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/plants.json

@@ -0,0 +1,17 @@
+{
+  "type": "Plant",
+  "defs": [
+    { "defName": "BaseTree", "abstract": true, "sizeCells": 2.0, "trunkRadiusCells": 0.28 },
+    { "defName": "TreeOakA",      "parent": "BaseTree", "label": "plant.oak",   "texture": "things/plant/treeoak/TreeOakA" },
+    { "defName": "TreeOakB",      "parent": "BaseTree", "label": "plant.oak",   "texture": "things/plant/treeoak/TreeOakB" },
+    { "defName": "TreeBirchA",    "parent": "BaseTree", "label": "plant.birch", "texture": "things/plant/treebirch/TreeBirchA" },
+    { "defName": "TreeGrayPineA", "parent": "BaseTree", "label": "plant.pine",  "texture": "things/plant/treegraypine/TreeGrayPineA" },
+
+    { "defName": "GrassA", "label": "plant.grass", "texture": "things/plant/grass/grassa", "sizeCells": 1.2 },
+
+    { "defName": "BaseBush", "abstract": true, "label": "plant.bush", "sizeCells": 1.4 },
+    { "defName": "BushA", "parent": "BaseBush", "texture": "things/plant/bush/BushA" },
+    { "defName": "BushB", "parent": "BaseBush", "texture": "things/plant/bush/BushB" },
+    { "defName": "BushC", "parent": "BaseBush", "texture": "things/plant/bush/BushC" }
+  ]
+}