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.

crates/chanora_bridge/src/api.rs

@@ -233,6 +233,39 @@ pub struct BridgeAudioStats {
     pub ptt_active: bool,
 }
 
+// ---------- Connectivity (A.6.1) ----------
+
+/// Coarse OS-reported network state. Mirrors
+/// [`chanora_core::NetworkState`] across the bridge.
+#[derive(Debug, Clone, Copy)]
+pub enum BridgeNetworkState {
+    /// No signal seen yet.
+    Unknown,
+    /// OS reports a usable network.
+    Online,
+    /// OS reports no network.
+    Offline,
+}
+
+impl From<BridgeNetworkState> for chanora_core::NetworkState {
+    fn from(s: BridgeNetworkState) -> Self {
+        match s {
+            BridgeNetworkState::Unknown => chanora_core::NetworkState::Unknown,
+            BridgeNetworkState::Online => chanora_core::NetworkState::Online,
+            BridgeNetworkState::Offline => chanora_core::NetworkState::Offline,
+        }
+    }
+}
+
+/// 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.
+#[frb(sync)]
+pub fn set_network_state(state: BridgeNetworkState) {
+    session().set_network_state(state.into());
+}
+
 // ---------- Events (A.6) ----------
 
 /// Lifecycle event surfaced to Dart. Schema-controlled mirror of