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).

core/src/audio/audio_engine.cpp

@@ -334,6 +334,17 @@ void AudioEngine::set_stream_noise_reduction(uint32_t ssrc, bool enable) {
     }
 }
 
+bool AudioEngine::get_stream_state(uint32_t ssrc, float& gain, bool& mute, bool& noise_reduction) {
+    std::lock_guard lk(streams_mu_);
+    auto it = streams_.find(ssrc);
+    if (it == streams_.end()) return false;
+    const auto& s = it->second;
+    gain             = s.gain;
+    mute             = s.mute;
+    noise_reduction  = s.noise_reduction_enabled;
+    return true;
+}
+
 void AudioEngine::remove_stream(uint32_t ssrc) {
     std::lock_guard lk(streams_mu_);
     streams_.erase(ssrc);

core/src/audio/audio_engine.h

@@ -158,6 +158,10 @@ class AudioEngine {
     // Listener-chosen, local-only noise reduction on a specific remote stream (docs/voice.md
     // §10) — lazily instantiates an ApmProcessor on first enable, frees it on disable.
     void set_stream_noise_reduction(uint32_t ssrc, bool enable);
+    // Read back a stream's current receive-side state. Returns true and fills *out if the
+    // stream is known (even if defaults — gain=1, mute=false, nr=false), false if it has
+    // never been seen (no RemoteStream entry yet).
+    bool get_stream_state(uint32_t ssrc, float& gain, bool& mute, bool& noise_reduction);
     void remove_stream(uint32_t ssrc);
 
     // Edge-triggered talk-state transitions since the last call (docs/voice.md §7: talk state