Bug: BarrierOK with FlushedLSN == 0 (legacy 1-byte response) was counted
as successful sync_all durability even though no authoritative durable
progress was established. This allowed a legacy replica to silently pass
through the sync_all barrier without proving any LSN was fsynced.
Fix (wal_shipper.go): BarrierOK with FlushedLSN == 0 now returns an
error instead of nil. Barrier success requires the replica to report a
non-zero FlushedLSN proving which LSN was durably persisted. This makes
the code match the CP13-3 contract: replicaFlushedLSN is the sole
authority for sync_all durability.
New test: TestBarrier_LegacyResponseRejectedBySyncAll — proves legacy
1-byte responses don't establish durable authority.
Contract review doc updated to reflect the code fix.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
