feat: device enumeration, VAD/PTT input gate, stereo playback, WASAPI loopback

Closes the three items PROGRESS.md's M3 section explicitly carried forward as
out of scope:

- Device enumeration (vc_list_devices) + input device selection
  (vc_set_input_device), backed by AudioEngine::enumerate_devices() via
  miniaudio's ma_context_get_devices. Device ids are opaque hex-encoded
  ma_device_id strings.
- VAD/PTT send-side input gate (vc_set_input_mode, vc_set_push_to_talk).
  webrtc-audio-processing (the originally-planned APM) has no working
  Windows/MSVC build upstream (GCC-only Meson, unfinished MinGW support, hard
  abseil-cpp dependency), so VAD is a new lightweight, dependency-free
  energy/RMS processor (EnergyVadProcessor) behind the existing ApmProcessor
  interface. Gating is MIC-only; SCREEN_AUDIO/AUX_DEVICE always bypass it.
- True stereo playback: AudioEngine's mixer and output device now carry
  stereo end-to-end (mono streams upmix L=R) instead of downmixing decoded
  stereo streams to mono before mixing.
- Real WASAPI loopback capture for SCREEN_AUDIO (Windows-only, via
  miniaudio's loopback device type), replacing test-only injection as the
  production capture path.

Also: vccli gains --list-devices, --input-device, --input-mode, and
--share-screen-audio flags, plus a stdin command loop (ptt on/off, mode
vad/ptt) for manual verification. New test_vad_ptt_devices.cpp covers all
four items (ABI-level + a white-box AudioEngine stereo-mix check).

Docs updated to match: voice.md, roadmap.md (decision-log entry superseding
the original webrtc-audio-processing choice), tech-stack.md, README.md,
architecture.md, CLAUDE.md, PROGRESS.md.

Still explicitly out of scope, documented not silently dropped: real
webrtc-audio-processing/AEC (no AEC/NS/AGC exists at all yet), macOS/iOS
SCREEN_AUDIO capture, process-specific loopback, and a pre-existing
RT-thread rule violation in the capture path that predates this work.

Verified: ctest 12/12 green across 3 consecutive full-suite runs (both dev
and m1-dev presets build clean); test_vad_ptt_devices passed 5 consecutive
standalone runs; manually verified live (vccli --list-devices against real
hardware, vccli --voice --input-mode vad streaming without incident).

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

core/src/audio/audio_engine.h

@@ -75,10 +75,21 @@ class JitterBuffer {
 // ── AudioParams ──────────────────────────────────────────────────────────────
 struct AudioParams {
     uint32_t    sample_rate        = 48000;
-    uint32_t    channels           = 1;
+    uint32_t    capture_channels   = 1;  // no stereo capture device (mic) in this pass
+    uint32_t    playback_channels  = 2;  // true stereo output (see audio_engine.cpp on_playback)
     uint32_t    frame_ms           = 20;
-    std::string capture_device_id;   // "" = default
-    std::string playback_device_id;  // "" = default
+    std::string capture_device_id;   // "" = default; opaque id from AudioEngine::enumerate_devices
+    std::string playback_device_id;  // "" = default; opaque id from AudioEngine::enumerate_devices
+};
+
+// One enumerated device, returned by AudioEngine::enumerate_devices(). `id` is an internal,
+// opaque hex-encoded ma_device_id — callers must always round-trip an id that came from
+// enumerate_devices(); never construct one by hand (names aren't guaranteed unique, so the id
+// is the only stable handle miniaudio accepts back for device selection).
+struct DeviceInfo {
+    std::string id;
+    std::string name;
+    bool        is_default = false;
 };
 
 // ── AudioEngine ──────────────────────────────────────────────────────────────
@@ -105,6 +116,18 @@ class AudioEngine {
 
     bool running() const { return running_.load(std::memory_order_acquire); }
 
+    // Enumerate input or output devices (for UI pickers / vc_list_devices). Static: works
+    // before any AudioEngine instance is running (device pickers need to populate pre-connect).
+    // Inits a throwaway ma_context if VOICECAT_HAS_AUDIO; returns {} otherwise. See DeviceInfo
+    // above for the `id` encoding contract.
+    static std::vector<DeviceInfo> enumerate_devices(bool capture);
+
+    // Real desktop-audio loopback capture (Windows/WASAPI only, VOICECAT_HAS_LOOPBACK). Feeds
+    // `kind`'s capture_cb_ directly, same pattern as the real mic capture device — NOT routed
+    // through inject_capture()'s test-only ring. No-op (returns false) when unsupported.
+    bool start_loopback_capture(int kind);
+    void stop_loopback_capture();
+
     // Inject synthetic PCM directly into the capture pipeline (bypasses real device).
     // Thread-safe; can be called from any thread including tests. `kind` selects which local
     // stream's injection tap to feed (each gets its own ring buffer); the 2-arg overload
@@ -139,6 +162,13 @@ class AudioEngine {
     void init_recv_stream(uint32_t ssrc, const codec::OpusParams& p);
 #endif
 
+#ifdef VOICECAT_HAS_AUDIO
+    // TEST-ONLY — exposes the playback mixer without a real ma_device, so tests can verify
+    // stereo mixing end-to-end (no audio hardware needed). Same logic the real playback
+    // callback uses; safe to call any time after start() (no ma_device touched).
+    void mix_for_test(int16_t* out, uint32_t frames) { on_playback(out, frames); }
+#endif
+
    private:
 #ifdef VOICECAT_HAS_AUDIO
     static void capture_data_cb(ma_device*, void*, const void*, ma_uint32);
@@ -150,6 +180,18 @@ class AudioEngine {
     ma_device playback_device_{};
     bool      capture_started_  = false;
     bool      playback_started_ = false;
+
+#ifdef VOICECAT_HAS_LOOPBACK
+    // Desktop-audio loopback capture (SCREEN_AUDIO) — own lifecycle, decoupled from
+    // capture_device_/playback_device_ start/stop (a screen-share can start/stop independently
+    // of the mic and of whether anything is currently playing back).
+    static void loopback_data_cb(ma_device*, void*, const void*, ma_uint32);
+    void        on_loopback(const int16_t* pcm, ma_uint32 frames);
+
+    ma_device loopback_device_{};
+    bool      loopback_started_ = false;
+    int       loopback_kind_    = 0;
+#endif
 #endif
 
     AudioParams      params_{};