fix(audio): add per-5s live-loop telemetry to name the silent-recording stage

The recorded audio is full-length silent AAC (−91dB, 1124 frames/23.95s) —
the named pipe carried ~9.1MB of zeros the entire take. The mixer's loop
ran, the pipe connected, ffmpeg read it, and the file is full-length
silence. Sources started fine ('Audio: using system default mic...' logged),
no failure callbacks fired, and the existing integration test
(Mix_WithFiltersDuckAndGain_Lands_On_AudioPipe) proves the loop→pipe path
carries real audio when fed — the fault is capture-side.

Added permanent per-5s live-loop telemetry to startup.log so the next take
names the exact stage without new code:

  Audio live: pipe connected= dropped writes= micLevel= loopLevel= drained
  N/N samples peakMix N (per 5s)

- AudioMixer.FillAndMix now returns (MicRms, MicDrained, LoopDrained) for
  the accumulation; StartLive/StopLive log start/stop lines.
- NamedPipeAudioWriter.DroppedWrites: nonzero = audio dropped before ffmpeg
  connected (names 'pipe never connected' stage).

Likely root cause: WASAPI loopback captures the default render endpoint —
if audio plays on a non-default device the recording is silently silent.
The fix requires device enumeration + selection (slice 13 candidate).

Suite 290/291 — same sole pre-existing compositor pixel failure.

Services/Audio/AudioMixer.cs

@@ -50,6 +50,10 @@ public sealed class AudioMixer : IDisposable
     private float[]? _mixBuffer;
     private float[]? _delayedMix;
     private DateTime _lastLoopErrorLogged = DateTime.MinValue;
+    private DateTime _nextStatsLog = DateTime.UtcNow + TimeSpan.FromSeconds(5);
+    private long _micDrainedTotal;
+    private long _loopDrainedTotal;
+    private float _peakMix;
 
     public AudioMixer(
         IAudioSource mic,
@@ -159,6 +163,7 @@ public sealed class AudioMixer : IDisposable
         _liveCts = cts;
         _pipe = new NamedPipeAudioWriter();
         _pipe.Start(pipeName);
+        _log?.Invoke($"Audio live: started, pipe '{pipeName}'");
         _ = Task.Run(() => LiveLoopAsync(_pipe, cts.Token));
     }
 
@@ -169,7 +174,11 @@ public sealed class AudioMixer : IDisposable
     {
         var cts = _liveCts;
         _liveCts = null;
-        cts?.Cancel();
+        if (cts != null)
+        {
+            _log?.Invoke("Audio live: stopped");
+            cts.Cancel();
+        }
         _pipe.Stop();
     }
 
@@ -274,8 +283,28 @@ public sealed class AudioMixer : IDisposable
             var nextTick = DateTime.UtcNow + _mixInterval;
             try
             {
-                FillAndMix(micChunk, loopbackChunk, mixBuffer);
+                var (_, micDrained, loopDrained) = FillAndMix(micChunk, loopbackChunk, mixBuffer);
                 await pipe.WriteAsync(mixBuffer, cancellationToken).ConfigureAwait(false);
+
+                _micDrainedTotal += micDrained;
+                _loopDrainedTotal += loopDrained;
+                for (var i = 0; i < mixBuffer.Length; i++)
+                {
+                    var abs = Math.Abs(mixBuffer[i]);
+                    if (abs > _peakMix) _peakMix = abs;
+                }
+
+                var now = DateTime.UtcNow;
+                if (now >= _nextStatsLog && _log != null)
+                {
+                    _nextStatsLog = now + TimeSpan.FromSeconds(5);
+                    _log($"Audio live: pipe connected={pipe.IsConnected} dropped writes={pipe.DroppedWrites} " +
+                         $"micLevel={_meter.Level:F3} loopLevel={_loopbackMeter.Level:F3} " +
+                         $"drained {_micDrainedTotal:N0}/{_loopDrainedTotal:N0} samples peakMix {_peakMix:F3} (per 5s)");
+                    _micDrainedTotal = 0;
+                    _loopDrainedTotal = 0;
+                    _peakMix = 0;
+                }
             }
             catch (OperationCanceledException)
             {
@@ -313,8 +342,11 @@ public sealed class AudioMixer : IDisposable
     }
 
     /// <summary>Drains one tick's worth of mic + loopback, silence-fills any
-    /// underrun, applies the ducker and the honest gains, and mixes to stereo.</summary>
-    private float FillAndMix(float[] micChunk, float[] loopbackChunk, float[] mix)
+    /// underrun, applies the ducker and the honest gains, and mixes to stereo.
+    /// Returns the pre-duck mic RMS and how many samples each buffer contributed
+    /// (the 5s live-loop telemetry reads those to name whether capture or the
+    /// pipe starved a silent recording).</summary>
+    private (float MicRms, int MicDrained, int LoopDrained) FillAndMix(float[] micChunk, float[] loopbackChunk, float[] mix)
     {
         var micCount = _micBuffer.Read(micChunk, micChunk.Length);
         var loopCount = _loopbackBuffer.Read(loopbackChunk, loopbackChunk.Length);
@@ -362,6 +394,6 @@ public sealed class AudioMixer : IDisposable
             delayed = _delayedMix = new float[mix.Length];
         _syncDelay.Process(mix, delayed);
         Array.Copy(delayed, mix, mix.Length);
-        return micRms;
+        return (micRms, micCount, loopCount);
     }
 }

Services/Audio/NamedPipeAudioWriter.cs

@@ -17,6 +17,12 @@ public interface IAudioPipeWriter : IDisposable
     /// <summary>True once ffmpeg has connected and the pipe is writable.</summary>
     bool IsConnected { get; }
 
+    /// <summary>Count of <see cref="WriteAsync"/> calls that wrote nothing because
+    /// the pipe wasn't connected yet or was already closed. Part of the silent-
+    /// recording diagnosis: nonzero means audio was dropped before ffmpeg ever
+    /// connected to the pipe.</summary>
+    long DroppedWrites { get; }
+
     /// <summary>Writes interleaved float samples; dropped until the client connects.</summary>
     Task WriteAsync(ReadOnlyMemory<float> samples, CancellationToken cancellationToken = default);
 
@@ -29,6 +35,9 @@ public sealed class NamedPipeAudioWriter : IAudioPipeWriter
     private readonly object _sync = new();
     private NamedPipeServerStream? _pipe;
 
+    /// <inheritdoc/>
+    public long DroppedWrites { get; private set; }
+
     public void Start(string pipeName)
     {
         lock (_sync)
@@ -64,7 +73,11 @@ public sealed class NamedPipeAudioWriter : IAudioPipeWriter
             pipe = _pipe;
         }
         if (pipe == null || !pipe.IsConnected || samples.IsEmpty)
+        {
+            if (pipe?.IsConnected == false || pipe == null)
+                DroppedWrites++;
             return;
+        }
 
         var bytes = MemoryMarshal.AsBytes(samples.Span).ToArray();
         await pipe.WriteAsync(bytes, cancellationToken).ConfigureAwait(false);