Run the script the export produces

### Task Summary

The other side of the comparison. This step adds `StandaloneRunner`: it writes the script the operator's generator emits, binds its inputs to the files the fixture wrote, runs it, and reads the frames it leaves behind.

The script is kept where it ran, so an operator whose two answers differ can be opened as generated rather than described second-hand.

Step 5 of 27 in #8325. It reads the generator trait, so it needs #8327.