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_RP/textures/terrain_texture.json

@@ -0,0 +1,20 @@
+{
+  "resource_pack_name": "lobby_transfer_RP",
+  "texture_name": "atlas.terrain",
+  "padding": 8,
+  "num_mip_levels": 4,
+  "texture_data": {
+    "portal_frame": {
+      "textures": "textures/blocks/end_portal_frame_top"
+    },
+    "portal_jamie": {
+      "textures": "textures/blocks/emerald_block"
+    },
+    "portal_lyla": {
+      "textures": "textures/blocks/amethyst_block"
+    },
+    "portal_mya": {
+      "textures": "textures/blocks/prismarine_bricks"
+    }
+  }
+}