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/NativeMethods.cs

@@ -81,6 +81,10 @@ internal static partial class NativeMethods
     internal static partial VcResult vc_set_remote_stream(nint c, uint userId, uint streamId,
         float gain, int muted, int noiseReduction);
 
+    [LibraryImport(LibName)]
+    internal static partial VcResult vc_get_remote_stream(nint c, uint userId, uint streamId,
+        out VcRemoteStreamStateNative outState);
+
     [LibraryImport(LibName)]
     internal static partial VcResult vc_get_stream_audio_config(nint c, uint userId,
         uint streamId, out VcAudioConfigNative outCfg);