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

[lklimek]

Summary

• Fixes a SIGILL crash on macOS during shutdown caused by _NSTouchBarFinderObservation KVO observers on inconsistent objects
• Calls [window orderOut:nil] on all windows as the first action in on_exit(), giving AppKit a chance to properly tear down display-related observations while windows and views are still alive
• No-op on non-macOS platforms

Root cause

During shutdown, winit's event handler teardown triggers a display cycle flush (CATransaction commit). This causes _NSTouchBarFinderObservation invalidate to remove KVO observers from objects that are already in an inconsistent state, throwing an NSException that AppKit catches and deliberately crashes on via _crashOnException:.

Fix

By ordering all windows out before returning from on_exit(), AppKit processes the window removal while everything is still alive and consistent — the KVO observers are properly detached during this orderly removal rather than during the chaotic winit teardown.

Test plan

• Launch on macOS, close the app window — no crash on exit
• Launch on macOS, verify the app still shuts down cleanly (background tasks stop, no hang)
• Build on Linux — compiles without warnings (no-op path)

_{🤖 Co-authored by Claudius the Magnificent AI Agent}

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