence tier. The main tier is then composed
+with the accumulated underlay policy, and the optional application version is
+checked against that composed main tier only. Underlay roots cannot claim that
+version path (`underlay.version`); imported fragments retain `import.version`
+protection. ConfigKit then applies the selected overlay and explicit CLI
+overrides. Scalar, sequence, and null definitions replace lower values between
+tiers unless a collection policy defines the sequence operation; mappings
+combine recursively. Syntax and strict-composition errors inside a tier cannot
+be masked by a higher-priority tier. Defaults, typed decoding, resolution,
+secret lookup, validation, and publication run once on the final assembled
+input.
 
 An active layered overlay requires `LayeredOptions.OverlayFilesystem` explicitly,
 independent of the main source's namespace. Its name and root-relative lookup
@@ -149,7 +153,10 @@ Ordinary `Load` continues to infer its selected-overlay context from its source.
 
 Source IDs follow acquisition order—main and its depth-first imports, then each
 underlay tier's inputs and depth-first imports, then overlay and CLI—even though
-history follows precedence order. Source `Layer` describes precedence while
+history follows precedence order. Default main-version checking happens after
+underlay composition because the main tier inherits the accumulated collection
+policy. Enabled per-source version checks retain acquisition order and source
+local headers. Source `Layer` describes precedence while
 `Kind` describes acquisition. Underlay records use `LayerUnderlay` and a
 one-based `UnderlayIndex` giving their original tier position; all other records
 have index zero. Explicit empty groups and `NoSource` consume no source ID.
@@ -824,8 +831,8 @@ headers are consumed before strict composition and cannot supply a registered
 record identity. Final typed decoding and application validation still run on the
 completed configuration.
 
-Ordinary sources and selected overlays may declare collection behavior under
-`configkit.sequences` at the document root. Each entry has a required literal
+Ordinary sources, layered inputs, and selected overlays may declare collection
+behavior under `configkit.sequences` at the document root. Each entry has a required literal
 `destination` and may specify `key` as one string or a nonempty list of strings,
 plus a `mode` of `replace`, `append`, or `upsert`. These controls are removed
 before application decoding. A declaration destination is relative to that
@@ -841,11 +848,14 @@ replaces a complete matched record at its existing position and appends new
 identities in incoming order; it does not merge record fields.
 
 Declarations govern ordinary strict source composition, including loads with
-imports disabled, and selected-overlay precedence. Overlay declarations apply
-after the base source graph is composed and must agree with retained collection
-identity and any application registration. They do not change layered
-precedence, inheritance, or CLI behavior. A root `configkit.imports`
-declaration still requires `Options.Imports`; `NoSource` has no declaration scope.
+imports disabled, and each strict scope in layered loading. Layered groups and
+main apply their resolved sequence operation between precedence tiers; a higher
+strict scope inherits the lower identity and explicit mode before peer
+composition. Overlay declarations apply after the base source graph is composed
+and must agree with retained collection identity and any application
+registration. These policies do not change inheritance or CLI behavior. A root
+`configkit.imports` declaration still requires `Options.Imports`; `NoSource` has
+no declaration scope.
 
 An empty mapping contributes no child definitions. Two empty mappings compose, and
 an empty mapping neither clears nor conflicts with another mapping's children.

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Sequence Identity And Combination Implementation Plan
 
-Status: implementation in progress; Stages 1–14 complete.
+Status: implementation in progress; Stages 1–15 complete.
 
 Audience: a Luna coding agent implementing one numbered stage per prompt, in
 order. This plan starts at commit
@@ -556,6 +556,12 @@ Do not absorb unfinished work from Stages 5–14 into this stage.
 Validation: layered/grouped, source-version, dependency scheduling, destination,
 and metadata tests plus the everyday check.
 
+**Completion:** Implemented lower collection-policy propagation across underlay
+groups, main composition, and layered overlays. Verified policy-only lower
+tiers, inherited identity and explicit mode, explicit replacement, source
+version behavior, sequence history, and consumer examples. Validation:
+`go test ./internal/engine -count=1` and `./scripts/check.sh`.
+
 ## Stage 16 — Verify Replacement And Nested Policy Ownership End To End
 
 Scope: the consequential lifetime cases across the now-active source assembly.