feat(core): A.6.1 — use OS connectivity signals to drive reconnect

Extends the A.6 supervisor with an OS-level connectivity hint so a
returning network triggers a redial immediately instead of waiting
out the current backoff slot (up to 60 s). The watchdog remains the
authoritative loss detector — the OS signal is advisory.

* `chanora_core::NetworkState` (Unknown / Online / Offline) is owned
  by `ChanoraSession` via a `tokio::sync::watch::Sender`.
  `set_network_state()` / `network_state()` are the public accessors.
* The supervisor's watch-phase `select!` gains a `network_rx`
  branch: Offline pre-charges watchdog misses (capped at
  `MAX_MISSES - 1`) so the next probe failure trips immediately;
  Online clears stale misses. This shrinks UI-banner latency on a
  Wi-Fi drop from ~15 s to ~5 s.
* The reconnect-loop's backoff sleep races against Online: a
  transition cuts the sleep short and resets the attempt counter so
  future losses start at the smallest backoff window again.
* `chanora_bridge` adds `BridgeNetworkState` (mirror enum) and a
  sync `set_network_state(state)` function. On platforms with no
  signal wired the supervisor stays at Unknown and falls back to
  pure watchdog/backoff — no behavioural regression vs A.6.
* Flutter adds `connectivity_plus ^6.1.0` and wires
  `_wireConnectivity()` in `main()`: seeds with `checkConnectivity()`
  then forwards every `onConnectivityChanged` to the bridge,
  mapping any non-`none` transport to Online.

Verified on Moto G Stylus 5G (Android 14): `svc wifi disable && svc
data disable` for ~40 s — reconnect banner appeared promptly
because the watchdog was pre-charged. After `svc wifi enable && svc
data enable` the supervisor woke from its 15 s backoff slot and
reconnected within seconds; the channel tree re-rendered without
user action.

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

@@ -10,7 +10,7 @@ import 'package:freezed_annotation/freezed_annotation.dart' hide protected;
 part 'api.freezed.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`, `clone`, `clone`, `fmt`, `fmt`, `fmt`, `fmt`, `fmt`, `from`, `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`, `clone`, `clone`, `fmt`, `fmt`, `fmt`, `fmt`, `fmt`, `fmt`, `from`, `from`, `from`
 
 /// Connect to a TeamSpeak-compatible server and return the initial
 /// state snapshot. Honours the DEC-006 single-connection invariant
@@ -37,6 +37,13 @@ Future<void> startAudio() => RustLib.instance.api.crateApiStartAudio();
 Future<void> setPtt({required bool active}) =>
     RustLib.instance.api.crateApiSetPtt(active: active);
 
+/// Notify the core of the latest OS-reported connectivity state.
+/// Called by the Flutter side from `connectivity_plus` callbacks.
+/// The core's supervisor uses this to (a) pre-charge the watchdog
+/// on Offline and (b) short-circuit reconnect backoff on Online.
+void setNetworkState({required BridgeNetworkState state}) =>
+    RustLib.instance.api.crateApiSetNetworkState(state: state);
+
 /// Subscribe to lifecycle events. Each call yields a fresh
 /// subscription; multiple subscribers are supported. On slow
 /// consumers, events are dropped rather than blocking the supervisor
@@ -185,6 +192,19 @@ sealed class BridgeEvent with _$BridgeEvent {
   const factory BridgeEvent.audioStopped() = BridgeEvent_AudioStopped;
 }
 
+/// Coarse OS-reported network state. Mirrors
+/// [`chanora_core::NetworkState`] across the bridge.
+enum BridgeNetworkState {
+  /// No signal seen yet.
+  unknown,
+
+  /// OS reports a usable network.
+  online,
+
+  /// OS reports no network.
+  offline,
+}
+
 /// Server snapshot as seen by Dart.
 class BridgeSnapshot {
   /// Server name.