feat(M3): multi-stream & per-channel tuning

Implements docs/roadmap.md M3: multiple concurrent streams per user (MIC +
SCREEN_AUDIO + AUX_DEVICE), independent per-stream receiver gain/mute/noise-
reduction, talk indicators, and enforced per-channel Opus configurability
(mono/stereo, bitrate, frame size, FEC/DTX, application).

Bugs fixed along the way (found while implementing, not pre-existing scope):
- Server hard-coded stream_id=1 for every announce, so a second stream from
  the same user silently overwrote the first in SessionRegistry::set_user_stream.
  Now a per-session counter (ConnSession::next_stream_id_); handle_stream_stop
  validates against announced_stream_ids_ before clearing.
- Client dropped mode/dtx/complexity/application from effective_audio even for
  the single M2 stream -- only sample_rate/bitrate_bps/frame_ms/fec were ever
  applied to OpusParams. Fixed on both the send (handle_stream_announce_result)
  and receive (sync_remote_streams) paths via a shared
  opus_params_from_audio_config() helper.
- OpusEncoder always used OPUS_APPLICATION_VOIP; added OpusParams::application
  and wired it through.
- on_playback's per-stream decode passed the wrong frame_size to opus_decode
  (total samples instead of samples-per-channel), which would have overflowed
  the decode buffer for any stereo stream.
- teardown_voice() raced when called concurrently from run_io()'s own cleanup
  and from disconnect() on a different thread -- both could see
  udp_thread_/talk_timer_thread_ as joinable() at once and race to join() the
  same std::thread (intermittent std::system_error under ctest). Fixed with a
  teardown_mu_ guard instead of carrying the flake forward.

New:
- Per-channel AudioConfig: SessionRegistry now seeds Lobby (mono/24kbps/VOIP/
  FEC+DTX) and a new "Music Room" channel (stereo/128kbps/AUDIO/no DTX);
  handle_stream_announce enforces the channel's config, clamping (not
  overriding) bitrate_bps to its ceiling.
- core/src/core/client.h/.cpp: local-stream state is now a
  std::unordered_map<int, LocalStream> keyed by vc_stream_kind, with
  request_id-correlated announce/result handling (request_id already
  round-tripped on the wire; just wasn't read before). on_capture_frame is
  kind-aware and upmixes mono capture to stereo when a stream's config calls
  for it. set_self_mute's mic_muted now only gates the MIC kind. NS is wired
  through set_remote_stream. New run_talk_timer() thread emits
  VC_EVENT_TALK_STATE from both remote and local edge detection.
- core/src/audio/audio_engine.h/.cpp: kind-keyed injection taps
  (inject_capture), stereo-to-mono downmix at the decode/mix boundary,
  RemoteStream gains recv_ns (lazy ApmProcessor) + noise_reduction_enabled
  and last_voice_ms/talking; new set_stream_noise_reduction() and
  poll_talk_transitions().
- core/src/session/session.h/.cpp: Stream now carries the full AudioConfig,
  not just sample_rate/frame_ms.
- New additive C ABI (core/include/voicecat.h): vc_audio_config +
  vc_get_stream_audio_config (effective Opus config for any stream you own or
  a peer's); vc_test_inject_capture (test-only synthetic PCM injection,
  clearly marked, mirrors AudioEngine::inject_capture).
- tests/test_m3_multistream.cpp: the M3 exit criterion through the real ABI
  (mirrors test_voice_client_abi.cpp's approach, not raw sockets) -- two
  concurrent local streams, independent gain/mute/NS control, per-channel
  config divergence via vc_get_stream_audio_config, talk indicators.

Explicitly out of scope for this pass (tracked in PROGRESS.md, not silently
dropped): VAD/PTT input gate + device enumeration; real WASAPI loopback
capture for SCREEN_AUDIO (synthetic injection only); true stereo playback
output (AudioEngine's mixer/output device stays mono -- Opus itself is fully
stereo-correct on the wire).

ctest --test-dir build/m1-dev: 11/11 green, verified across 3 consecutive
full-suite runs plus 8 standalone runs of the new test.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/session_registry.cpp

@@ -17,7 +17,44 @@ void SessionRegistry::init_default_channels() {
     lobby.proto.set_name("Lobby");
     lobby.proto.set_type(voicecat::v1::CHANNEL_PERMANENT);
     lobby.proto.set_order(0);
+    {
+        // Speech profile: mono, low bitrate, FEC+DTX on for resilience/silence-suppression.
+        auto* a = lobby.proto.mutable_audio();
+        a->set_codec(0);
+        a->set_mode(voicecat::v1::MODE_MONO);
+        a->set_sample_rate(48000);
+        a->set_bitrate_bps(24000);
+        a->set_frame_ms(20);
+        a->set_application(voicecat::v1::OPUS_VOIP);
+        a->set_fec(true);
+        a->set_expected_packet_loss(10);
+        a->set_dtx(true);
+        a->set_complexity(5);
+    }
     channels_[1] = std::move(lobby);
+
+    ChannelEntry music;
+    music.proto.set_id(2);
+    music.proto.set_name("Music Room");
+    music.proto.set_type(voicecat::v1::CHANNEL_PERMANENT);
+    music.proto.set_order(1);
+    {
+        // Music/screen-audio profile: stereo, high bitrate, FEC/DTX off (continuous signal).
+        auto* a = music.proto.mutable_audio();
+        a->set_codec(0);
+        a->set_mode(voicecat::v1::MODE_STEREO);
+        a->set_sample_rate(48000);
+        a->set_bitrate_bps(128000);
+        a->set_frame_ms(20);
+        a->set_application(voicecat::v1::OPUS_AUDIO);
+        a->set_fec(false);
+        a->set_expected_packet_loss(0);
+        a->set_dtx(false);
+        a->set_complexity(8);
+    }
+    channels_[2] = std::move(music);
+
+    next_channel_id_ = 3;  // 1 and 2 are now reserved (Lobby, Music Room)
 }
 
 uint64_t SessionRegistry::register_session(std::weak_ptr<ConnSession> session) {
@@ -203,6 +240,14 @@ uint32_t SessionRegistry::user_channel(uint32_t user_id) const {
     return (it == users_.end()) ? 0 : it->second.proto.channel_id();
 }
 
+std::optional<voicecat::v1::AudioConfig> SessionRegistry::channel_audio_config(
+    uint32_t channel_id) const {
+    std::shared_lock lk(mu_);
+    auto it = channels_.find(channel_id);
+    if (it == channels_.end()) return std::nullopt;
+    return it->second.proto.audio();
+}
+
 }  // namespace voicecat::server
 
 #endif  // VOICECAT_HAS_NET