fix: null the cleared batch timer on last-listener unsubscribe

`Timeout.refresh()` is a no-op on a handle already passed to
`clearTimeout`. `unsubscribeLocalWatcher` cleared `root.batch.timer`
without nulling it, so a re-subscribe inside the teardown grace window
reused the root with a dead handle and `scheduleLocalBatchFlush` never
re-armed — fs change events for that root stopped reaching the renderer.

- Null `root.batch.timer` after clearing it in the unsubscribe path.
- Add a real-timer regression test covering unsubscribe + re-subscribe
  within the grace window; it fails on the previous PR head.

src/main/ipc/filesystem-watcher-local-events.test.ts

@@ -16,6 +16,11 @@ vi.mock('./parcel-watcher-process', () => ({ subscribeViaWatcherProcess: subscri
 
 import { createLocalWatcher } from './filesystem-watcher-local-events'
 import { cancelLocalBatchFlush } from './filesystem-watcher-batch-control'
+import {
+  subscribeLocalWatcher,
+  unsubscribeLocalWatcher
+} from './filesystem-watcher-local-subscription'
+import { watcherLifecycleState } from './filesystem-watcher-lifecycle-state'
 
 function deferred<T>(): { promise: Promise<T>; resolve: (value: T) => void } {
   let resolve!: (value: T) => void
@@ -326,4 +331,26 @@ describe('local filesystem watcher flush serialization', () => {
       { kind: 'update', absolutePath: otherPath, isDirectory: false }
     ])
   })
+
+  it('re-arms the debounce window after a re-subscribe inside the teardown grace period', async () => {
+    // Why real timers: fake-timers' refresh() revives a cleared handle, but Node's is a no-op — the bug only shows on real Timeouts.
+    vi.useRealTimers()
+    statMock.mockResolvedValue({ isDirectory: () => true })
+    const listener = { ...sender, id: 7, once: vi.fn() }
+    try {
+      await subscribeLocalWatcher('/repo', listener as never)
+      watcherCallback?.(null, [{ type: 'delete', path: '/repo/file.ts' }])
+      unsubscribeLocalWatcher('/repo', listener.id)
+      await subscribeLocalWatcher('/repo', listener as never)
+      watcherCallback?.(null, [{ type: 'delete', path: '/repo/file.ts' }])
+      await new Promise((resolve) => setTimeout(resolve, WATCH_BATCH_TRAILING_MS + 50))
+      expect(sender.send).toHaveBeenCalledTimes(1)
+    } finally {
+      for (const teardown of watcherLifecycleState.pendingTeardowns.values()) {
+        clearTimeout(teardown)
+      }
+      watcherLifecycleState.pendingTeardowns.clear()
+      watcherLifecycleState.watchedRoots.clear()
+    }
+  })
 })

src/main/ipc/filesystem-watcher-local-subscription.ts

@@ -199,6 +199,8 @@ export function unsubscribeLocalWatcher(worktreePath: string, senderId: number):
   if (root.listeners.size === 0) {
     if (root.batch.timer) {
       clearTimeout(root.batch.timer)
+      // Why: a cleared handle can't be refresh()ed; null it so a grace-window re-subscribe arms a fresh window.
+      root.batch.timer = null
     }
     // Why: duplicate unwatch calls for a root would leak overwritten grace timers; keep just one.
     if (watcherLifecycleState.pendingTeardowns.has(rootKey)) {