feat: add multi-world hub system with lobby portals and hub-return addon

Lobby addon detects players in portal zones at X: -15/0/15 and transfers
them to Jamie/Lyla/Mya survival worlds. Hub-return addon gives players a
recovery compass and chat commands (!hub, !lobby) to return to the lobby.

Includes docker-compose.yml for 4 Bedrock servers (lobby + 3 child worlds),
spark pet behavior/resource packs, and updated .gitignore.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

addon/spark_pet_RP/models/entity/dragon_gravestone.geo.json

@@ -0,0 +1,55 @@
+{
+  "format_version": "1.12.0",
+  "minecraft:geometry": [
+    {
+      "description": {
+        "identifier": "geometry.dragon_gravestone",
+        "texture_width": 32,
+        "texture_height": 32,
+        "visible_bounds_width": 1.0,
+        "visible_bounds_height": 1.5,
+        "visible_bounds_offset": [0, 0.5, 0]
+      },
+      "bones": [
+        {
+          "name": "root",
+          "pivot": [0, 0, 0]
+        },
+        {
+          "name": "base",
+          "parent": "root",
+          "pivot": [0, 0, 0],
+          "cubes": [
+            {
+              "origin": [-4, 0, -2],
+              "size": [8, 2, 4],
+              "uv": [0, 20]
+            }
+          ]
+        },
+        {
+          "name": "stone",
+          "parent": "base",
+          "pivot": [0, 2, 0],
+          "cubes": [
+            {
+              "origin": [-3, 2, -1],
+              "size": [6, 8, 2],
+              "uv": [0, 0]
+            },
+            {
+              "origin": [-2, 10, -1],
+              "size": [4, 2, 2],
+              "uv": [0, 10]
+            },
+            {
+              "origin": [-1, 12, -1],
+              "size": [2, 1, 2],
+              "uv": [0, 14]
+            }
+          ]
+        }
+      ]
+    }
+  ]
+}