Monotone event frontiers need atomic minimum selection and writer-cutover fencing

A serializable event closure must select and validate the current minimum eligible queue key inside the same transaction (or protect the equivalent queue-revision/predicate range). Selecting a minimum outside the transaction and later rereading only that row lets an earlier concurrent ingress commit, after which closure can advance the monotone frontier past it. Writer handoff must also preserve the frontier, simulation time, ordered queue, schedule revisions, receipts, and authority epoch as one cutover state; every old-writer mutation must validate the epoch at commit. Replay checks should cover already queued identities as well as terminal receipts.