Remove mountains from the map (forest covers the high band for now)

Dropped the Mountain terrain def and extended Forest to maxHeight 1.01, so the
former mountain elevations generate forest instead. No terrain blocks light now;
the occluder mechanism stays generic (mature trees still cast shadows) -- updated
the stale "mountain" comments. Removed the unused terrain.mountain localization.
Build + --check-content clean.

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

Mods/Core/Languages/ru/ui.json

@@ -146,7 +146,6 @@
   "terrain.sand": "песок",
   "terrain.grass": "трава",
   "terrain.forest": "лес",
-  "terrain.mountain": "горы",
   "plant.oak": "дуб",
   "plant.birch": "берёза",
   "plant.pine": "сосна",