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;
     }