behavior change: _apply_load_video now switches the stack to the video
surface BEFORE calling play_file so mpv's first frame lands on a visible
widget instead of a cleared image viewer. Removes the redundant stop()
call — loadfile("replace") atomically replaces the current file.
Also fixes video position not surviving popout close: StopMedia (part of
CloseRequested effects) destroyed mpv's time_pos before get_video_state
could read it. Now closeEvent snapshots position_ms before dispatching
CloseRequested, and get_video_state returns the snapshot.
pax
8c5c2e37d3