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/Marshaling.cs

@@ -81,6 +81,11 @@ internal static class Marshaling
         return result;
     }
 
+    public static RemoteStreamState ToManaged(in VcRemoteStreamStateNative native) => new(
+        native.Gain,
+        native.Muted != 0,
+        native.NoiseReduction != 0);
+
     public static AudioConfigInfo ToManaged(in VcAudioConfigNative native) => new(
         native.Codec,
         native.Mode != 0,