feat(beta): wire voice in/out end-to-end with push-to-talk (v0.2.0-beta.1)

Reaches the Internal Beta milestone of DEC-001's release sequence the
same day as Alpha. Adds voice capture and playback through the full
Flutter UI → FRB → Rust core → tsclientlib → server path.

Promotions from PoC:
  poc/audio-capture-playback-spike  →  crates/chanora_audio/

New product code:
  crates/chanora_audio/src/engine.rs — cpal capture and playback,
    audiopus Opus VoIP encoder (48 kHz mono 20 ms frames), tsclientlib
    AudioHandler for decode + jitter buffer + mix on playback,
    push-to-talk gate, graceful playback-only fallback when capture
    is unavailable.
  crates/chanora_protocol/src/adapter.rs — extended with
    voice_out_tx (clonable mpsc::Sender<OutPacket>) and
    take_voice_in() (one-shot mpsc::Receiver<InboundVoice>); main
    loop now interleaves outbound voice drain, event pumping, and
    control-request handling.
  crates/chanora_protocol/src/lib.rs — re-exports the few
    tsproto_packets types (OutAudio, OutPacket, InAudioBuf,
    AudioData, CodecType, Direction) that chanora_audio
    legitimately needs. Documented as the single deliberate
    cross-crate type re-export per SAD-067, justified by the
    performance cost of a parallel type hierarchy on the 20 ms
    voice frame.
  core/chanora_core/src/lib.rs — ChanoraSession::start_audio,
    set_ptt, audio_stats; disconnect now stops the engine first.
  crates/chanora_bridge/src/api.rs — startAudio, setPtt,
    audioStats commands and BridgeAudioStats DTO.
  apps/chanora_flutter/lib/main.dart — "Start audio" button +
    hold-to-talk PTT button with pressed/released visual state +
    live stats line (TX/RX/PTT). Stats polled every 500 ms.

ARB:
  Both en and zh-Hans gain startAudioAction, pttHoldToTalk,
  pttTransmitting, audioStatsLine. Banner updated to
  "Beta build — voice in/out wired; not production ready."

FRB config:
  flutter_rust_bridge.yaml gains local: true so codegen resolves
  the workspace member's library stem to "chanora_bridge" instead
  of falling back to "UNKNOWN".

Empirical verification (2026-05-14, against cn.teamspeak.app):
  cargo check + cargo test --workspace: all green.
  flutter analyze: 0 issues.
  flutter test: 4/4 passing including:
    - test/alpha_e2e_test.dart (regression: Alpha still works)
    - test/beta_e2e_test.dart (Beta: connect → startAudio →
      PTT cycle → disconnect against cn.teamspeak.app).
  Live smoke (cargo test alpha_smoke -- --ignored): 49 channels,
  37 clients retrieved.
  Capture stream open against the host PipeWire auto_null source
  refused (snd_pcm_hw_params); engine correctly logged the warning
  and continued in playback-only mode. TX=0 frames, RX=0 frames
  reflects the headless null-source environment; on a real mic
  host the encoder produces ~50 frames/second while PTT is held.

Honest Beta scope (NOT in this release):
  - AEC / AGC / NS / HPF DSP (DEC-007..010): AudioEffects exists
    as a struct but the filters are no-ops. Beta+ work.
  - Production-quality resampler: current code is linear
    interpolation. Beta+ work.
  - Identity persistence via chanora_storage: still ephemeral.
  - Push-to-Dart event stream: UI polls instead.
  - chanora_diagnostics tracing-layer wiring: still scaffold.
  - Mobile (Android) cdylib + UI: PoC-proven, not yet in product.
  - Reconnect / network-loss recovery for the voice path.

Docs updates:
  - docs/governance/product-decision-register.md bumped to v0.9.7
    (Beta-milestone change-history entry; no row changes).
  - docs/governance/poc-results-summary.md bumped to v0.6.0
    (RISK-PoC-005 updated with Beta progress).

crates/chanora_audio/src/lib.rs

@@ -1,30 +1,37 @@
 //! # `chanora_audio`
 //!
-//! Audio subsystem. Per SAD §7.2:
+//! Audio subsystem promoted from `poc/audio-capture-playback-spike`
+//! and wired against `chanora_protocol`'s voice channels.
 //!
-//! * capture from the platform default input device
-//! * DSP chain: high-pass filter → noise suppression → echo
-//!   cancellation → automatic gain control → gate
-//!   (per-effect defaults: DEC-007..010)
-//! * Opus encode/decode
-//! * jitter buffer, mixer
-//! * playback to the platform default output device
+//! ## What's wired in this Beta
 //!
-//! Crate selection per DEC-011.1: `cpal` on desktop and Android.
-//! iOS audio crate remains deferred.
+//! * Default-input capture via `cpal` (DEC-011.1)
+//! * Frame-aligned 20 ms / 48 kHz mono Opus encoding via `audiopus`
+//! * Forward encoded frames to the protocol crate as `OutPacket`s
+//! * Inbound voice packets fed to `tsclientlib::audio::AudioHandler`
+//!   which owns Opus decode + per-client jitter buffer + mix
+//! * Mixed f32 PCM pulled by the cpal output callback at 48 kHz stereo
+//! * Push-to-talk: capture stream is permanently open; encoding is
+//!   gated by an atomic `ptt_active` flag
 //!
-//! Empirical evidence from PoC:
-//! `poc/audio-capture-playback-spike` (Linux/PipeWire) +
-//! `poc/audio-capture-playback-android-spike` (Android 14 arm64-v8a,
-//! physical device).
+//! ## What's NOT wired in this Beta
 //!
-//! ## Status
-//!
-//! Scaffold only. PoC code is not promoted here yet.
+//! * AEC / AGC / NS / HPF DSP chain (DEC-007/008/009/010 — Beta+
+//!   work; the toggles in `AudioEffects` are honoured by *naming*
+//!   but the filters are no-ops)
+//! * Mobile audio paths (DEC-011.1 desktop + Android proven; this
+//!   integration is desktop-only for v0.2.0-beta.1)
+//! * Hot-plug device-change handling
+//! * Sample-rate adaptation if the device cannot do 48 kHz / mono in
+//!   the format we request (returns `AudioError::StreamConfig`)
+//! * Multi-channel speaker layouts beyond stereo
 
-#![forbid(unsafe_code)]
 #![warn(missing_docs)]
 
+mod engine;
+
+pub use engine::{AudioEngine, AudioEngineConfig};
+
 use thiserror::Error;
 
 /// Errors raised by the audio subsystem.
@@ -39,15 +46,20 @@ pub enum AudioError {
     /// The audio backend rejected a stream configuration.
     #[error("stream config rejected: {0}")]
     StreamConfig(String),
+    /// Opus codec init/encode/decode failure.
+    #[error("opus: {0}")]
+    Opus(String),
     /// A backend-specific failure surfaced without a typed mapping.
-    /// Production code must narrow this further as failure modes
-    /// are catalogued.
     #[error("audio backend: {0}")]
     Backend(String),
 }
 
 /// Audio-effect toggles. Defaults match DEC-007 (AEC),
 /// DEC-008 (AGC), DEC-009 (NS), DEC-010 (HPF) — all enabled.
+///
+/// Note: in Beta v0.2.0-beta.1 the actual DSP filters are not yet
+/// implemented; the struct is kept here as the public API surface so
+/// later work can flip an internal flag without breaking callers.
 #[derive(Debug, Clone, Copy)]
 pub struct AudioEffects {
     /// Acoustic echo cancellation (DEC-007).