Cancel a schedule

Cancels a schedule and settles its occurrence cancelled, releasing the timer. Terminal: a cancelled schedule cannot be resumed or replaced, and cancelling one that is already cancelled changes nothing and returns the same version. A cancel that races its own occurrence becoming due still wins — the occurrence settles cancelled and no work is delivered — which is why this is a refusal-free retry rather than a race a client has to reason about. The schedule stays readable; archive it to take it out of the listing.