feat(core): A.6 — supervisor reconnect with watchdog and event stream

Adds an end-to-end auto-reconnect path so a brief network outage no
longer leaves the client wedged in a half-dead state. The flow has
three layers, each motivated by a real failure mode observed on the
Moto G live test:

* `chanora_protocol::DisconnectReason` (`UserRequested` /
  `StreamEnded` / `Error(String)`) is reported on a `oneshot` when
  the per-connection task exits, so the supervisor can tell user
  intent apart from a real loss.
* `chanora_core` spawns a supervisor task per `ChanoraSession`. It
  listens for the loss notifier AND runs a watchdog that issues
  `snapshot()` probes every 5s with a 4s timeout — three consecutive
  misses synthesise a `DisconnectReason::Error(...)` and trigger the
  reconnect path. The watchdog catches the "ghost connected" case
  where tsclientlib silently resets internal state but the event
  stream never errors. Backoff schedule: 1s, 2s, 5s, 15s, 30s, 60s
  (capped). On success the supervisor swaps the dead `ProtocolClient`
  for the new one in place and, if audio was running, restarts the
  audio engine bound to the new `voice_in`/`voice_out` channels.
* `SessionEvent` (Connected / Lost / Reconnecting / Disconnected /
  AudioStarted / AudioStopped) is broadcast on a 64-slot channel.
  `chanora_bridge` re-exports it as `BridgeEvent` and exposes
  `events_stream(StreamSink)`; the Flutter side subscribes from
  `initState` and renders a reconnect banner with attempt count and
  delay. New `SnapshotProbe` exposes a clone-friendly snapshot path
  so the watchdog can probe without holding `&self` across awaits.

Localization adds `statusReconnecting` and `statusConnectionLost`
keys to `app_en.arb` and `app_zh.arb`.

Verified on Moto G Stylus 5G (Android 14) against cn.teamspeak.app:
killed Wi-Fi + cellular for ~70 s; watchdog declared loss at three
misses, supervisor walked the backoff schedule, and the UI
reconnected automatically once the radios came back. Snapshot tree
re-rendered without user action.

apps/chanora_flutter/lib/l10n/generated/app_localizations.dart

@@ -181,6 +181,18 @@ abstract class AppL10n {
   /// **'Error: {message}'**
   String statusError(String message);
 
+  /// No description provided for @statusReconnecting.
+  ///
+  /// In en, this message translates to:
+  /// **'Reconnecting… attempt {attempt}, in {delay}s'**
+  String statusReconnecting(int attempt, int delay);
+
+  /// No description provided for @statusConnectionLost.
+  ///
+  /// In en, this message translates to:
+  /// **'Connection lost: {reason}'**
+  String statusConnectionLost(String reason);
+
   /// No description provided for @audioStatsLine.
   ///
   /// In en, this message translates to:

apps/chanora_flutter/lib/l10n/generated/app_localizations_en.dart

@@ -55,6 +55,16 @@ class AppL10nEn extends AppL10n {
     return 'Error: $message';
   }
 
+  @override
+  String statusReconnecting(int attempt, int delay) {
+    return 'Reconnecting… attempt $attempt, in ${delay}s';
+  }
+
+  @override
+  String statusConnectionLost(String reason) {
+    return 'Connection lost: $reason';
+  }
+
   @override
   String audioStatsLine(int sent, int received, String ptt) {
     return 'TX $sent frames • RX $received frames • PTT $ptt';

apps/chanora_flutter/lib/l10n/generated/app_localizations_zh.dart

@@ -54,6 +54,16 @@ class AppL10nZh extends AppL10n {
     return '错误：$message';
   }
 
+  @override
+  String statusReconnecting(int attempt, int delay) {
+    return '正在重新连接……第 $attempt 次尝试，$delay 秒后';
+  }
+
+  @override
+  String statusConnectionLost(String reason) {
+    return '连接已断开：$reason';
+  }
+
   @override
   String audioStatsLine(int sent, int received, String ptt) {
     return '发送 $sent 帧 • 接收 $received 帧 • PTT $ptt';