fix: persist audio track selection across songs (preferredTrackIndex)

Sources/PlayerBridge.swift

@@ -70,6 +70,9 @@ final class PlayerBridge: ObservableObject {
     // 缓存时长
     private var cachedDuration: Double = 0
     
+    // 用户偏好音轨（跨歌曲保持）
+    private var preferredTrackIndex: Int = 0
+    
     // MARK: - 初始化
     
     func setup() {
@@ -340,14 +343,17 @@ final class PlayerBridge: ObservableObject {
             
             let options = group.options
             availableTracks = options.enumerated().map { idx, _ in "Track \(idx + 1)" }
-            currentTrackIndex = 0
             
-            // 选择默认音轨
-            if let defaultOpt = group.defaultOption,
-               let idx = options.firstIndex(of: defaultOpt) {
-                currentTrackIndex = idx
+            // 使用用户偏好音轨（如果存在），否则用默认
+            var targetIndex = preferredTrackIndex
+            if targetIndex >= options.count {
+                targetIndex = 0
             }
             
+            // 应用选中的音轨
+            item.select(options[targetIndex], in: group)
+            currentTrackIndex = targetIndex
+            
             updateTrackLabel()
         }
     }
@@ -361,6 +367,7 @@ final class PlayerBridge: ObservableObject {
             
             item.select(group.options[index], in: group)
             currentTrackIndex = index
+            preferredTrackIndex = index  // 记住用户偏好
             updateTrackLabel()
         }
     }