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/main.dart

@@ -60,9 +60,57 @@ class _BetaHomeState extends State<_BetaHome> {
   bool _audioStarted = false;
   rust.BridgeAudioStats? _audioStats;
   Timer? _statsTimer;
+  StreamSubscription<rust.BridgeEvent>? _eventsSub;
+
+  // A.6 reconnect banner state.
+  String? _lostReason;
+  int? _reconnectAttempt;
+  int? _reconnectDelay;
+
+  @override
+  void initState() {
+    super.initState();
+    _eventsSub = rust.eventsStream().listen(_onEvent);
+  }
+
+  void _onEvent(rust.BridgeEvent evt) {
+    if (!mounted) return;
+    switch (evt) {
+      case rust.BridgeEvent_Connected():
+        setState(() {
+          _phase = _Phase.connected;
+          _lostReason = null;
+          _reconnectAttempt = null;
+          _reconnectDelay = null;
+        });
+      case rust.BridgeEvent_Lost(:final reason):
+        setState(() {
+          _lostReason = reason;
+          _reconnectAttempt = null;
+          _reconnectDelay = null;
+        });
+      case rust.BridgeEvent_Reconnecting(:final attempt, :final delaySecs):
+        setState(() {
+          _reconnectAttempt = attempt;
+          _reconnectDelay = delaySecs;
+        });
+      case rust.BridgeEvent_Disconnected():
+        setState(() {
+          _phase = _Phase.idle;
+          _lostReason = null;
+          _reconnectAttempt = null;
+          _reconnectDelay = null;
+        });
+      case rust.BridgeEvent_AudioStarted():
+        setState(() => _audioStarted = true);
+      case rust.BridgeEvent_AudioStopped():
+        setState(() => _audioStarted = false);
+    }
+  }
 
   @override
   void dispose() {
+    _eventsSub?.cancel();
     _statsTimer?.cancel();
     _hostCtl.dispose();
     _nickCtl.dispose();
@@ -201,6 +249,42 @@ class _BetaHomeState extends State<_BetaHome> {
             ),
             const SizedBox(height: 12),
             Text(statusText(), style: theme.textTheme.titleMedium),
+            if (_lostReason != null || _reconnectAttempt != null) ...[
+              const SizedBox(height: 8),
+              Container(
+                padding: const EdgeInsets.all(10),
+                decoration: BoxDecoration(
+                  color: theme.colorScheme.errorContainer,
+                  borderRadius: BorderRadius.circular(8),
+                ),
+                child: Row(
+                  children: [
+                    SizedBox(
+                      width: 16,
+                      height: 16,
+                      child: CircularProgressIndicator(
+                        strokeWidth: 2,
+                        color: theme.colorScheme.onErrorContainer,
+                      ),
+                    ),
+                    const SizedBox(width: 10),
+                    Expanded(
+                      child: Text(
+                        _reconnectAttempt != null
+                            ? l10n.statusReconnecting(
+                                _reconnectAttempt!,
+                                _reconnectDelay ?? 0,
+                              )
+                            : l10n.statusConnectionLost(_lostReason ?? ''),
+                        style: TextStyle(
+                          color: theme.colorScheme.onErrorContainer,
+                        ),
+                      ),
+                    ),
+                  ],
+                ),
+              ),
+            ],
             const SizedBox(height: 12),
             if (_phase == _Phase.idle) ...[
               _ConnectForm(