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.

tasks/sud.py

@@ -61,6 +61,14 @@ class SudTask(ATask):
 		ramping = self.sud.state == SudState.RAMPING
 		phase = ramp if ramping and ramp is not None else hold
 
+		# A ramp step whose target is below the current temperature can
+		# only be reached by passive cooling - force the heater off for
+		# its whole duration (decided once, here, rather than re-checked
+		# every tick) instead of relying on the controller's own FSM to
+		# notice and idle out.
+		cooling = ramping and ramp is not None and ramp['temp'] < self.tc.get_theta_ist() - TEMP_REACHED_TOLERANCE
+		self.tc.set_cooling(cooling)
+
 		if step is not None:
 			self.apply_plant_params(step)
 			if ramping and ramp is not None:
@@ -76,6 +84,7 @@ class SudTask(ATask):
 			'Rate': ramp.get('rate') if ramp else None,
 			'Duration': hold.get('duration') if (hold is not None and not ramping) else None,
 			'WaitForUser': step.get('user_wait_for_continue', False) if step else None,
+			'Cooling': cooling,
 		}}))
 
 	def on_state_changed(self, value):