Harden Voice History writer convergence

[MarcusJRLee]

Summary

• share a five-second SQLite writer-coordination bound across session and retention connections
• require concurrent-retention tests to observe filesystem and metadata convergence
• prove finalization survives a deterministic 2.25-second writer lock
• document the persistence boundary in architecture and ADR 0031

Verification

• 476 tests in 71 suites passed
• deterministic contention regression passed
• release build passed
• HID soak: p50 0.010 ms, p95 0.015 ms, p99 0.018 ms, max 0.146 ms
• strict Swift format, Clang format, shell syntax, and diff checks passed

[MarcusJRLee]

Integrated into dev through rebase-merged rollup PR #34. This intermediate PR remains preserved as the focused review record for its original slice.