fix: StopBgMusic强制Stop+中断FadeInBg协程，彻底解决bg不退

- StopBgMusic去掉isPlaying检查，AudioSource存在就强制Stop
- 保存FadeInBg协程引用到_fadeCoroutine
- StopBgMusic时先StopCoroutine(_fadeCoroutine)再Stop音频

Assets/Scripts/UI/PageController.cs

@@ -319,6 +319,8 @@ public class PageController : MonoBehaviour
     [SerializeField] private AudioClip _experimentBgClip;
     [SerializeField] private AudioClip[] _womanClips;
     private AudioSource _bgAudioSource;
+    private Coroutine _fadeCoroutine;
+    private Coroutine _fadeCoroutine;
 
     private void PlayHomeBg()
     {
@@ -326,13 +328,14 @@ public class PageController : MonoBehaviour
         if (_bgAudioSource == null) { _bgAudioSource = gameObject.AddComponent<AudioSource>(); _bgAudioSource.loop = true; _bgAudioSource.playOnAwake = false; }
         _bgAudioSource.clip = _homeBgClip;
         _bgAudioSource.Play();
-        StartCoroutine(FadeInBg());
+        _fadeCoroutine = StartCoroutine(FadeInBg());
         Debug.Log("PageController: 播放首页背景音乐");
     }
 
     private void StopBgMusic()
     {
-        if (_bgAudioSource != null && _bgAudioSource.isPlaying)
+        if (_fadeCoroutine != null) { StopCoroutine(_fadeCoroutine); _fadeCoroutine = null; }
+        if (_bgAudioSource != null)
         {
             _bgAudioSource.Stop();
             Debug.Log("PageController: 停止背景音乐");
@@ -346,7 +349,7 @@ public class PageController : MonoBehaviour
         if (_bgAudioSource == null) { _bgAudioSource = gameObject.AddComponent<AudioSource>(); _bgAudioSource.loop = true; _bgAudioSource.playOnAwake = false; }
         _bgAudioSource.clip = _experimentBgClip;
         _bgAudioSource.Play();
-        StartCoroutine(FadeInBg());
+        _fadeCoroutine = StartCoroutine(FadeInBg());
         Debug.Log("PageController: 播放实验背景音乐");
     }