`src/cancel.rs` — `CancelSignal`

The cooperative cancellation flag, shareable as Arc<CancelSignal>, honored at every level of the engine.

Key items

• cancel() — fire; idempotent; wakes all waiters.
• is_cancelled() — non-blocking poll.
• notified().await — race-free async wait; completes immediately if already fired.
• reset() — swap in a fresh token (the underlying tokio CancellationToken is one-shot by design).

Behavior notes

• Wraps tokio_util::sync::CancellationToken precisely to avoid the check-then-act race of hand-rolled AtomicBool + Notify.
• The engine re-arms in finalize() — after the run observed the cancel — so a cancel arriving between runs is observed by exactly one run, then cleared. One cancel can never permanently kill an agent.
• A task already waiting on the old token when reset() happens still completes against the old (fired) token.
• Lock poisoning is recovered (single-value data — the recover_guard policy).

Deep dive: Cancellation.