fix: avoid interrupting exit cleanup

[SingLinkNetwork]

What changed

• Replaced the uncancellable 3-second forced-exit delay with a cancellable 10-second cleanup watchdog.
• Cancels the watchdog after cleanup succeeds or fails, while retaining a logged forced-exit fallback for a genuine hang.
• Preserves the existing macOS IP-forwarding restore and desktop cleanup order.
• Adds regression coverage for successful, failed, and stalled cleanup.

Root cause

The old delayed exit could terminate the process while Windows proxy, tray, window, or core cleanup was still in progress, leaving a core process or system proxy behind.

Verification

• git diff --check passed.
• Local Flutter/Dart is unavailable on the macOS host, so Flutter tests and analysis have not been claimed as run.
• This draft PR must pass the complete cross-platform GitHub Actions suite before merge.

Closes #123