Weight system: body mass, carry capacity, Kleiber metabolism

Mass is derived by a cubic law: mass = BaseMassKg * (size / reference)^3, so a
juvenile/seedling weighs far less than an adult/mature and the size gene moves
mass too. New Mass helper + BaseMassKg on AnimalDef/PlantDef (deer 70, wolf 45,
boar 80, chicken 2.5; trees ~450, grass 0.4, etc.) and MassKg per unit on
ProductDef (foundation for hauling/inventory).

Wired into the sim (as chosen):
- Corpse meat is now proportional to body mass (Mass.MeatFraction), not raw
  bodySize; Corpse gained a BodySize field so sprite scale stays independent of
  meat (save/load updated).
- Kleiber metabolism: need decay x= (mass/refMass)^0.75, normalized per species,
  so juveniles are cheaper to feed and heavier individuals eat a bit more,
  without cross-species balance blowups (adults of typical size are unchanged).
- Carry capacity = mass x fraction x carry capability (Moving for quadrupeds;
  Manipulation for future humans), shown in inspector/console. A dormant Carrier
  component + Mass.LoadSpeedFactor hook slow an overloaded animal — the
  foundation for item hauling.

Inspector shows body/plant weight (+carry); `animal`/`plant` console print mass.
Build + --check-content clean (42 genes, 41 traits). Not GUI-verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Mods/Core/Defs/Plants/Grass.json

@@ -3,6 +3,7 @@
   // Трава: быстрый почвопокров, основной скаттер лугов и леса.
   "defs": [
     { "defName": "GrassA", "label": "plant.grass", "texture": "things/plant/grass/grassa", "sizeCells": 1.5,
+      "baseMassKg": 0.4,
       "harvestProduct": "ProductGrass",
       "genome": { "optimalLight": 0.85, "lightTolerance": 0.35, "optimalTemperature": 19, "temperatureTolerance": 8,
                   "coldHardiness": 9, "heatHardiness": 6,