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/entity/dragon_gravestone.entity.json

@@ -0,0 +1,32 @@
+{
+  "format_version": "1.10.0",
+  "minecraft:client_entity": {
+    "description": {
+      "identifier": "silverlabs:dragon_gravestone",
+      "materials": {
+        "default": "entity_alphatest"
+      },
+      "textures": {
+        "default": "textures/entity/dragon_gravestone"
+      },
+      "geometry": {
+        "default": "geometry.dragon_gravestone"
+      },
+      "render_controllers": [
+        "controller.render.dragon_gravestone"
+      ],
+      "animations": {
+        "idle": "animation.dragon_gravestone.idle",
+        "controller": "controller.animation.dragon_gravestone",
+        "beacon_controller": "controller.animation.dragon_gravestone.beacon"
+      },
+      "scripts": {
+        "animate": ["controller", "beacon_controller"]
+      },
+      "particle_effects": {
+        "memorial_glow": "minecraft:endrod",
+        "beacon_beam": "minecraft:endrod"
+      }
+    }
+  }
+}