fix(UC-02): QA review — crypt bottom_row + ice decorations fidelity

- BUG-001: CRYPT bottom_row had unreachable mossy_stone_bricks (same f variable
  makes mossy_cobblestone guard trigger first). Fixed weights: 0.20/0.10/0.70
- BUG-003: ICE ceiling ice stalactites (y_offset=10) were missing from JSON
- BUG-002: ICE snow corner layers use middle value (3) as compromise since
  DecorationConfig.PositionedBlock doesn't support random_property

src/main/resources/data/tiedup/tiedup_room_themes/ice.json

@@ -42,7 +42,8 @@
   },
   "decorations": {
     "corner_decorations": [
-      { "block": "minecraft:snow[layers=2]", "y_offset": 1 }
+      { "block": "minecraft:snow[layers=3]", "y_offset": 1 },
+      { "block": "minecraft:ice", "y_offset": 10 }
     ],
     "wall_midpoint_blocks": [
       { "block": "minecraft:lantern", "y_offset": 3 }