GPUI's process harness: how an application is built, configured and started.
[Application] is what a host program holds in order to bring GPUI up — the
main function of a desktop app, or an embedder driving its own run loop.
Everything an application is once it is running lives in
[gpui_authoring], which this crate drives rather than contains. So do the
test and bench harnesses: they build a real [gpui_authoring::App] and drive
real frames, so they stay beside the code they exercise.
The gpui crate is a thin facade over both, so applications keep importing
gpui.