Design: improve select and regenerate flow

[teleaziz]

• Reserve pending node rewrites before accepting candidates
• Use transactional multi-key compare-and-set for proposal publication and cleanup
• Prevent new regeneration requests from overwriting an active acceptance
• Atomically clean up superseded and cancelled proposals
• Add regression coverage for accept, reject, refinement, and cancellation races

[github-actions]

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[builder-io-integration]

Builder reviewed your changes — looks good ✅

Review Details

Code Review Summary

The latest PR update addresses the previously reported D1 transaction issue by adding a dialect-specific atomicBatch execution path and routing D1 multi-key CAS operations through guard statements plus atomic mutations. The non-D1 transaction path remains intact, and the added tests cover D1 batch wiring, mixed update/insert/delete transitions, mismatch rollback, and error propagation.

The guard statements intentionally insert the shared guard key only when a CAS precondition fails; because the guard row is inserted first, that path produces a unique-constraint failure and causes D1's batch to roll back. Successful mutations are then checked for exactly one affected row before events are emitted. The design action changes and race-focused tests continue to provide good coverage for acceptance reservations and superseding requests.

No new high-confidence issues were found. The previously open D1 compatibility comment was verified as fixed and resolved before this review.

🧪 Browser testing: Skipped — browser testing infrastructure is unavailable (Chrome MCP tools were absent); all 11 grounded UI cases were couldnt_verify due to environment limitations.