fix: 音视频同步——音频从 firstFrameTime 开始读取

之前音频用 player.currentTime() 作为起始时间，视频用
AVPlayerItemVideoOutput 的 firstFrameTime，两者可能不一致
导致音视频不同步。

改为:
- prepareAudioCapture: 只预加载音频轨道，不立即开始读取
- captureVideoFrame: 第一帧时设置 firstFrameTime 后调用
  beginAudioReading(startTime: firstFrameTime)
- 音频和视频共用同一个时间基准，确保精确同步
- 移除 startRecording 的 startTime 参数

Sources/PlayerBridge.swift

@@ -122,8 +122,7 @@ final class PlayerBridge: ObservableObject {
             }
             guard !queue.isEmpty else { return }
             let sourceURL = queue[currentIndex].url
-            let startTime = player.currentTime()
+            screenRecorder.startRecording(from: output, sourceURL: sourceURL)
-            screenRecorder.startRecording(from: output, sourceURL: sourceURL, startTime: startTime)
         }
         #endif
     }

Sources/PlayerRecorder.swift

@@ -22,6 +22,10 @@ final class PlayerRecorder: NSObject {
     /// 音频线程读取 firstFrameTime 的镜像
     nonisolated(unsafe) private var audioFirstFrameTime: CMTime?
     
+    // 音频预加载状态
+    private var pendingAudioAsset: AVURLAsset?
+    private var pendingAudioTrack: AVAssetTrack?
+    
     @Published var isRecording = false
     @Published var durationText = "00:00"
     
@@ -38,7 +42,7 @@ final class PlayerRecorder: NSObject {
     }
     
     /// 开始录制
-    func startRecording(from output: AVPlayerItemVideoOutput, sourceURL: URL, startTime: CMTime) {
+    func startRecording(from output: AVPlayerItemVideoOutput, sourceURL: URL) {
         guard !isRecording else { return }
         
         weakOutput = output
@@ -97,8 +101,8 @@ final class PlayerRecorder: NSObject {
         startTimer()
         startCaptureLoop()
         
-        // 启动音频读取（从源 URL）
+        // 准备音频读取（等待 firstFrameTime 后才真正开始）
-        startAudioCapture(sourceURL: sourceURL, startTime: startTime, audioWriterInput: aInput)
+        prepareAudioCapture(sourceURL: sourceURL)
     }
     
     /// 停止录制
@@ -151,6 +155,8 @@ final class PlayerRecorder: NSObject {
             audioFirstFrameTime = itemTime  // 同步给音频线程
             writer?.startSession(atSourceTime: itemTime)
             isRunning = true
+            // 音频预加载完成后，用 firstFrameTime 作为起始时间开始读取
+            beginAudioReading(startTime: itemTime)
         }
         
         // 相对时间戳
@@ -182,8 +188,12 @@ final class PlayerRecorder: NSObject {
     }
     
     // MARK: - 音频读取（从源 URL）
-    private func startAudioCapture(sourceURL: URL, startTime: CMTime, audioWriterInput: AVAssetWriterInput) {
+    
+    /// 预加载音频轨道（异步），不立即开始读取
+    private func prepareAudioCapture(sourceURL: URL) {
         let asset = AVURLAsset(url: sourceURL)
+        pendingAudioAsset = asset
+        pendingAudioTrack = nil
         
         Task {
             do {
@@ -192,7 +202,27 @@ final class PlayerRecorder: NSObject {
                     print("[Recorder] No audio track in source")
                     return
                 }
+                self.pendingAudioTrack = audioTrack
+                // 如果 firstFrameTime 已经设置好了，立即开始读取
+                if let fft = self.firstFrameTime {
+                    self.beginAudioReading(startTime: fft)
+                }
+            } catch {
+                print("[Recorder] Audio track load failed: \(error)")
+            }
+        }
+    }
     
+    /// 用 firstFrameTime 作为起始时间，创建 reader 并开始后台读取
+    private func beginAudioReading(startTime: CMTime) {
+        guard let asset = pendingAudioAsset, let audioTrack = pendingAudioTrack else {
+            return  // 轨道还没加载好，prepareAudioCapture 的回调会再次调用
+        }
+        guard let aInput = audioInput else { return }
+        // 防止重复启动
+        guard audioReader == nil else { return }
+        
+        do {
             let reader = try AVAssetReader(asset: asset)
             let output = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: [
                 AVFormatIDKey: kAudioFormatLinearPCM,
@@ -205,7 +235,7 @@ final class PlayerRecorder: NSObject {
             ])
             output.alwaysCopiesSampleData = false
             
-                // 从播放位置开始读取
+            // 从视频第一帧时间开始读取，确保音视频同步
             reader.timeRange = CMTimeRange(start: startTime, duration: .positiveInfinity)
             
             if reader.canAdd(output) {
@@ -219,14 +249,17 @@ final class PlayerRecorder: NSObject {
                 let queue = DispatchQueue(label: "recorder.audio.capture")
                 self.audioCaptureQueue = queue
                 queue.async { [weak self] in
-                        guard let self = self, let aInput = self.audioInput else { return }
+                    guard let self = self else { return }
                     self.readAudioLoop(reader: reader, output: output, audioInput: aInput)
                 }
             }
         } catch {
             print("[Recorder] Audio capture setup failed: \(error)")
         }
-        }
+        
+        // 清理预加载状态
+        pendingAudioAsset = nil
+        pendingAudioTrack = nil
     }
     
     private nonisolated func readAudioLoop(reader: AVAssetReader, output: AVAssetReaderTrackOutput, audioInput: AVAssetWriterInput) {