Add shared stage checkpoint persistence

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Implementation Plan: Shared Stage Checkpoint Mechanics
 
-Status: Not started.
+Status: Implementation in progress; Stage 1 complete.
 
 ## Scope and Authority
 
@@ -152,7 +152,16 @@ to simplify copying.
 
 ## Stage 1: Add the Shared Checkpoint Operation
 
-Status: Pending.
+Status: Complete.
+
+Completion marker (2026-09-22): Added the private stage-level checkpoint
+save-and-promote helper and bounded candidate copying without migrating either
+production caller. Focused real-store coverage proves live isolation through
+updates, validation, save-time normalization and timestamps, save failures,
+missing-target creation, pointer-preserving promotion, unrelated aggregate
+preservation, caller-copy isolation, context/error identity, and exact one-save
+behavior. Verified with `go test ./internal/manifest ./internal/stage`,
+`go test -race ./internal/stage`, and `git diff --check`.
 
 Preparation: `internal/manifest/store.go`, `manifest.go`, `analyze_state.go`,
 `transcribe_state.go`; `internal/stage/analyze_execution_state.go`,