Adds sync.RWMutex (ioMu) to BlockVol enforcing mutual exclusion
between normal I/O and destructive state operations.
Shared (RLock): WriteLBA, ReadLBA, Trim, SyncCache, replica
applyEntry, rebuild applyRebuildEntry — concurrent I/O safe.
Exclusive (Lock): RestoreSnapshot, ImportSnapshot, Expand,
PrepareExpand, CommitExpand, CancelExpand — drains all in-flight
I/O before modifying extent/WAL/dirtyMap.
Scope rule: RLock covers local data-structure mutation only.
Replication shipping is asynchronous and outside the lock, so
exclusive holders block only behind local I/O, not network stalls.
Lock ordering: ioMu > snapMu > assignMu > mu.
Closes the critical ER item: restore/import vs concurrent WriteLBA
silent data corruption gap.
3 new tests: concurrent writes allowed, real restore-vs-write
contention with data integrity check, close coordination.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
