Plot the Sud demo's plant M/C over the run

Adds a third figure tracking plant.M/plant.C alongside the existing
temperature/state plots, making the per-step mass changes (malt going
in, water boiling off) visible.

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

scripts/demos/sud/demo_sud.py

@@ -102,6 +102,8 @@ if __name__ == '__main__':
 	_stirrer_speed = []
 	_heater_power_set = []
 	_heater_power_eff = []
+	_plant_M = []
+	_plant_C = []
 
 	wait_user_elapsed = 0.0
 	t = 0.0
@@ -142,6 +144,8 @@ if __name__ == '__main__':
 		_stirrer_speed.append(stirrer.speed * stirrer.isOn)
 		_heater_power_set.append(heater.power_set)
 		_heater_power_eff.append(heater.power_eff)
+		_plant_M.append(plant.M)
+		_plant_C.append(plant.C)
 
 		t += dt
 		steps += 1
@@ -181,6 +185,16 @@ if __name__ == '__main__':
 	legend(["stirrer speed (effective)"])
 	grid(True)
 
+	figure(3)
+	subplot(2, 1, 1)
+	plot(_t_arr, _plant_M, '-b', linewidth=1)
+	legend(["plant M (kg)"])
+	grid(True)
+	subplot(2, 1, 2)
+	plot(_t_arr, _plant_C, '-b', linewidth=1)
+	legend(["plant C (J/(kg*K))"])
+	grid(True)
+
 	show()
 
 	print("End of program")