Cancel Capture

Cancel a capture, whether or not the worker has already claimed it.

Three outcomes, and the caller must handle all three:

• {cancelled: True, mode: "stopped"} — the job had not started (or was paused for input), so it is already terminal. Nothing ever ran.
• {cancelled: True, mode: "requested"} — the worker owns the run, so we stamp cancel_requested_at and the runner stops cooperatively at its next checkpoint (before any contact is written).
• {cancelled: False, status: "finished"} — too late, it already completed. The caller should resolve its row from GET, not hang.

The worker claims within ~0.5s, so "requested" is the common path in practice; the guarded pre-claim flip below is the rare fast case.