Use dirty flags to coalesce tile updates in SAB ring

- Extend SharedTileRing to include a shared dirtyFlags buffer alongside header and data
- Pass shared dirty buffer through WorkerClient/WorkerMessages and initialize views in Worker.worker
- In SAB mode, mark tiles dirty via Atomics.compareExchange before enqueuing to ensure each tile is queued at most once until processed
- On the main thread, clear dirty flags when draining the ring and build packedTileUpdates from distinct tile refs
- Keep non-SAB behaviour unchanged while reducing ring pressure and making overflows reflect true backlog, not duplicate updates

src/core/worker/WorkerMessages.ts

@@ -38,6 +38,7 @@ export interface InitMessage extends BaseWorkerMessage {
   sharedTileRingHeader?: SharedArrayBuffer;
   sharedTileRingData?: SharedArrayBuffer;
   sharedStateBuffer?: SharedArrayBuffer;
+  sharedDirtyBuffer?: SharedArrayBuffer;
 }
 
 export interface TurnMessage extends BaseWorkerMessage {