fix(stepdown): restart the audio pipe around encoder changes

A quality step-down now closes the named audio pipe before stopping/flushing the
pump, then opens a fresh server before restarting the pump. This prevents ffmpeg
from waiting on the old audio input and avoids reconnecting to a broken pipe.

EncoderRestartTests pins the audio-stop → video-stop → audio-start → video-start
order. 616 tests green, clean build with 0 warnings.

ytLive.Tests/EncoderRestartTests.cs

@@ -26,6 +26,7 @@ public sealed class EncoderRestartTests
         var tier = (Width: 1920, Height: 1080, Fps: 60, Bitrate: 8.0);
         var started = new List<EncoderOptions>();
         var frames = new List<CapturingEncoder>();
+        var order = new List<string>();
 
         var scene = new Scene { Name = "Live" };
         scene.Elements.Add(new Source { Name = "Background", Type = SourceType.Background, IsBackground = true });
@@ -49,7 +50,12 @@ public sealed class EncoderRestartTests
             encoderFactory: () =>
             {
                 var encoder = new CapturingEncoder();
-                encoder.Started += options => started.Add(options);
+                encoder.Started += options =>
+                {
+                    started.Add(options);
+                    order.Add($"encoder-start-{options.Fps}");
+                };
+                encoder.Stopped = () => order.Add("encoder-stop");
                 frames.Add(encoder);
                 return encoder;
             },
@@ -63,7 +69,9 @@ public sealed class EncoderRestartTests
         // options func now reports the new tier — which the restart must re-read.
         tier = (1920, 1080, 30, 6.4);
 
-        Assert.True(await pump.ChangeEncoderAsync());
+        Assert.True(await pump.ChangeEncoderAsync(
+            stopAudio: () => order.Add("audio-stop"),
+            startAudio: () => order.Add("audio-start")));
         Assert.True(await frames[1].FirstFrame.Task.WaitAsync(TimeSpan.FromSeconds(5)), "restarted session never produced a frame");
 
         Assert.Equal(2, started.Count);
@@ -71,6 +79,7 @@ public sealed class EncoderRestartTests
         Assert.Equal(30, started[1].Fps);
         Assert.Equal(6400, started[1].BitrateKbps);
         Assert.NotSame(frames[0], frames[1]);
+        Assert.Equal(new[] { "encoder-start-60", "audio-stop", "encoder-stop", "audio-start", "encoder-start-30" }, order);
 
         await pump.StopAsync();
     }
@@ -79,6 +88,7 @@ public sealed class EncoderRestartTests
     {
         public readonly TaskCompletionSource<bool> FirstFrame = new(TaskCreationOptions.RunContinuationsAsynchronously);
         public event Action<EncoderOptions>? Started;
+        public Action? Stopped { get; set; }
 
         public int DroppedFrames => 0;
 
@@ -97,7 +107,11 @@ public sealed class EncoderRestartTests
             return Task.CompletedTask;
         }
 
-        public Task StopAsync(CancellationToken cancellationToken = default) => Task.CompletedTask;
+        public Task StopAsync(CancellationToken cancellationToken = default)
+        {
+            Stopped?.Invoke();
+            return Task.CompletedTask;
+        }
 
         public void Dispose() { }
     }