Warn-only replay/snapshot divergence check (D7, promoted from
plans/defer/replay_validation.md). The log is authoritative and snapshots
are best-effort, so a mismatch is diagnostic, never fatal — but divergence
between “what the log replays to” and “what the snapshot recorded” is
exactly the class of bug the determinism guarantees (RFC-MACP-0003) forbid,
so it must be visible. Returns the number of mismatched fields
(0 = consistent).