feat(portals): add craftable portal blocks replacing hardcoded coordinates

Portal frame crafted from obsidian + ender pearl, combined with crystals
(emerald/amethyst/prismarine) to create world-specific portal blocks.
Stepping on a portal block triggers transfer. Includes resource pack with
vanilla textures, block definitions, crafting recipes, and updated script.

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

lobby-addon/lobby_transfer_BP/blocks/portal_mya.json

@@ -0,0 +1,28 @@
+{
+  "format_version": "1.21.40",
+  "minecraft:block": {
+    "description": {
+      "identifier": "silverlabs:portal_mya",
+      "menu_category": {
+        "category": "items",
+        "group": "itemGroup.name.decorations"
+      }
+    },
+    "components": {
+      "minecraft:destructible_by_mining": {
+        "seconds_to_destroy": 5.0
+      },
+      "minecraft:destructible_by_explosion": {
+        "explosion_resistance": 1200.0
+      },
+      "minecraft:light_emission": 10,
+      "minecraft:map_color": "#4FC1E9",
+      "minecraft:material_instances": {
+        "*": {
+          "texture": "portal_mya",
+          "render_method": "opaque"
+        }
+      }
+    }
+  }
+}