Delegate transcription checkpoints to shared persistence

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implementation Plan: Shared Stage Checkpoint Mechanics
 
-Status: Implementation in progress; Stages 1-2 complete.
+Status: Implementation in progress; Stages 1-3 complete.
 
 ## Scope and Authority
 
@@ -229,7 +229,17 @@ selection, and failure behavior remain unchanged.
 
 ## Stage 3: Migrate Transcribe and Remove Duplicate Machinery
 
-Status: Pending.
+Status: Complete.
+
+Completion marker (2026-09-22): Delegated transcription's locked checkpoint
+operation to the shared helper while retaining typed validation, direct-call
+record initialization, stage timestamp updates, post-save working collection
+refresh, and the existing caller-owned mutex and failure latch. Removed the
+duplicate production manifest clone; the read-only resume assertion now uses
+the shared bounded snapshot. Existing real-store, concurrency, force/retry,
+interruption, aggregate-repair, and zero-sized-audio coverage passed unchanged.
+Verified with `go test ./internal/stage ./internal/app ./internal/manifest`,
+`go test -race ./internal/stage ./internal/app`, and `git diff --check`.
 
 Preparation: `internal/stage/transcribe_execution_state.go`, `transcribe.go`,
 `transcribe_resume.go`, `transcribe_checkpoint_test.go`,