Writer cutover needs a commit-time fence and exact scheduler snapshot

Stopping new admissions is insufficient for a single-writer transition because an old ingress, interval advance, event closure, or physical mutation may already be in flight. Persist a transition status and fencing revision first; every mutator must revalidate writer identity, authority epoch, status, and fence at commit. During the fenced drain, snapshot or losslessly migrate the clock, monotone event frontier, ordered queue, every discontinuity schedule and revision, active process/passive-law set, stochastic crossing inputs, and identity/content/procedure digests. The final switch or abort must validate the same fenced state atomically. Otherwise a stale old commit can cross the switch or a migrated scheduler can reopen, retime, lose, or duplicate pending work.