Derive Pot's M/C from pot/grain/water mass instead of hardcoding them

Add Sud.derive_plant_params(), which combines pot_mass/pot_material/
grain_mass/water_mass into a single lumped (mass, specific heat) pair
using approximate specific-heat constants for water, grain, and (by a
small material lookup table) the pot itself. demo_sud.py now builds
its plant_params from this instead of hardcoded C/M values, keeping
L/Td as the only manually-tuned plant parameters.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/demos/sud/demo_sud.py

@@ -33,14 +33,14 @@ if __name__ == '__main__':
 		}
 	}
 
+	sud = Sud("sude/sud_0010.json")
+
 	plant_params = {
-		"C"			: 4190,
-		"M"			: 20,
+		**sud.derive_plant_params(),
 		"L"			: 0.2,
 		"Td"		: 30
 	}
 
-	sud = Sud("sude/sud_0010.json")
 	ctrl = TempController(dt, ctrl_params, plant_params, theta_amb)
 	plant = Pot(dt, plant_params, theta_amb)
 	stirrer = StirrerSim(dt)