fix: 录制停止时音频未同步停止导致输出文件超长

根因: readAudioLoop 从文件读音频速度远快于实时，每帧 dispatch 到
MainActor 造成大量 Task 积压。用户点停止时，积压的 Task 在
isRunning=false 生效前全部执行，导致整个源音频被写入。

修复:
- 音频直接在后台队列写入，不再 dispatch 到 MainActor
- 增加 nonisolated(unsafe) audioStopped 标志，stopAudioCapture
  先设标志让 loop 立即退出
- 增加 nonisolated(unsafe) audioFirstFrameTime 供后台线程读取
- audioInput 作为参数传入 readAudioLoop，避免跨 actor 访问

Sources/PlayerRecorder.swift

@@ -17,6 +17,10 @@ final class PlayerRecorder: NSObject {
     private var audioReaderOutput: AVAssetReaderTrackOutput?
     private var audioCaptureQueue: DispatchQueue?
     private var isRunning = false
+    /// 线程安全的音频停止标志，后台队列直接读取
+    nonisolated(unsafe) private var audioStopped: Bool = false
+    /// 音频线程读取 firstFrameTime 的镜像
+    nonisolated(unsafe) private var audioFirstFrameTime: CMTime?
     
     @Published var isRecording = false
     @Published var durationText = "00:00"
@@ -40,6 +44,7 @@ final class PlayerRecorder: NSObject {
         weakOutput = output
         firstFrameTime = nil
         isRunning = false
+        audioStopped = false
         
         // 临时文件
         let formatter = DateFormatter()
@@ -143,6 +148,7 @@ final class PlayerRecorder: NSObject {
         // 第一帧：用它的时间作为 session 起点
         if firstFrameTime == nil {
             firstFrameTime = itemTime
+            audioFirstFrameTime = itemTime  // 同步给音频线程
             writer?.startSession(atSourceTime: itemTime)
             isRunning = true
         }
@@ -213,7 +219,8 @@ final class PlayerRecorder: NSObject {
                     let queue = DispatchQueue(label: "recorder.audio.capture")
                     self.audioCaptureQueue = queue
                     queue.async { [weak self] in
-                        self?.readAudioLoop(reader: reader, output: output)
+                        guard let self = self, let aInput = self.audioInput else { return }
+                        self.readAudioLoop(reader: reader, output: output, audioInput: aInput)
                     }
                 }
             } catch {
@@ -222,47 +229,45 @@ final class PlayerRecorder: NSObject {
         }
     }
     
-    private nonisolated func readAudioLoop(reader: AVAssetReader, output: AVAssetReaderTrackOutput) {
+    private nonisolated func readAudioLoop(reader: AVAssetReader, output: AVAssetReaderTrackOutput, audioInput: AVAssetWriterInput) {
-        while reader.status == .reading {
+        while !audioStopped && reader.status == .reading {
             guard let sampleBuffer = output.copyNextSampleBuffer() else {
                 break
             }
             
-            // 在 MainActor 上调整时间戳并写入
+            // 直接在后台队列处理，不走 MainActor（避免队列积压）
-            Task { @MainActor [weak self] in
+            guard let firstTime = audioFirstFrameTime else {
-                guard let self = self, self.isRunning else { return }
+                // 视频第一帧还没到，跳过这个音频 buffer
-                guard let firstTime = self.firstFrameTime else { return }
+                continue
-                guard let input = self.audioInput, input.isReadyForMoreMediaData else { return }
+            }
-                
+            guard audioInput.isReadyForMoreMediaData else { continue }
-                let pts = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
+            
-                let relativeTime = CMTimeSubtract(pts, firstTime)
+            let pts = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
-                guard relativeTime.seconds >= 0 else { return }
+            let relativeTime = CMTimeSubtract(pts, firstTime)
-                
+            guard relativeTime.seconds >= 0 else { continue }
-                // 重建 sample buffer with new time
+            
-                var count: CMItemCount = 0
+            // 重建 sample buffer with new time
-                CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: 0, arrayToFill: nil, entriesNeededOut: &count)
+            var count: CMItemCount = 0
-                var timingInfo = [CMSampleTimingInfo](repeating: CMSampleTimingInfo(), count: count)
+            CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: 0, arrayToFill: nil, entriesNeededOut: &count)
-                CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: count, arrayToFill: &timingInfo, entriesNeededOut: nil)
+            var timingInfo = [CMSampleTimingInfo](repeating: CMSampleTimingInfo(), count: count)
-                
+            CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: count, arrayToFill: &timingInfo, entriesNeededOut: nil)
-                for i in 0..<count {
+            
-                    timingInfo[i].decodeTimeStamp = .invalid
+            for i in 0..<count {
-                    timingInfo[i].presentationTimeStamp = CMTimeSubtract(timingInfo[i].presentationTimeStamp, firstTime)
+                timingInfo[i].decodeTimeStamp = .invalid
-                }
+                timingInfo[i].presentationTimeStamp = CMTimeSubtract(timingInfo[i].presentationTimeStamp, firstTime)
-                
-                var newBuffer: CMSampleBuffer?
-                CMSampleBufferCreateCopyWithNewTiming(allocator: kCFAllocatorDefault, sampleBuffer: sampleBuffer, sampleTimingEntryCount: count, sampleTimingArray: &timingInfo, sampleBufferOut: &newBuffer)
-                
-                if let nb = newBuffer {
-                    _ = input.append(nb)
-                }
             }
             
-            // 等待 MainActor 处理完
+            var newBuffer: CMSampleBuffer?
-            Thread.sleep(forTimeInterval: 0.005)
+            CMSampleBufferCreateCopyWithNewTiming(allocator: kCFAllocatorDefault, sampleBuffer: sampleBuffer, sampleTimingEntryCount: count, sampleTimingArray: &timingInfo, sampleBufferOut: &newBuffer)
+            
+            if let nb = newBuffer {
+                _ = audioInput.append(nb)
+            }
         }
     }
     
     private func stopAudioCapture() {
+        audioStopped = true          // 先设标志，loop 立即退出
         audioReader?.cancelReading()
         audioReader = nil
         audioReaderOutput = nil