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_BP/entities/dragon_gravestone.json

@@ -0,0 +1,84 @@
+{
+  "format_version": "1.21.0",
+  "minecraft:entity": {
+    "description": {
+      "identifier": "silverlabs:dragon_gravestone",
+      "is_spawnable": false,
+      "is_summonable": true,
+      "is_experimental": false
+    },
+    "component_groups": {
+      "silverlabs:active": {
+        "minecraft:interact": {
+          "interactions": [
+            {
+              "on_interact": {
+                "filters": {
+                  "all_of": [
+                    { "test": "is_family", "subject": "other", "value": "player" }
+                  ]
+                },
+                "event": "silverlabs:break_gravestone",
+                "target": "self"
+              },
+              "use_item": false,
+              "interact_text": "Collect Memorial",
+              "play_sounds": "random.anvil_land"
+            }
+          ]
+        }
+      },
+      "silverlabs:breaking": {
+        "minecraft:loot": {
+          "table": "loot_tables/gameplay/gravestone_break.json"
+        },
+        "minecraft:instant_despawn": {}
+      }
+    },
+    "components": {
+      "minecraft:type_family": {
+        "family": ["dragon_gravestone"]
+      },
+      "minecraft:physics": {},
+      "minecraft:collision_box": {
+        "width": 0.5,
+        "height": 0.8
+      },
+      "minecraft:pushable": {
+        "is_pushable": false,
+        "is_pushable_by_piston": false
+      },
+      "minecraft:knockback_resistance": {
+        "value": 1.0
+      },
+      "minecraft:damage_sensor": {
+        "triggers": [
+          { "cause": "all", "deals_damage": false }
+        ]
+      },
+      "minecraft:persistent": {},
+      "minecraft:scale": {
+        "value": 0.8
+      },
+      "minecraft:health": {
+        "value": 999,
+        "max": 999
+      }
+    },
+    "events": {
+      "minecraft:entity_spawned": {
+        "add": {
+          "component_groups": ["silverlabs:active"]
+        }
+      },
+      "silverlabs:break_gravestone": {
+        "remove": {
+          "component_groups": ["silverlabs:active"]
+        },
+        "add": {
+          "component_groups": ["silverlabs:breaking"]
+        }
+      }
+    }
+  }
+}