TempControllerBase.cooling/set_cooling() forced y=0 in advance of a
Sud ramp-down step; redundant now that the FSM's own COOL state
detects and handles a negative diff itself. Removed from the
controller, SudTask's anticipatory detection (and the Step message's
now-unused "Cooling" field), and demo_sud.py's mirror of it.
gui: the "Cool down" status-bar indicator now reflects the
controller's actual State ("States.COOL") via the TempCtrl channel,
rather than Sud's removed anticipatory flag - renamed
sud_cooling/set_sud_cooling to tc_cooling/set_tc_cooling to match.