Game as the Core mod: data-driven content and localization

The whole game is now described by Mods/Core — the first consumer of
the engine's new MrGameEng.Mods module:

- textures/ moved to Mods/Core/Textures; atlases are built at game
  start from the merged texture tree of all active mods into Cache/
  (gitignored, incremental) instead of being committed, and the
  GameAssets atlas handles are gone with them
- terrain, plants and pawns are JSON defs (Mods/Core/Defs) with parent
  inheritance; scenes read TerrainDef/PlantDef/PawnDef instead of
  hardcoded arrays, and the TerrainKind enum is retired
- HUD strings come from Mods/Core/Languages (ru default, en fallback)
  through LanguageManager; the language switches at runtime
- new console commands: mods, lang [code], defs [type]; atlas now
  inspects the runtime-built cache
- bump engine: MrGameEng.Mods module and the explicit-sources
  AtlasBuilder overload

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Mods/Core/Defs/terrain.json

@@ -0,0 +1,22 @@
+{
+  "type": "Terrain",
+  "defs": [
+    { "defName": "DeepWater", "label": "terrain.deepwater", "maxHeight": 0.30, "color": [23, 63, 95] },
+    { "defName": "Water",     "label": "terrain.water",     "maxHeight": 0.42, "color": [32, 99, 155] },
+    { "defName": "Sand",      "label": "terrain.sand",      "maxHeight": 0.48, "color": [237, 201, 120],
+      "isLand": true, "surface": "terrain/surfaces/sand" },
+    { "defName": "Grass",     "label": "terrain.grass",     "maxHeight": 0.68, "color": [90, 160, 70],
+      "isLand": true, "surface": "terrain/surfaces/mossy",
+      "scatter": [
+        { "chance": 0.35, "options": ["GrassA"] },
+        { "chance": 0.07, "options": ["BushA", "BushB", "BushC"] }
+      ] },
+    { "defName": "Forest",    "label": "terrain.forest",    "maxHeight": 0.85, "color": [44, 110, 50],
+      "isLand": true, "surface": "terrain/surfaces/soil",
+      "scatter": [
+        { "chance": 0.35, "options": ["TreeOakA", "TreeOakB", "TreeBirchA", "TreeGrayPineA"] }
+      ] },
+    { "defName": "Mountain",  "label": "terrain.mountain",  "maxHeight": 1.01, "color": [136, 132, 128],
+      "surface": "terrain/surfaces/roughhewnrock" }
+  ]
+}