Force the heater off and show a "Cool down" indicator during a passive cooldown

A ramp step whose target is below the current actual temperature can
only be reached by ambient cooling - the heater can't actively cool.
SudTask now detects this once per ramp phase and calls the controller's
new set_cooling() override (forces y=0, bypassing the FSM) instead of
waiting for its hysteresis-based IDLE transition. The GUI shows a
blinking blue "Cool down" label in the status bar while this is active.
Progression already only advances once theta_ist matches theta_soll
regardless of direction, so no change was needed there.

scripts/demos/sud/demo_sud.py

@@ -77,6 +77,15 @@ if __name__ == '__main__':
 		if step is not None:
 			ramping = sud.state == SudState.RAMPING
 			phase = step['ramp'] if ramping and 'ramp' in step else step.get('hold', {})
+
+			# A ramp step whose target is below the current temperature can
+			# only be reached by passive cooling - force the heater off,
+			# mirroring tasks/sud.py's SudTask.
+			cooling = ramping and 'ramp' in step and step['ramp']['temp'] < ctrl.get_theta_ist() - TEMP_REACHED_TOLERANCE
+			ctrl.set_cooling(cooling)
+			if cooling:
+				print("  -> cooling down to {}".format(step['ramp']['temp']))
+
 			print(f"Step {step['number']}: {step['descr']}")
 			apply_plant_params(step)
 			if ramping and 'ramp' in step: