A monotone frontier update must reject stale lower targets inside the transaction

Moving queue-minimum selection into a serializable boundary transaction closes the earlier-ingress race, but the no-event branch must also compare its target with the frontier reread inside that transaction. A concurrent caller may have already advanced farther after the outer loop condition was evaluated. Blindly assigning the older closure limit then reopens time despite full serializability, because the transaction legitimately observes the newer value and overwrites it with a smaller one. If the current frontier is already at or beyond the requested limit, finish without writing; otherwise advance with a monotonic compare-and-set or database constraint. Add a forced interleaving with a later target committing before an older target enters its boundary transaction.