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.

internal/engine/compose.go

@@ -239,7 +239,7 @@ func validateRegisteredSequences(inputs []*document.Node, claims []compositionCl
 		start := lengthsByPath[pathKey]
 		contribution := validatedSequence{items: node.Sequence}
 		for itemIndex, item := range node.Sequence {
-			itemPath := claim.path.Index(itemIndex)
+			itemPath := claim.path.Index(start + itemIndex)
 			if item == nil || item.Kind != document.Mapping {
 				location := (*Location)(nil)
 				if item != nil {

internal/engine/compose_test.go

@@ -339,6 +339,20 @@ func TestComposeDocumentsValidatesRegisteredSequenceSingletons(t *testing.T) {
 	}
 }
 
+func TestComposeDocumentsReportsAssignedPathForMalformedRegisteredSequenceContribution(t *testing.T) {
+	_, diagnostics := composeDocumentsWithPolicy(
+		sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+		parseCompositionDocument(t, "source-1", "records: [{id: first}]\n"),
+		parseCompositionDocument(t, "source-2", "records: [{name: malformed}]\n"),
+	)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid || diagnostics[0].Path.String() != "/records/1" {
+		t.Fatalf("diagnostics = %#v, want invalid item at /records/1", diagnostics)
+	}
+	if diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" {
+		t.Fatalf("malformed item location = %#v, want source-2", diagnostics[0].Primary)
+	}
+}
+
 func TestComposeDocumentsRejectsRegisteredDuplicateIdentities(t *testing.T) {
 	tests := []struct {
 		name   string