Report malformed sequence items at assigned paths

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implementation Plan: Sequence Placement and Composition
 
-Status: implementation in progress; stages 1–13 completed.
+Status: implementation complete; all 14 stages completed.
 
 ## Purpose and authority
 
@@ -796,3 +796,20 @@ No environmental limitation affected validation; checks ran on Linux/amd64.
 Completion: the repository is clean, the feature and its evidence agree, and no
 known in-scope implementation work remains. No product decisions remain open;
 private helper names and equivalent mechanics are ordinary implementation work.
+
+Completion evidence: final review covered the complete implementation diff from
+baseline `2051177` against the settled behavior and public-contract boundaries.
+It found one malformed-record diagnostic using a contribution-local rather than
+assigned sequence index. The composer now reports the effective assigned item
+path, with a regression for a malformed record in the second contribution at
+`/records/1`. Review also confirmed registered identity checks remain limited to
+strict composition, precedence still replaces complete sequences, both opt-ins
+are explicit, and source ownership, physical locations, contexts, and detached
+evidence remain intact. Final checks passed with Go `go1.26.9 linux/amd64`:
+
+- `GOTOOLCHAIN=local GOPROXY=off go test ./internal/engine -run 'ComposeDocuments(ReportsAssignedPathForMalformedRegisteredSequenceContribution|ValidatesRegisteredSequenceSingletons|RejectsRegisteredSequence)' -count=1`
+- `GOTOOLCHAIN=local GOPROXY=off go test -race ./...`
+- `GOTOOLCHAIN=local GOPROXY=off ./scripts/check.sh`
+- `git diff --check 2051177`
+
+No environmental limitation affected validation; checks ran on Linux/amd64.