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>

docs/README.md

@@ -19,7 +19,7 @@ that implementation can start from a shared, agreed plan.
 | Media transport | **UDP** secured by **TLS-exported keys + ChaCha20-Poly1305 AEAD** — mandatory, no plaintext mode (see [security.md](security.md)) |
 | Crypto libraries | **mbedTLS** (TLS 1.3) + **libsodium** (AEAD, Argon2id, Ed25519) — both permissive, **no GPL/LGPL anywhere** |
 | Voice codec | **Opus** (libopus 1.6), per-channel configurable mono/stereo, bitrate, frame size, FEC/DTX |
-| Audio DSP | **webrtc-audio-processing (APM)** — AEC/NS/AGC/VAD. NR is **two-sided**: sender can denoise, and each listener can denoise a *specific* other user locally. Input gate supports **VAD and PTT**, client-configurable. |
+| Audio DSP | **webrtc-audio-processing (APM)** was the plan for AEC/NS/AGC/VAD, but has no working Windows/MSVC build upstream — v1 ships a lightweight energy/RMS VAD instead, no AEC/NS/AGC yet (see [tech-stack.md](tech-stack.md), [roadmap.md](roadmap.md) §2). NR is **two-sided**: sender can denoise, and each listener can denoise a *specific* other user locally — this plumbing exists but is currently inert pending a real DSP backend. Input gate supports **VAD and PTT**, client-configurable. |
 | Identity | **Guests + admin-provisioned local accounts** (Argon2id, SQLite). No self-serve registration; guests toggleable per server. |
 | Text | **Ephemeral** — live relay, no server-side history in v1. |
 | Serialization | **Protocol Buffers** for the control plane; **custom binary** for voice frames |