Step 3

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 2 complete.
+- Phase 6 implementation: Step 3 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:
@@ -218,9 +218,16 @@ Introduce a worker thread or queued task owner.
 
 Initial target:
 
-- repeated runtime-state requests coalesce
-- worker writes only latest pending snapshot
-- tests cover coalescing without filesystem where possible
+- [x] repeated runtime-state requests coalesce
+- [x] worker writes only latest pending snapshot
+- [x] tests cover coalescing without filesystem where possible
+
+Current implementation:
+
+- `PersistenceWriter::EnqueueSnapshot(...)` starts a worker lazily and debounces snapshots by `debounceKey`.
+- Runtime-state saves enqueue debounced snapshots, so routine mutation paths no longer write the runtime-state file directly.
+- Synchronous `PersistenceWriter::WriteSnapshot(...)` remains for stack preset saves and transitional direct writes.
+- `PersistenceWriterTests` use an injected in-memory sink to verify coalescing and non-coalesced immediate requests without touching the filesystem.
 
 ### Step 4. Add Atomic Write And Failure Reporting