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).

apps/chanora_flutter/lib/src/rust/api.dart

@@ -8,7 +8,7 @@ import 'lib.dart';
 import 'package:flutter_rust_bridge/flutter_rust_bridge_for_generated.dart';
 
 // These functions are ignored because they are not marked as `pub`: `runtime`, `session`
-// These function are ignored because they are on traits that is not defined in current crate (put an empty `#[frb]` on it to unignore): `clone`, `clone`, `clone`, `fmt`, `fmt`, `fmt`, `from`
+// These function are ignored because they are on traits that is not defined in current crate (put an empty `#[frb]` on it to unignore): `clone`, `clone`, `clone`, `clone`, `fmt`, `fmt`, `fmt`, `fmt`, `from`
 
 /// Connect to a TeamSpeak-compatible server and return the initial
 /// state snapshot. Honours the DEC-006 single-connection invariant
@@ -27,6 +27,49 @@ Future<void> disconnect() => RustLib.instance.api.crateApiDisconnect();
 /// True if a connection is currently active.
 Future<bool> isConnected() => RustLib.instance.api.crateApiIsConnected();
 
+/// Start the audio engine on the active connection. Requires a
+/// connection; idempotent (will replace any previous engine).
+Future<void> startAudio() => RustLib.instance.api.crateApiStartAudio();
+
+/// Set the push-to-talk state.
+Future<void> setPtt({required bool active}) =>
+    RustLib.instance.api.crateApiSetPtt(active: active);
+
+/// Read audio statistics. Errors if no connection or audio not started.
+Future<BridgeAudioStats> audioStats() =>
+    RustLib.instance.api.crateApiAudioStats();
+
+/// Statistics from the audio engine.
+class BridgeAudioStats {
+  /// Number of Opus frames sent since audio started.
+  final int framesSent;
+
+  /// Number of inbound voice packets decoded.
+  final int framesReceived;
+
+  /// Current push-to-talk state.
+  final bool pttActive;
+
+  const BridgeAudioStats({
+    required this.framesSent,
+    required this.framesReceived,
+    required this.pttActive,
+  });
+
+  @override
+  int get hashCode =>
+      framesSent.hashCode ^ framesReceived.hashCode ^ pttActive.hashCode;
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is BridgeAudioStats &&
+          runtimeType == other.runtimeType &&
+          framesSent == other.framesSent &&
+          framesReceived == other.framesReceived &&
+          pttActive == other.pttActive;
+}
+
 /// Channel as seen by Dart. Matches `chanora_protocol::ChannelInfo`
 /// but with primitive `u64` ids so the Dart side gets `BigInt`s
 /// without any wrapper-type ceremony.