Pause a schedule

Suspends a schedule without discarding it: its occurrence stays planned, its timer is released, and resume re-arms the original planned instant rather than silently moving it. If that instant has passed, the schedule's lateness policy decides whether the first arrival becomes due or settles skipped_stale; a pause defers the decision, it does not promise delivery. A schedule with a dispatching or ambiguous callback is refused until that attempt settles, because pausing cannot strand its watchdog or turn a proven success into an unreachable completed state. Pausing an already paused schedule changes nothing and returns the same version, so a retry is free. A cancelled or completed schedule cannot be paused.