Birth type gene (live-bearing vs egg-laying) + hybrid-creation foundation

GeneEggLaying (species-fixed) selects the conception cycle: viviparous animals
carry to a live birth (existing path); oviparous animals, after the same gravid
period, LAY eggs instead. New Egg entity (carries the already-bred offspring
genome) + EggSystem incubates and hatches it into a baby. AnimalPregnancySystem
branches on the mother's birth type. Adds an oviparous showcase species, Chicken.
Eggs are serialized (WorldSave.Eggs) like corpses, so clutches survive save/load.

Also lays the foundation for creating any species or hybrid (full command/UI
deferred): AnimalSet.HybridGenome(base, other) produces a cross anchored to a
base species' body/gene-set with shared genes blended Mendelian-style — so
Create(species, GenerateGenome) makes any species and Create(species,
HybridGenome) makes any hybrid.

EggTexture on AnimalDef (defaults to the seed dot placeholder); reproduction line
added to the `animal` console command. Build + --check-content clean (41 genes,
40 traits). Not GUI-verified.

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

Mods/Core/Defs/Animals/Animals.json

@@ -101,6 +101,36 @@
         "GeneOmnivory": 0.85,
         "GeneToxinTolerance": 0.25
       }
+    },
+
+    {
+      "defName": "Chicken",
+      "label": "pawn.chicken",
+      "kind": "animal",
+      "texture": "things/pawn/animal/chicken/Chicken_east",
+      "body": "Quadruped",
+      "diet": ["plant"],
+      "spawnPer1000Cells": 1.5,
+      "baseSpeed": 22,
+      "visionCells": 10,
+      "genome": {
+        "GeneMaxBodySize": 0.5,
+        "GeneMetabolism": 1.3,
+        "GeneMoveSpeed": 0.9,
+        "GeneBloodVolume": 0.7,
+        "GeneVision": 1.1,
+        "GeneBrainSize": 0.3,
+        "GeneInsulation": 0.45,
+        "GeneFurColor": 0.6,
+        "GeneMaturityAge": 50,
+        "GeneLifespan": 180,
+        "GeneBreedingSeason": 1,
+        "GeneGestationDays": 12,
+        "GeneLitterSize": 6,
+        "GeneHerbivory": 0.8,
+        "GeneToxinTolerance": 0.15,
+        "GeneEggLaying": 1.0
+      }
     }
   ]
 }