fix(storage): a dead flush watch falls back to the 500ms poll, not the 30s sweep

The safety sweep is armed alongside the watch, and startFlushRequestPolling()
declines to arm over an existing interval — so when a watch died mid-life the
fallback did nothing and the store quietly answered flush requests on a 30s
cadence instead of the 500ms one the door promises. The sweep is cleared first.
A degrade nobody asked for is still a degrade.

src/storage/adapters/fileSystemStorage.ts

@@ -2467,6 +2467,15 @@ export class FileSystemStorage extends BaseStorage {
             )
             this.flushWatcher?.close()
             this.flushWatcher = undefined
+            // The SAFETY sweep must go first. It is already armed at 30s, and
+            // startFlushRequestPolling() declines to arm over an existing
+            // interval — so leaving it would quietly leave this store answering
+            // flush requests on a 30s cadence instead of the 500ms one the door
+            // promises. A degrade nobody asked for is still a degrade.
+            if (this.flushWatcherInterval) {
+              clearInterval(this.flushWatcherInterval)
+              this.flushWatcherInterval = undefined
+            }
             this.startFlushRequestPolling(sweep)
           })
           if (typeof watcher.unref === 'function') watcher.unref()