Implement crash-safe persistence for the Bitbox hash table:
- wal.go: WAL format with START/CLEAR/UPDATE/END entries, builder/reader
- sync.go: 3-phase sync protocol (BeginSync → WriteWAL → CommitSync)
- recover.go: WAL replay for crash recovery
The WAL records page diffs (not full pages) for compact logging. The
3-phase protocol ensures: WAL fsynced before HT modification, HT fsynced
before WAL truncation, providing at-least-once delivery of page updates.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
