[Data Integrity] Implement atomic commit with write-ahead logging (WAL)

[bbopen]

Summary

Database commits are not atomic - a crash during commit can leave the database in an inconsistent state with orphaned blocks. Implement a write-ahead log (WAL) for atomic commits with crash recovery.

Problem

In core/blockstore/commitor.ts, blocks are written individually. If a crash occurs mid-commit:

• Some blocks may be written, others not
• The meta record may point to non-existent blocks
• Orphaned blocks consume storage without being reachable

Proposed Solution

1. Design WAL entry format for commit operations
2. Create WALService at core/blockstore/wal-service.ts
3. Implement IndexedDB WAL storage
4. Implement two-phase commit protocol:
   • Begin: Write WAL entry with commit intent
   • Record: Write blocks with WAL reference
   • Commit: Update meta record
   • Finalize: Remove WAL entry
5. Implement rollback with orphan cleanup
6. Implement crash recovery on database open
7. Integrate WAL into commitor

Expected Outcomes

• All blocks in commit succeed or fail together
• Partial failures trigger complete rollback
• Crash recovery restores consistent state
• Optional feature flag for WAL

Dependencies

Should be implemented after the validateBlocks async fix (PR #1494).