namida and Forestidia, thanks for the input.
(Harvester runs replay to end) is moderately easy. The unrelated replay verifier is adaptable to take the existing physics map and gamestate instead of creating them from scratch.
Re 1), yes, cancelling a map must stay fast, it's so common. Long solutions take a noticeable time to run in the replay verifier already. We would have to test this on slow machines. Forestidia has two laptops that run Lix, and the slower would be good to test.
Lix has a level that solves with 0 skills, but not obviously. Thus Forestidia's 3) is a concern: Cancelling the map produces a solution checkmark, that would look like a bug.
Before I adapt the harvester, I should work on the quicksave icon. The bad quicksave icon (floppy disk) is the prime culprit for the lost data. This icon must change. Everything else feels like a bonus in comparison, or like overengineering, I'd have to sleep over it.

-- Simon