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

components/plant/pot.py

@@ -56,6 +56,10 @@ class Pot(APlant):
 	def set_ambient_temperature(self, theta_amb):
 		self.theta_amb = theta_amb
 
+	def set_thermal_params(self, M, C):
+		self.M = M
+		self.C = C
+
 	def set_power(self, power):
 		self.p_in = power