When a second server reports the same volume during master restart,
UpdateFullHeartbeat now uses epoch-based tie-breaking instead of
first-heartbeat-wins:
1. Higher epoch wins as primary — old entry demoted to replica
2. Same epoch — higher WALHeadLSN wins (heuristic, warning logged)
3. Lower epoch — added as replica
Applied in both code paths: the auto-register branch (no entry
exists yet for this name) and the unlinked-server branch (entry
exists but this server is not in it).
This is a deterministic reconstruction improvement, not ground
truth. The long-term fix is persisting authoritative volume state.
5 new tests covering all reconciliation scenarios.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
