[AUD-D02] Add cross-process concurrency control for storage transactions

[ndycode]

Deferred Audit Finding

• Finding ID: AUD-D02
• Severity: P2 (data consistency)
• Target milestone: next patch release

Problem

Current storage transaction protection is process-local. Concurrent multi-process writes can still race into last-writer-wins outcomes.

Scope

• Add advisory file lock or compare-and-swap conflict detection
• Add bounded retry/merge strategy for conflict recovery
• Add integration tests that simulate multi-process concurrent writes

Acceptance Criteria

1. Concurrent process writes cannot silently lose updates.
2. Conflict path is deterministic and observable.
3. Existing WAL/backup behavior remains intact.
4. Full validation gate remains green.