feat(audio): per-stream mix controls in Windows client + vc_get_remote_stream getter

PerUserTuningDialog previously broadcast one gain/mute/NR set to *all* of a
user's streams, even though the core mixer (AudioEngine::RemoteStream) and
the C ABI (vc_set_remote_stream) were already per-stream. The UI had no
per-mix controls anywhere.

Reworks the dialog to enumerate ListUserStreams on open and render one row
per stream (kind + label + Gain + Mute + NR), each wiring only to its own
stream_id. Adds a read-back ABI counterpart, vc_get_remote_stream, so the
dialog opens at the listener's actual current per-stream settings (defaults
1.0/unmuted/NR-off) rather than always 100%. Additive ABI change only; no
existing symbols touched.

Tests: test_m3_multistream extended with getter round-trip assertions; new
C# smoke test exercises the full P/Invoke marshaling path with two clients.
Docs: voice.md §10 notes the getter. NR checkbox keeps its honest
'passthrough' label (NS DSP still unbuilt per §8).

clients/windows/VoiceCat.Interop/VoiceCatClient.cs

@@ -208,6 +208,13 @@ public sealed class VoiceCatClient : IDisposable
         NativeMethods.vc_set_remote_stream(_handle.DangerousGetHandle(), userId, streamId, gain,
             muted ? 1 : 0, noiseReduction ? 1 : 0);
 
+    public (VcResult Result, RemoteStreamState? State) GetRemoteStream(uint userId, uint streamId)
+    {
+        var r = NativeMethods.vc_get_remote_stream(_handle.DangerousGetHandle(), userId,
+            streamId, out var native);
+        return (r, r == VcResult.Ok ? Marshaling.ToManaged(in native) : null);
+    }
+
     public (VcResult Result, AudioConfigInfo? Config) GetStreamAudioConfig(uint userId, uint streamId)
     {
         var r = NativeMethods.vc_get_stream_audio_config(_handle.DangerousGetHandle(), userId,