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.

Services/Encoder/FramePump.cs

@@ -248,17 +248,24 @@ public sealed class FramePump : IDisposable
         }
     }
 
-    /// <summary>TASK 33: relaunch the pump and encoder at a new tier, over the same URL and audio pipe.
+    /// <summary>TASK 33: relaunch the pump and encoder at a new tier, over the same URL and named audio pipe.
     ///
     /// <para>Full stop + start rather than an encoder swap, deliberately: <see cref="StopAsync"/> awaits
     /// the loop and flushes the encoder, so no in-flight submit can land on a disposed process. A bare
     /// swap races exactly that and would fault the loop — which ends the broadcast, the one thing a
-    /// quality step-down must never do. The cost is a short freeze, already accepted by the ladder.</para>
+    /// quality step-down must never do. The audio pipe must be closed before the pump stops and freshly
+    /// opened before it starts again; otherwise ffmpeg waits on the still-open audio input, then the new
+    /// process tries to attach to a broken server. The cost is a short freeze, already accepted by the ladder.</para>
     /// </summary>
-    public async Task<bool> ChangeEncoderAsync(CancellationToken cancellationToken = default)
+    public async Task<bool> ChangeEncoderAsync(Action stopAudio, Action startAudio,
+        CancellationToken cancellationToken = default)
     {
+        ArgumentNullException.ThrowIfNull(stopAudio);
+        ArgumentNullException.ThrowIfNull(startAudio);
+        stopAudio();
         await StopAsync(cancellationToken);
         if (cancellationToken.IsCancellationRequested) return false;
+        startAudio();
         await StartAsync(cancellationToken);
         return IsRunning;
     }

ViewModels/MainViewModel.StepDown.cs

@@ -74,7 +74,9 @@ public partial class MainViewModel
 
             _encoderTierOverride = landed;
             AppLog.Write($"Quality step-down: {decision.Why}");
-            await _framePump.ChangeEncoderAsync();
+            await _framePump.ChangeEncoderAsync(
+                _audioMixer.StopLive,
+                () => _audioMixer.StartLive(InstanceProfile.AudioPipeName(EncoderOptions.DefaultAudioPipeName)));
 
             // Show what is actually going out. The dropdown keeps the creator's choice — the policy
             // does not get to rewrite their intent, only the encoder's output.

docs/ai/output.md

@@ -248,8 +248,10 @@ devices, no timers).
   mixer at construction, read live each tick, so the volume sliders and mute buttons are stream-honest.
   (Before this wiring the seams defaulted to unity and the controls were decorative.) `StopStream` →
   `StopLive()` (closes the pipe → ffmpeg audio EOF) **BEFORE** stopping the frame pump (video EOF) —
-  the reverse order stalls on pipe backpressure. `FfmpegEncoder` itself is untouched; the VM owns the
-  pipe lifecycle.  **Recordings are fragmented MP4 (TASK 32·4, 2026-11-05):** the record block carries
+  the reverse order stalls on pipe backpressure. Encoder step-down follows the same order: close the
+  audio pipe, flush/stop the pump, create a fresh server on the same name, then restart the pump so ffmpeg
+  connects to a live server. `FfmpegEncoder` itself is untouched; the VM owns the pipe lifecycle.
+  **Recordings are fragmented MP4 (TASK 32·4, 2026-11-05):** the record block carries
   `-movflags +frag_keyframe+empty_moov+default_base_moof`, so the index is written first and a playable
   fragment is flushed at every keyframe. A plain MP4 puts its index at the end, which is why a power cut
   or a killed process previously left the whole take unreadable. The FLV stream block never gets it.

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() { }
     }