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 state or
dedup-count 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).