P2-04: coalesce facade autosaves

src/facade/projectStore.ts

@@ -35,6 +35,35 @@ export class PersistenceWriteQueue {
   }
 }
 
+export class ProjectAutosaveScheduler {
+  private timer: ReturnType<typeof setTimeout> | null = null
+  private pending: DocumentSnapshot | null = null
+  private flushPromise: Promise<ProjectSaveResult | null> | null = null
+
+  constructor(private readonly save: (document: DocumentSnapshot) => Promise<ProjectSaveResult>, private readonly idleMs = 800) {}
+
+  schedule(document: DocumentSnapshot) {
+    this.pending = cloneDocument(document)
+    if (this.timer) clearTimeout(this.timer)
+    this.timer = setTimeout(() => { this.timer = null; void this.flush() }, this.idleMs)
+  }
+
+  flush() {
+    if (this.flushPromise) return this.flushPromise
+    const document = this.pending
+    this.pending = null
+    if (!document) return Promise.resolve(null)
+    this.flushPromise = this.save(document).finally(() => { this.flushPromise = null; if (this.pending) void this.flush() })
+    return this.flushPromise
+  }
+
+  cancel() {
+    if (this.timer) clearTimeout(this.timer)
+    this.timer = null
+    this.pending = null
+  }
+}
+
 export class SqliteProjectPersistence implements ProjectPersistenceClient {
   private readonly worker: Worker | null
   private nextRequestId = 0