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/apm_processor.cpp

@@ -1,7 +1,18 @@
 #include "audio/apm_processor.h"
 
+#include <chrono>
+#include <cmath>
+
 namespace voicecat::audio {
 
+namespace {
+int64_t steady_now_ms() {
+    return std::chrono::duration_cast<std::chrono::milliseconds>(
+               std::chrono::steady_clock::now().time_since_epoch())
+        .count();
+}
+}  // namespace
+
 // ── ApmPassthrough ────────────────────────────────────────────────────────────
 // No-op: returns true (VAD always open), does not modify PCM.
 // Replaced by WebrtcApmProcessor when VOICECAT_HAS_APM is defined.
@@ -11,11 +22,47 @@ class ApmPassthrough final : public ApmProcessor {
     bool process_capture(int16_t*, int, int) override { return true; }
 };
 
+// ── EnergyVadProcessor ──────────────────────────────────────────────────────
+// Lightweight, dependency-free energy/RMS VAD — see apm_processor.h's create_vad() doc comment
+// for why this exists instead of a real APM. No AEC (process_render is a no-op); doesn't modify
+// the PCM it's given, only inspects it.
+class EnergyVadProcessor final : public ApmProcessor {
+   public:
+    EnergyVadProcessor(float rms_threshold, int64_t hang_time_ms)
+        : threshold_(rms_threshold), hang_time_ms_(hang_time_ms) {}
+
+    void process_render(const int16_t*, int, int) override {}
+
+    bool process_capture(int16_t* pcm, int samples, int /*sample_rate*/) override {
+        if (samples > 0) {
+            double sum_sq = 0.0;
+            for (int i = 0; i < samples; ++i) {
+                double s = static_cast<double>(pcm[i]) / 32768.0;
+                sum_sq += s * s;
+            }
+            double rms = std::sqrt(sum_sq / samples);
+            if (rms >= threshold_) last_voice_ms_ = steady_now_ms();
+        }
+        return (steady_now_ms() - last_voice_ms_) < hang_time_ms_;
+    }
+
+   private:
+    float   threshold_;
+    int64_t hang_time_ms_;
+    int64_t last_voice_ms_ = 0;  // epoch start -> gate begins closed until first loud frame
+};
+
 std::unique_ptr<ApmProcessor> ApmProcessor::create() {
 #ifdef VOICECAT_HAS_APM
-    // TODO(M3): return std::make_unique<WebrtcApmProcessor>();
+    // TODO: return std::make_unique<WebrtcApmProcessor>(); — see create_vad()'s doc comment for
+    // why this isn't wired up yet (no working Windows/MSVC build upstream).
 #endif
     return std::make_unique<ApmPassthrough>();
 }
 
+std::unique_ptr<ApmProcessor> ApmProcessor::create_vad(float rms_threshold,
+                                                        int64_t hang_time_ms) {
+    return std::make_unique<EnergyVadProcessor>(rms_threshold, hang_time_ms);
+}
+
 }  // namespace voicecat::audio