feat: BGMController独立音频脚本—彻底脱离PageController复杂编辑历史

新增BGMController.cs挂在[Managers]上:
- Awake()中创建AudioSource(spatialBlend=0,vol=0.5)
- AssetDatabase兜底加载_homeBgClip/_experimentBgClip
- 静态方法PlayHomeBg/PlayExperimentBg/StopBgMusic
PageController调用改为BGMController.xxx

Assets/Scenes/main.unity

@@ -3054,6 +3054,7 @@ GameObject:
   - component: {fileID: 441640512}
   - component: {fileID: 441640511}
   - component: {fileID: 441640516}
+  - component: {fileID: 441640517}
   m_Layer: 0
   m_Name: '[Managers]'
   m_TagString: GameController
@@ -3275,6 +3276,20 @@ MonoBehaviour:
   _enableOpeningFlow: 1
   _basketballClip: {fileID: 8300000, guid: 25ff553be9c84452ab0a39fbfd06be77, type: 3}
   _manCallClip: {fileID: 8300000, guid: bc5816d37e79f4461a243f63cd20c925, type: 3}
+--- !u!114 &441640517
+MonoBehaviour:
+  m_ObjectHideFlags: 0
+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
+  m_GameObject: {fileID: 441640509}
+  m_Enabled: 1
+  m_EditorHideFlags: 0
+  m_Script: {fileID: 11500000, guid: d5a0e43c7217447f9b4be5e2434520e4, type: 3}
+  m_Name: 
+  m_EditorClassIdentifier: 
+  _homeBgClip: {fileID: 0}
+  _experimentBgClip: {fileID: 0}
 --- !u!1 &458363765
 GameObject:
   m_ObjectHideFlags: 0