Add application-owned writer lock foundations

[TheSentinel454]

Summary

Buzz is moving database invariants from triggers and foreign keys into application-owned transaction protocols. This draft adds the first transition foundations while preserving all current database backstops.

• Add shared community-write locks and matching exclusive deletion locks, with stable ordering for multi-community writes.
• 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

None found. 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

[github-actions]

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