Close cross-cutting sequence regression gaps

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implementation Plan: Sequence Placement and Composition
 
-Status: implementation in progress; stages 1–11 completed.
+Status: implementation in progress; stages 1–12 completed.
 
 ## Purpose and authority
 
@@ -706,6 +706,23 @@ Validation: targeted new regressions, `go test -race ./...`, and
 `./scripts/check.sh`. Completion: no known evidence, ownership, or accounting gap
 remains hidden behind a correct final tree.
 
+Completion evidence: cross-cutting regressions now cover a typed main-tier
+sequence replacement after underlay item appends, including presence and
+zero/null distinctions, callback metadata and resource context, final origins,
+removed-item explanation, and historical redaction in explanation and effective
+rendering. Additional tests verify shared aggregate node accounting across a
+nested item glob and sibling contribution, native file closure after nested item
+composition failure, and retained sequence-history applications storing source
+fragments rather than cumulative snapshots. Existing sequence placement, strict
+diagnostic, version-evidence, and rendering tests continue to cover the other
+settled risks. These checks passed with Go `go1.26.9 linux/amd64`:
+
+- `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 13 — Document implemented contracts and provide a maintained example
 
 Scope: consumer documentation and a tested in-repository adoption fixture.