Make grain_mass/water_mass per-step and re-derive plant params on change

grain_mass and water_mass now live on each step (defaulted from
default.step) instead of being fixed for the whole brew, since both
change over a mash (malt going in, water boiling off).
Sud.derive_plant_params() takes them as arguments so it can be
recomputed per step; the demo re-applies the resulting M/C to both the
real plant and the controller's Smith-predictor model on every step
change via the new Pot.set_thermal_params()/
TempController.set_model_params().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgR9tPaSzFkAwRAUyeaaCD

scripts/demos/sud/demo_sud.py

@@ -35,8 +35,9 @@ if __name__ == '__main__':
 
 	sud = Sud("sude/sud_0010.json")
 
+	first_step = sud.schedule[0]
 	plant_params = {
-		**sud.derive_plant_params(),
+		**sud.derive_plant_params(first_step['grain_mass'], first_step['water_mass']),
 		"L"			: 0.2,
 		"Td"		: 60
 	}
@@ -65,9 +66,15 @@ if __name__ == '__main__':
 			stirrer.set_duty_cycle(1.0 if speed > 0 else 0.0)
 		stirrer.set_speed(speed)
 
+	def apply_plant_params(step):
+		params = sud.derive_plant_params(step['grain_mass'], step['water_mass'])
+		plant.set_thermal_params(params['M'], params['C'])
+		ctrl.set_model_params(params['M'], params['C'])
+
 	def on_step_changed(step):
 		if step is not None:
 			print(f"Step {step['number']}: {step['descr']}")
+			apply_plant_params(step)
 			if 'ramp' in step:
 				ctrl.set_theta_soll(step['ramp']['temp'])
 				ctrl.set_heatrate_soll(step['ramp']['rate'])