#2891 gatorwalk-factory studio: Simulate follow-ups from #2808's reviews (stuck Run tab on an unrunnable walk, settling states)

Low findings from the verification reviews of #2808 (Simulate mode), left for after it shipped.

1. A walk the engine cannot run leaves the Run tab stuck: it shows no Discard button, and the Scenarios tab's Copy as scenario says 'Checking the walk on the engine…' forever. Show the walk banner (with Discard) whenever a walk exists, and say the walk could not run instead of Checking (studio/src/simulate_view.tsx, RunTab, NoRun, CopyAsScenario).
2. While a step is settling, the panel can briefly show a false 'The walk could not run' alert, or the previous walk's frames, which still take clicks (state.ts takeStep / settleWalk). Keep the frames being replaced visibly pending, and ignore actions until the walk settles.
3. The override a person takes always carries the note 'one more pass' (simulate.ts actionsAt). Let the Copy output say so or ask for a note.
4. takeStep does nothing, silently, when the picked scenario did not run (parse error); say why or hide the actions.
5. settleWalk retries without a bound while the walk or the definition keeps changing; cap it or yield between rounds.
6. Copy names every entry -walk, which can match a saved scenario's name; pick a free name.

Also noted and not a bug: a global exit's taken count matches by name, which the schema keeps unambiguous (a stage transition cannot share a global transition's name).