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/Animals/Animals.json

@@ -16,6 +16,7 @@
       "body": "Quadruped",
       "diet": ["plant"],
       "spawnPer1000Cells": 2.5,
+      "baseMassKg": 70,
       "genome": {
         "GeneMaxBodySize": 1.5,
         "GeneMetabolism": 1.0,
@@ -51,6 +52,7 @@
       "attackDamage": 80,
       "attackBleed": 3.0,
       "attackBloodLoss": 5.0,
+      "baseMassKg": 45,
       "genome": {
         "GeneMaxBodySize": 1.6,
         "GeneMetabolism": 1.1,
@@ -84,6 +86,7 @@
       "attackDamage": 45,
       "attackBleed": 1.8,
       "attackBloodLoss": 3.0,
+      "baseMassKg": 80,
       "genome": {
         "GeneMaxBodySize": 1.3,
         "GeneMetabolism": 1.2,
@@ -116,6 +119,7 @@
       "spawnPer1000Cells": 1.5,
       "baseSpeed": 22,
       "visionCells": 10,
+      "baseMassKg": 2.5,
       "genome": {
         "GeneMaxBodySize": 0.5,
         "GeneMetabolism": 1.3,