MPVBackend.stop() clears the PiP bridge callbacks on macOS to prevent
crashes during window close, but the backend is reused across videos
and setupPiPIfNeeded is guarded by isPiPSetUp, so the callbacks were
never wired again. The next PiP session then started without notifying
PlayerService (no onPiPStatusChanged), leaving the player window
visible alongside the PiP window, both playing.
Extract the callback wiring into wirePiPBridgeCallbacks() and re-run it
in startPiP() and in setupPiPIfNeeded when already set up.
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