ref(workerchild): move checking future completion into bg thread

[bmckerry]

Previously, future completion was only checked at the start of the worker child loop. A common pattern we saw was:

• a task executes, produces futures, next loop iteration runs immediately and those futures aren't complete yet
• next task executes, and since some tasks can take 5-60s to complete, we're stuck 5-60s until we check future completion again
  • this meant that TaskProducer can increase the latency between a worker receiving a task and marking the task as complete from N to (N+M), where N is the execution time of the original task and M is the execution time of the next task

This didn't affect task throughput (since checking doneness is non-blocking), but did mean tasks would sometime exceed their processing deadline which is bad.

This PR tries to deal with that by moving this logic into a separate thread in the worker child.

[untitaker]

cursor comment seems legit, but rubberstamping otherwise

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit b7e2223. Configure here.}