#22432: feat(memory): harden writes with cross-process file locking

## Summary - harden memory write critical sections with cross-process file locking - apply to both `memory_write` and `memory_upsert` - preserve concurrent upsert e2e safety coverage ## Stacking / review notes - **This is intentionally stacked on #21003** (memory tools foundation). - Until #21003 merges, this PR diff will include shared foundation changes. - After #21003 lands, this should collapse to the incremental lock-hardening delta. ## Why In-process mutexes are not sufficient when multiple workers/processes can touch memory files (for example cron + active user session). Cross-process locking prevents write interleaving and corruption risk.