Replace the incremental batch-insert approach in doEnsureVisited with
the new refresh lifecycle:
1. BeginRefresh - start buffering subscription events
2. ReadDirAllEntries - fetch full listing (no lock held)
3. CommitRefresh - atomically replace cache + replay buffered events
This ensures that any creates, deletes, or updates that arrive via the
subscription handler during the filer listing are not lost. The snapshot
replaces all stale entries, and buffered events are replayed on top to
bring the cache up to date.
e91d43ef08