Adds sync.Mutex (superMu) to BlockVol, shared between group commit's
syncWithWALProgress() and flusher's updateSuperblockCheckpoint().
Both paths now serialize superblock mutation + persist, preventing
WALTail/WALCheckpointLSN regression when flusher and group commit
write the full superblock concurrently.
persistSuperblock() also guarded for consistency.
Removes temporary log.Printf lines in the open/recovery path that
were added during BUG-RESTART-ZEROS investigation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>