fix(audio): apply receive-side NR to stereo mic streams

The per-listener noise-reduction toggle (vc_set_remote_stream) did nothing
on Windows/macOS/iOS. The decode loop gated the RNNoise pass on
dec_channels == 1 as a proxy for "this stream is voice" (assuming
stereo => screen-share). The stereo-mic capture commit broke that: a stereo
mic with send-side NR off transmits stereo Opus, so the receiver decoded
two channels and skipped NR entirely. gain/mute have no channel guard, which
is why only NR appeared broken.

Thread the stream kind through init_recv_stream into RemoteStream::is_voice
(set from si.kind() == STREAM_MIC), gate receive NR on is_voice instead of
channel count, and fold a stereo voice frame to mono -> denoise -> duplicate
back across both channels in place (symmetric with the send-side downmix;
RNNoise is mono-only). Screen-audio shares are never denoised.

New test test_recv_noise_reduction drives AudioEngine and asserts a stereo
voice stream's noise floor collapses with NR on (RMS 1046 -> 0.1) while a
screen-audio share stays unchanged. ctest --preset dev green 29/29.
Docs: voice.md section 10. Shared-core fix; clients need only a rebuild.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_jitter_depth.cpp

@@ -67,7 +67,7 @@ int main() {
     CHECK(opus_len > 0);
 
     const uint32_t ssrc = 1;
-    engine.init_recv_stream(ssrc, op, /*user_id=*/0, /*stream_id=*/0);
+    engine.init_recv_stream(ssrc, op, /*user_id=*/0, /*stream_id=*/0, /*is_voice=*/false);
 
     const uint32_t pb_frames = 480;  // 10 ms hardware period
     const int      out_n     = static_cast<int>(pb_frames) * 2;  // stereo interleaved