fix: replace blocking waitUntilExit() with async continuation

proc.waitUntilExit() blocks cooperative thread pool threads and
deadlocks in Swift Concurrency Task context because NSRunLoop
notifications aren't processed. Replace with terminationHandler +
withCheckedContinuation pattern.

Sources/StreamRecorderKit/FFmpegRecorder.swift

@@ -106,8 +106,8 @@ public final class FFmpegRecorder: StreamRecorderEngine {
             throw RecordingError.unknown("无法启动 ffmpeg: \(error.localizedDescription)")
         }
         
-        // 等待完成
+        // 异步等待进程完成（不阻塞 cooperative thread）
-        proc.waitUntilExit()
+        await waitForProcess(proc)
         stderrPipe.fileHandleForReading.readabilityHandler = nil
         isRecording = false
         
@@ -136,6 +136,22 @@ public final class FFmpegRecorder: StreamRecorderEngine {
     
     // MARK: - Private
     
+    /// Async-friendly process wait — uses terminationHandler + continuation
+    /// instead of blocking waitUntilExit() which deadlocks in Task context
+    private func waitForProcess(_ proc: Process) async {
+        guard proc.isRunning else { return }
+        await withCheckedContinuation { (continuation: CheckedContinuation<Void, Never>) in
+            proc.terminationHandler = { _ in
+                continuation.resume()
+            }
+            // Race condition: process may have exited between isRunning check and setting handler
+            if !proc.isRunning {
+                proc.terminationHandler = nil
+                continuation.resume()
+            }
+        }
+    }
+    
     private func findFFmpeg() -> String {
         let paths = [
             "/opt/homebrew/bin/ffmpeg",
@@ -186,7 +202,7 @@ public final class FFmpegRecorder: StreamRecorderEngine {
         
         do {
             try proc.run()
-            proc.waitUntilExit()
+            await waitForProcess(proc)
         } catch {
             throw RecordingError.unknown("无法分析输出文件")
         }
@@ -269,7 +285,7 @@ public final class FFmpegRecorder: StreamRecorderEngine {
         
         do {
             try proc.run()
-            proc.waitUntilExit()
+            await waitForProcess(proc)
         } catch {
             return nil
         }