P1: Guard cancellation state transitions and racing completion

[vamgan]

Problem

cancelTask() can mutate terminal tasks, and runTask() can still mark a task succeeded while cancellation is in progress.

Relevant code:

• src/runtime.ts:133-150
• src/runtime.ts:196-205

Issues:

• A completed or failed task can be moved to cancelling, then cancelled/failed.
• runTask() only treats cancelled/failed as terminal before marking success; cancelling can still become succeeded.

Impact

Final task state is nondeterministic and can regress from terminal states.

Suggested fix

• Make cancelTask() no-op or throw for succeeded, failed, and cancelled.
• Treat cancelling as a finalization blocker in runTask().
• Consider atomic/compare-and-set style state transitions in the store contract.

Acceptance criteria

• Cancelling a terminal task cannot change its terminal status.
• Race tests cover cancellation while runTask() is near success finalization.