fix: 重连PageController和UIManager的FixationPanel按钮引用

修复原因：用户将BtnBack/BtnLimbFixation/BtnSplintFixation移入FixationPanel子物体后，
Inspector中的SerializeField引用自动断开，导致按钮点击无响应。

重连引用：
- _btnFixationBack → Canvas/FixationPanel/BtnBack
- _btnLimbFixation → Canvas/FixationPanel/BtnLimbFixation
- _btnSplintFixation → Canvas/FixationPanel/BtnSplintFixation
- _experimentPanel → Canvas/ExperimentPanel
- _btnExperimentBack → Canvas/ExperimentPanel/BtnBack
- UIManager._btnLimbFixation / _btnSplintFixation 同步重连

Assets/Scenes/main.unity

@@ -5702,8 +5702,8 @@ MonoBehaviour:
   _btnBleeding: {fileID: 730628646}
   _btnTransport: {fileID: 955229752}
   _fixationPanel: {fileID: 1813870029}
-  _btnLimbFixation: {fileID: 0}
-  _btnSplintFixation: {fileID: 0}
+  _btnLimbFixation: {fileID: 1913973802}
+  _btnSplintFixation: {fileID: 1015007396}
 --- !u!114 &1642393471
 MonoBehaviour:
   m_ObjectHideFlags: 0
@@ -5719,8 +5719,8 @@ MonoBehaviour:
   _homePagePanel: {fileID: 1037678401}
   _modeSelectPanel: {fileID: 1819847500}
   _fixationPanel: {fileID: 1813870029}
-  _experimentPanel: {fileID: 0}
-  _btnExperimentBack: {fileID: 0}
+  _experimentPanel: {fileID: 200058300}
+  _btnExperimentBack: {fileID: 617526394}
   _uiManager: {fileID: 1642393470}
   _btnStart: {fileID: 1701102418}
   _btnExit: {fileID: 1171419708}
@@ -5728,9 +5728,9 @@ MonoBehaviour:
   _btnPracticeMode: {fileID: 1604978690}
   _btnExamMode: {fileID: 1196237764}
   _modeDescText: {fileID: 0}
-  _btnFixationBack: {fileID: 0}
-  _btnLimbFixation: {fileID: 0}
-  _btnSplintFixation: {fileID: 0}
+  _btnFixationBack: {fileID: 1329058529}
+  _btnLimbFixation: {fileID: 1913973802}
+  _btnSplintFixation: {fileID: 1015007396}
   _practiceModeDesc: "\u25CF \u6A21\u62DF\u7EC3\u4E60\u6A21\u5F0F\uFF1A\u4E0D\u9650\u65F6\u957F\uFF0C\u53EF\u53CD\u590D\u7EC3\u4E60\u3002\u7CFB\u7EDF\u4E0D\u8BC4\u5206\uFF0C\u5E2E\u52A9\u60A8\u719F\u6089\u5B9E\u9A8C\u6D41\u7A0B\u548C\u64CD\u4F5C\u6B65\u9AA4\u3002"
   _examModeDesc: "\u25CF \u8003\u6838\u8BC4\u4F30\u6A21\u5F0F\uFF1A\u9650\u5B9A\u65F6\u95F4\uFF0C\u5B8C\u6210\u540E\u7CFB\u7EDF\u81EA\u52A8\u8BC4\u5206\uFF0C\u5E76\u63D0\u4F9B\u8BE6\u7EC6\u53CD\u9988\u62A5\u544A\u3002"
   _editorPreview: 0