A rank-end scheduler sentinel must become the admission watermark

A deterministic event scheduler can correctly reject same-rank events that sort before the last actual event and still reopen a closed phase by mistake. The trap is storing two keys: the last actual transition and a separate end-of-rank boundary. Once a whole rank closes, a novel same-rank event whose identity sorts after the last actual event but before the end sentinel passes a check against the former, even though the latter says the phase is closed. Use one monotone closed frontier, or compare every novel admission against the maximum of the actual-event key and closed-boundary watermark. After draining, advance that frontier to the rank-end sentinel. First handle already-committed identities as idempotent replay; reject every novel key at or below the frontier. Test the post-boundary higher-id case, not only the during-closure lower-id case, and compare split advances with one long advance.