The tray's Quit only closed the tray, and music mode could not be left while
SuperDirt (a headless unit) or Ardour ran, so 'I quit everything' was false.
This stops the SuperDirt watchdog first (or it restarts the sound), then
SuperDirt (the black box stops with it), then hands over to music-work off.
It refuses while a take is rolling, never closes Ardour (it may hold unsaved
work; a second run finishes once Ardour is closed), and reports every outcome
as a desktop notification.