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>

sude/sud_0010.json

@@ -1,10 +1,10 @@
 {
 	"Name" : "Sud-0010",
 	"Description"		: "Rotfraenkisch, Dunkles Lager",
-	"pot_weight_kg"		: 5.960,
+	"pot_mass"		 	: 5.960,
 	"pot_material"		: "Edelstahl 18/10",
-	"Schuettung_kg"		: 5.21,
-	"Wasser_kg"			: 22,
+	"grain_mass"		: 5.21,
+	"water_mass"		: 22,
 	"schedule":
 	[
 		{