[P3][adapters] Document OPFS flush() weaker power-loss guarantees

[cevheri]

Summary

OPFS flush() may not provide same power-loss guarantees as POSIX fsync. Adapter maps fsync() → flush() without documenting weaker contract.

Audit ID: Section 4 Low — Wave 3
Location: src/adapter/opfs.ts:69-71
Related: #10 OPFS E2E

Tasks

1. Research OPFS sync handle durability docs (Chrome/Firefox).
2. Document in docs/RELIABILITY.md and adapter JSDoc what flush guarantees.
3. Verify OPFS browser persistence end-to-end on Chrome, Firefox, and Safari #10 E2E should verify best-effort persistence where possible.

Acceptance criteria

• Weaker browser contract documented honestly.
• No false parity with node-fs in README.
• Docs-only changeset usually not needed.