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/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);