fix(scenes): JoinRoom_Layer 修 c.Prefab.data.__id__=0 自引用 bug (修复 working tree)

build-joinroom.py 两处 bug:
1. group5_idx=0 hardcoded (实际应为 1, c.Prefab 占 data[0])
2. group-5 node _parent={\"__id__\": 1} (指向自己, group-5 应是 root 无 _parent)

修复: group5_idx=1, group-5 _parent=None
verify: c.Prefab data.__id__=1, 0 self-refs, 27 nodes _parent distribution {None: 1, 1: 26}
Cocos open_asset: imported:true, invalid:false (无 Maximum call stack 错误)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

cocoscreator_projects/YouleNexus/assets/framework/ui/prefabs/_playbook_demo/JoinRoom_Layer.prefab

@@ -6,7 +6,7 @@
     "__editorExtras__": {},
     "_native": "",
     "data": {
-      "__id__": 0
+      "__id__": 1
     },
     "optimizationPolicy": 0,
     "persistent": false
@@ -16,9 +16,7 @@
     "_name": "JoinRoom_Layer",
     "_objFlags": 0,
     "__editorExtras__": {},
-    "_parent": {
-      "__id__": 1
-    },
+    "_parent": null,
     "_children": [
       {
         "__id__": 1