remove hidden window abstraction

[jackkav]

follow up from #7790

we used hidden window as a stop gap so we could move fast but it causes possible execution difference with inso, so this change will align implementations

todo:

• remove forks
• remove unused code
• remove unused config
• decide what to do about concurrency
• decide whether to run in utility process or elsewhere
• decide what to do about waiting until promises have settled
• environment caching issue that causes set env in pre req script to re-render without context

notes:

concurrency doesn't appear to have a test
runs in main now, but working on running elsewhere

whats next:

• keep trying the utility process approach
• we should document the reasons for change and results of the investigation before moving this forward.