Canvas fixes are saved even when they take many steps

Fixing a bug in a large canvas no longer stops halfway: the assistant can read more of the canvas in one reply and can always save the fix it found before the reply ends. A reply that only looked at your canvas and was then cut off by the time limit is not charged, and it says so plainly instead of reporting steps as completed. Saying “continue” picks up from what was already read instead of starting over.