## Summary
Buzz is moving database invariants from triggers and foreign keys into
application-owned transaction protocols. This PR adds the first
transition foundations while preserving all current database backstops.
- Add a shared community-write lock for single-community writer
transactions, and make deletion lifecycle transitions take the matching
exclusive lock. A supported serving transaction never spans communities.
- Add shared event-writer and exclusive replica-probe lock ordering
around the existing heartbeat publication model.
- Document the supported writer contract, reviewed backfill path, and
transition gates.
- Keep one replica-floor publication concept and remove an unrelated
reconciliation-script edit found during review. This narrows the change
rather than adding parallel protocols.
The blast radius is limited to new opt-in writer helpers plus lock
acquisition in deletion and replica probing. Existing trigger and GUC
enforcement remains authoritative, so callers can migrate in later pull
requests.
### Related issue
[BUZZ-175](https://linear.app/squareup/issue/BUZZ-175). This is the
first implementation slice from the OSC migration plan.
### Testing
No manual testing. CI covers the Rust checks and database tests added
here.
Generated with [Codex](https://openai.com/codex)
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Signed-off-by: tornquist <tornquist@squareup.com>
Co-authored-by: Codex <noreply@openai.com>
