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/app_zh.arb

@@ -20,6 +20,9 @@
   "statusConnected": "已连接到 {server}",
   "statusError": "错误：{message}",
 
+  "statusReconnecting": "正在重新连接……第 {attempt} 次尝试，{delay} 秒后",
+  "statusConnectionLost": "连接已断开：{reason}",
+
   "audioStatsLine": "发送 {sent} 帧 • 接收 {received} 帧 • PTT {ptt}",
 
   "channelsHeading": "频道",