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>

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