Rework world scene into a textured tile terrain on engine WorldGen

Bump the engine submodule to 5365945 (MrGameEng.WorldGen) and drive
terrain from it: WorldGenerator is now a thin adapter over the engine's
seed-based Perlin/fBm/island HeightmapGenerator instead of the old
random+smoothing pass.

Strip WorldScene down to terrain + camera: render the world as a single
Tilemap entity using per-biome surface textures from the atlas (water
falls back to a tinted tile), and remove pawns, plants, AI systems,
population and save-restore for now. Delete the TerrainScene demo (its
tile approach now lives in WorldScene) along with its console command
and the hud.terrain/population localization strings.

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

Mods/Core/Languages/en/ui.json

@@ -1,6 +1,5 @@
 {
-  "hud.world": "LittleSim | seed {0} | population {1} | {2} FPS",
-  "hud.terrain": "LittleSim | terrain | seed {0} | animals: {1}",
+  "hud.world": "LittleSim | seed {0} | {2} FPS",
   "hud.controls": "WASD — camera, wheel — zoom, ` — console ('help' lists commands)",
   "hud.paused": "PAUSED",