Preserve append history across tiers

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implementation Plan: Sequence Placement and Composition
 
-Status: ready for implementation; no stages completed.
+Status: implementation in progress; stages 1–4 completed.
 
 ## Purpose and authority
 
@@ -333,6 +333,21 @@ Scope: private history operations, replay, and precedence handoff.
 Validation: history/explanation/overlay tests and `./scripts/check.sh`.
 Completion: strict accumulation is distinguishable from precedence replacement.
 
+Completion evidence: history applications now carry owned per-collection
+contribution ranges and distinguish each strict tier's first sequence
+contribution (replacement) from later appends. Replay preserves source
+applications through underlay handoff, maps appended record queries back to
+source-local indexes, and supports semantic indexed helper paths. Tests cover
+appended and independent collections, origin attribution, detached paths,
+explicit empty-list replacement, ordinary whole-list replacement, and tier
+handoff. These checks passed with Go `go1.26.9 linux/amd64`:
+
+- `GOTOOLCHAIN=local GOPROXY=off go test ./internal/engine -run 'History|Explain|Overlay' -count=1`
+- `GOTOOLCHAIN=local ./scripts/check.sh`
+- `git diff --check`
+
+No environmental limitation affected validation; checks ran on Linux/amd64.
+
 ## Stage 5 — Integrate projected paths with inspection and sensitivity
 
 Scope: metadata, retained history traversal, and safe inspection.