Add max-bytes retention budget alongside existing timeout budget:
- shipper_group.go: EvaluateRetentionBudgets now checks both timeout
(last contact time) and max-bytes (entry lag * 4KB > maxBytes).
Either exceeding budget → NeedsRebuild state transition.
- blockvol.go: add walRetentionMaxBytes (64MB default), pass to
EvaluateRetentionBudgets with primaryHeadLSN.
TestWalRetention_MaxBytesTriggersNeedsRebuild upgraded from PASS*
(log-only placeholder) to real PASS: asserts State()==NeedsRebuild
after lag exceeds configured max-bytes budget.
Retention contract: hold-back blocks reclaim for recoverable replicas,
timeout and max-bytes budgets escalate to NeedsRebuild and release hold.
Full rebuild lifecycle remains CP13-7 scope.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>