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/Languages/ru/ui.json

@@ -0,0 +1,28 @@
+{
+  "hud.world": "LittleSim | сид {0} | население {1} | {2} FPS",
+  "hud.terrain": "LittleSim | террейн | сид {0} | животных: {1}",
+  "hud.controls": "WASD — камера, колесо — зум, ` — консоль ('help' — список команд)",
+
+  "terrain.deepwater": "глубокая вода",
+  "terrain.water": "вода",
+  "terrain.sand": "песок",
+  "terrain.grass": "трава",
+  "terrain.forest": "лес",
+  "terrain.mountain": "горы",
+
+  "plant.oak": "дуб",
+  "plant.birch": "берёза",
+  "plant.pine": "сосна",
+  "plant.grass": "пучок травы",
+  "plant.bush": "куст",
+
+  "pawn.being": "житель",
+  "pawn.bear": "медведь",
+  "pawn.deer": "олень",
+  "pawn.fox": "лиса",
+  "pawn.hare": "заяц",
+  "pawn.boar": "кабан",
+  "pawn.wolf": "волк",
+  "pawn.muffalo": "муффало",
+  "pawn.squirrel": "белка"
+}