feat(UC-02): data-driven room themes — 6 JSON + consumer migration + cleanup

Phase 3: Extract 6 themes into JSON (oubliette, inferno, crypt, ice, sculk, sandstone)
Phase 4: Migrate HangingCagePiece to use RoomThemeRegistry.pickRandomOrFallback()
  - Move 7 shared static methods from RoomTheme into HangingCagePiece
  - Replace per-enum placeDecorations() with generic DecorationConfig-based placement
Phase 5: Delete RoomTheme.java (-1368L)

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

@@ -0,0 +1,58 @@
+{
+  "weight": 10,
+  "wall_palette": {
+    "default": [
+      { "block": "minecraft:ice", "weight": 0.10 },
+      { "block": "minecraft:blue_ice", "weight": 0.20 },
+      { "block": "minecraft:packed_ice", "weight": 0.70 }
+    ]
+  },
+  "floor_palette": {
+    "default": [
+      { "block": "minecraft:snow_block", "weight": 0.20 },
+      { "block": "minecraft:packed_ice", "weight": 0.80 }
+    ],
+    "edge": [
+      { "block": "minecraft:blue_ice", "weight": 1.0 }
+    ]
+  },
+  "ceiling_palette": {
+    "default": [
+      { "block": "minecraft:blue_ice", "weight": 0.20 },
+      { "block": "minecraft:packed_ice", "weight": 0.80 }
+    ]
+  },
+  "wall_shell": "minecraft:packed_ice",
+  "wall_accent": "minecraft:blue_ice",
+  "pillar_palette": {
+    "default": [
+      { "block": "minecraft:packed_ice", "weight": 0.50 },
+      { "block": "minecraft:blue_ice", "weight": 0.50 }
+    ],
+    "cap": [
+      { "block": "minecraft:blue_ice", "weight": 1.0 }
+    ]
+  },
+  "scatter_palette": {
+    "default": [
+      { "block": "minecraft:snow[layers=1]", "weight": 0.50, "random_property": { "name": "layers", "min": 1, "max": 2 } },
+      { "block": "minecraft:powder_snow", "weight": 0.20 },
+      { "block": "minecraft:ice", "weight": 0.30 }
+    ]
+  },
+  "decorations": {
+    "corner_decorations": [
+      { "block": "minecraft:snow[layers=2]", "y_offset": 1 }
+    ],
+    "wall_midpoint_blocks": [
+      { "block": "minecraft:lantern", "y_offset": 3 }
+    ],
+    "furniture_cluster": [
+      { "block": "minecraft:powder_snow_cauldron[level=3]", "y_offset": 1 },
+      { "block": "minecraft:blue_ice", "y_offset": 1 },
+      { "block": "minecraft:lantern", "y_offset": 2 }
+    ],
+    "use_torch_lighting": false,
+    "has_ceiling_chain": true
+  }
+}