Document shared checkpoint persistence

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implementation Plan: Shared Stage Checkpoint Mechanics
 
-Status: Implementation in progress; Stages 1-3 complete.
+Status: Complete.
 
 ## Scope and Authority
 
@@ -273,7 +273,24 @@ copy implementation, with their different models and synchronization intact.
 
 ## Stage 4: Document, Review Scope, and Validate
 
-Status: Pending.
+Status: Complete.
+
+Completion marker (2026-09-22): Documented the private shared checkpoint
+persistence boundary and linked the stage guides back to it while retaining
+their separate analyze and transcribe progress policies. The refactor removed
+the two local candidate/save/promotion sequences and the transcription-only
+bounded clone; the retained thin wrappers own typed validation, updates,
+working-state refresh, and their existing serialization and failure behavior.
+The collective test review retained focused helper integrity coverage plus
+stage/application recovery tests, without duplicating the helper's isolation
+matrix. A cumulative scope audit found no schema, validation-policy,
+scheduling, force, concurrency, output-selection, stage-order, adapter-call,
+configuration, or public-interface change.
+
+Verified with `go test ./...`, `go test -race ./...`, `go vet ./...`,
+`go build ./...`, `go test ./internal/doccheck`,
+`go test ./internal/config -run '^TestExamplesLoadAndValidate$'`, and
+`git diff --check`; all passed.
 
 Preparation: `docs/internal/manifest.md`, `docs/internal/stage-transcribe.md`,
 `docs/internal/stage-analyze.md`, documentation/testing policies, and the