Document sequence composition for consumers

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implementation Plan: Sequence Placement and Composition
 
-Status: implementation in progress; stages 1–12 completed.
+Status: implementation in progress; stages 1–13 completed.
 
 ## Purpose and authority
 
@@ -756,6 +756,24 @@ Validation: executable example/consumer tests, local documentation-link tests,
 whitespace review, and `./scripts/check.sh`.
 Completion: current docs and tested public examples describe what actually lands.
 
+Completion evidence: the canonical configuration contract and architecture
+policy now describe placement, registered strict composition, version/header
+boundaries, source containers, history, and unchanged precedence. README,
+consumer, development, and internal guides link to those contracts and the
+maintained prompt-catalog example. The example uses disposable in-repository
+fixtures to demonstrate path/glob item imports, a helper-supplied identity,
+strict composition, whole-sequence replacement of defaults, and consumer-owned
+validation. The feature roadmap and ADR status point to the implemented contract
+and delivery record; broader composite-identity behavior remains deferred.
+These checks passed with Go `go1.26.9 linux/amd64`:
+
+- `GOTOOLCHAIN=local GOPROXY=off go test ./examples/consumers`
+- `GOTOOLCHAIN=local GOPROXY=off go test -race ./...`
+- `GOTOOLCHAIN=local GOPROXY=off ./scripts/check.sh`
+- `git diff --check`
+
+No environmental limitation affected validation; checks ran on Linux/amd64.
+
 ## Stage 14 — Review the completed unit and hand off
 
 - Read the complete implementation diff against `2051177` and the feature