Three fixes:
1. TestWalRetention_RequiredReplicaBlocksReclaim: rewritten from log-only
placeholder to hard assertion (checkpointLSN <= replicaFlushedLSN)
2. TestWalRetention_TimeoutTriggersNeedsRebuild: rewritten from log-only
to hard assertion (State() == NeedsRebuild after 1ns timeout)
3. EvaluateRetentionBudgets: uses RetentionBudgetParams struct with
actual BlockSize from volume config instead of hardcoded 4096
All 3 retention tests now have real state/progress assertions.
No placeholder or log-only evidence remains in CP13-6 proof package.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>