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/core/client.h

@@ -147,9 +147,16 @@ struct vc_client {
         // vc_get_stream_audio_config() has something to read back for our own streams.
         voicecat::codec::OpusParams effective_params;
 
-        // Talk-indicator edge detection (docs/voice.md §7) — updated in on_capture_frame.
+        // Talk-indicator edge detection (docs/voice.md §7) — updated in on_capture_frame, after
+        // the VAD/PTT gate so a gated-closed frame doesn't show as "talking".
         std::atomic<int64_t> last_capture_ms{0};
         bool                  talking = false;
+
+        // Device-enumeration follow-up: the device this stream's capture should use ("" =
+        // default). Only meaningful for VC_STREAM_MIC today (the real capture device); set via
+        // vc_set_input_device. Opaque id from AudioEngine::enumerate_devices — see
+        // audio_engine.h's DeviceInfo doc comment.
+        std::string capture_device_id;
     };
     mutable std::mutex                            local_streams_mu_;
     std::unordered_map<int, LocalStream>          local_streams_;       // keyed by vc_stream_kind
@@ -174,6 +181,14 @@ struct vc_client {
     std::atomic<bool> self_mic_muted_{false};
     std::atomic<bool> self_deafened_{false};
 
+    // Follow-up to M3: send-side input gate (docs/voice.md §11). MIC-only — SCREEN_AUDIO/
+    // AUX_DEVICE are never gated (see PROGRESS.md for the rationale). Pure local state, no
+    // protocol traffic. mic_vad_ is constructed once the MIC stream's StreamAnnounceResult
+    // lands (handle_stream_announce_result, on io_thread_ — not the RT capture callback).
+    std::atomic<vc_input_mode> current_input_mode_{VC_INPUT_VOICE_ACTIVATION};
+    std::atomic<bool>          ptt_active_{false};
+    std::unique_ptr<voicecat::audio::ApmProcessor> mic_vad_;
+
     // teardown_voice() is called both from run_io()'s own cleanup (on the io_thread_, when
     // the read loop exits) and from disconnect() (on the caller's thread) -- without
     // serializing those two call sites, both can see udp_thread_/talk_timer_thread_ as