#24719 fix(next): clean up UI inspector from nonisolated deinit

#24707 introduced the UI inspector's @MainActor cleanup, and the next Release compile exposed Swift 6.2 errors in its deinit. This forward fix makes deinit nonisolated and performs AppKit monitor/notification removal only under MainActor.assumeIsolated on the main thread, matching the existing package pattern. It is required to get a Release compile green at the current tip after Aziz's WebRTC notice fix 83cb35f5b2b.

Validation:

• Release compile failure reproduced on run 40055959790 (UIInspectorController.swift:44-45).
• Nonisolated deinit follows existing MainActor.assumeIsolated cleanup pattern.
• git diff --check.

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Summary by cubic

Fixes the Release compile error in the UI inspector's deinit: cleanup is now nonisolated and only removes AppKit monitors/observers when already on the main thread, matching the existing MainActor.assumeIsolated pattern. The Release build previously failed at UIInspectorController.swift:44-45 under Swift 6.2.

CI

• Adds an advisory warning step that flags missing or unmapped bundled binaries in Release builds; the signed nightly check remains authoritative.