1. TestWalRetention_TimeoutTriggersNeedsRebuild: add hard assertion that
checkpoint advances past replicaFlushedLSN after NeedsRebuild (proves
hold is actually released, not just state transition)
2. TestWalRetention_RequiredReplicaBlocksReclaim: remove stale "EXPECTED
TO FAIL" / duplicate comment block
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>