fix(macos): order windows out on exit to prevent KVO crash - #820

…the process

On a Mac with a Touch Bar, `_NSTouchBarFinder` observes `nextResponder` on every
responder in the active window's chain — the window, its views, and the transient
views AppKit puts there while editing — and retracts those observations later,
from its own display-cycle block. When a retraction is not the first one for the
same observation, Foundation raises `NSRangeException`, AppKit catches it and
calls `-[NSApplication _crashOnException:]`, whose entire purpose is to abort:
the process dies with SIGILL, with no Rust frame on the stack and nothing for us
to catch.

That abort is not caused by anything we release. Retiring the native window
instead of releasing it, and a grace period before the release, were both shipped
and both still aborted; the same exception is reported by apps that share neither
GPUI nor our window lifetime, and one of them root-caused it to queued
invalidations retracting an observation that was already gone:

- longbridge/gpui-kit#3192 — intermittent, on switching the frontmost app
- kodezine/RustyCAN#95 — root cause: queued invalidations, already-removed observer
- dashpay/dash-evo-tool#820 — mitigated by ordering windows out before exit
- johnlindquist/kit#1550 — Electron, reported on the window object
- emilk/egui#2768 — eframe, on quitting the app
- feigeCode/navop#268 and feigeCode/navop#308 — the reports this exists for

`touch_bar_guard` takes over `-[NSApplication _crashOnException:]` and swallows
exactly one exception: an `NSRangeException` whose reason names
`_NSTouchBarFinderObservation`. Nothing is left to clean up in that case — the
retraction it failed on had already happened, so the state AppKit wanted is the
state it has. Every other exception is forwarded to the original implementation,
which aborts exactly as AppKit intended.

Release Notes:

- Fixed a crash on Macs with a Touch Bar, where AppKit aborted the process for an
  exception its own Touch Bar finder raised while retracting its observations.