step 2

docs/PHASE_6_BACKGROUND_PERSISTENCE_DESIGN.md

@@ -7,7 +7,7 @@ Phases 1-5 separate durable state, coordination policy, render-facing snapshots,
 ## Status
 
 - Phase 6 design package: proposed.
-- Phase 6 implementation: Step 1 complete.
+- Phase 6 implementation: Step 2 complete.
 - Current alignment: `RuntimeStore` owns durable serialization, config, package metadata, preset IO, and persistence requests; `CommittedLiveState` owns the current committed/session layer state; and `RuntimeCoordinator` publishes typed persistence requests for persisted mutations. The remaining issue is that actual disk writes are still synchronous store work rather than queued, debounced, atomic background writes.
 
 Current persistence footholds:
@@ -201,9 +201,16 @@ Move file-write mechanics behind a helper while keeping serialization ownership
 
 Initial target:
 
-- `RuntimeStore` can build serialized runtime-state snapshots
-- `PersistenceWriter` writes the snapshot
-- existing synchronous save path can call through the writer/helper during transition
+- [x] `RuntimeStore` can build serialized runtime-state snapshots
+- [x] `PersistenceWriter` writes the snapshot
+- [x] existing synchronous save path can call through the writer/helper during transition
+
+Current implementation:
+
+- `RuntimeStore::BuildRuntimeStatePersistenceSnapshot(...)` captures serialized runtime-state content and target path.
+- `PersistenceWriter::WriteSnapshot(...)` owns the temp-file and replace write mechanics.
+- `RuntimeStore::SavePersistentState(...)` still behaves synchronously, but now writes through `PersistenceWriter`.
+- Stack preset saves also use `PersistenceWriter` synchronously; preset async policy remains a later decision.
 
 ### Step 3. Add Debounced Background Worker