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.

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.