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

sude/sud_0010.json

@@ -3,14 +3,14 @@
 	"Description"		: "Rotfraenkisch, Dunkles Lager",
 	"pot_mass"		 	: 5.960,
 	"pot_material"		: "Edelstahl 18/10",
-	"grain_mass"		: 5.21,
-	"water_mass"		: 22,
 	"default":
 	{
 		"step" : {
 			"descr": "Put description here",
 			"user_message": "Put user message here",
 			"user_wait_for_continue": false,
+			"grain_mass"		: 5.21,
+			"water_mass"		: 22,
 			"ramp": {
 				"rate": 1.0,
 				"temp": 0,
@@ -33,6 +33,7 @@
 	"steps":
 	[
 		{
+			"grain_mass"		: 0,
 			"descr": "Heizen für Einmaischen",
 			"ramp":	{
 				"rate": 1.8,
@@ -101,6 +102,7 @@
 			}
 		},
 		{
+			"water_mass"		: 21,
 			"descr": "Halten Abmaischen",
 			"user_message": "Quittieren zum Abmaischen",
 			"user_wait_for_continue": true,