feat(portals): add portal signs, transfer notifications, and welcome titles

Lobby portals now have oak wall signs showing whose world each portal leads to
(Jamie/Lyla/Mya with color-coded text). Players see a title notification when
entering a portal. Child worlds show a welcome title with the world name on
arrival, read from variables.json via @minecraft/server-admin.

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

lobby-addon/lobby_transfer_BP/manifest.json

@@ -4,7 +4,7 @@
     "name": "Lobby Portal Transfer",
     "description": "Auto-transfers players when they step into portal areas",
     "uuid": "a1b2c3d4-1111-2222-3333-abcdef123456",
-    "version": [1, 0, 3],
+    "version": [1, 0, 4],
     "min_engine_version": [1, 21, 0]
   },
   "modules": [
@@ -12,7 +12,7 @@
       "type": "script",
       "language": "javascript",
       "uuid": "a1b2c3d4-4444-5555-6666-abcdef789012",
-      "version": [1, 0, 3],
+      "version": [1, 0, 4],
       "entry": "scripts/main.js"
     }
   ],