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/l10n/app_en.arb

@@ -1,60 +1,43 @@
 {
   "@@locale": "en",
-  "@@x-source-of-truth": "DEC-015 (register v0.9.5). Template ARB for English. Other locales must reference these keys. Server-provided content is NOT translated (ADR-008 / DEC-015).",
+  "@@x-source-of-truth": "DEC-015 (register v0.9.5). Template ARB for English. Server-provided content is NOT translated (ADR-008 / DEC-015).",
 
   "appTitle": "Chanora",
-  "@appTitle": {
-    "description": "Application title shown in launchers and the app bar. The product name `Chanora` is fixed by DEC-018 and must not be translated."
-  },
 
-  "homeNotProductionReadyBanner": "Alpha build — not production ready.",
-  "@homeNotProductionReadyBanner": {
-    "description": "Plain-language banner informing testers that this build is not for end-user use. Mirrors README.md's status line."
-  },
+  "homeNotProductionReadyBanner": "Beta build — voice in/out wired; not production ready.",
 
   "fieldServerHost": "Server address",
-  "@fieldServerHost": {
-    "description": "Label for the server-address input on the connect form."
-  },
   "fieldNickname": "Nickname",
-  "@fieldNickname": {
-    "description": "Label for the nickname input on the connect form."
-  },
 
   "connectAction": "Connect",
-  "@connectAction": {
-    "description": "Label for the button that initiates a connection."
-  },
   "disconnectAction": "Disconnect",
-  "@disconnectAction": {
-    "description": "Label for the button that ends the active connection."
-  },
   "refreshAction": "Refresh",
-  "@refreshAction": {
-    "description": "Label for the button that re-fetches the server snapshot."
-  },
+  "startAudioAction": "Start audio",
+  "pttHoldToTalk": "Hold to talk",
+  "pttTransmitting": "Transmitting…",
 
   "statusIdle": "Not connected",
-  "@statusIdle": {},
   "statusConnecting": "Connecting…",
-  "@statusConnecting": {},
   "statusConnected": "Connected to {server}",
   "@statusConnected": {
-    "placeholders": {
-      "server": { "type": "String" }
-    }
+    "placeholders": { "server": { "type": "String" } }
   },
   "statusError": "Error: {message}",
   "@statusError": {
+    "placeholders": { "message": { "type": "String" } }
+  },
+
+  "audioStatsLine": "TX {sent} frames • RX {received} frames • PTT {ptt}",
+  "@audioStatsLine": {
     "placeholders": {
-      "message": { "type": "String" }
+      "sent": { "type": "int" },
+      "received": { "type": "int" },
+      "ptt": { "type": "String" }
     }
   },
 
   "channelsHeading": "Channels",
-  "@channelsHeading": {},
   "clientsHeading": "Online clients",
-  "@clientsHeading": {},
   "countChannelsAndClients": "{channels} channels • {clients} online",
   "@countChannelsAndClients": {
     "placeholders": {