hold cleanup until signal on `procKilledCh`

[sethbrasile]

Fixes #534 on my 2021 M1 Pro MBP - Sonoma 14.4.1

#542 got it to a solid starting point so this was pretty simple? Thanks to whatever @ghost wrote that.

Hopefully this fixes for everyone on a Mac? 🤞🏻

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There's still a downside: When you reach a non zero exit code, you are trapped and have to kill the pid via external means. It does at least let you ctrl+c under normal circumstances.