feat(events): A.4 — emit SnapshotChanged from the watchdog probe

Adds a new variant to the lifecycle event catalogue so the UI can
auto-refresh the channel/client tree without an independent polling
timer on the Dart side. The supervisor's existing 5 s snapshot probe
is the source of truth: it already pulls a full snapshot to keep
the watchdog honest, so we piggyback on it.

* `chanora_core::SessionEvent::SnapshotChanged { channels, clients }`
  carries the latest channel and client counts.
* The supervisor compares the probe result to `last_counts` and
  fires the event only when the count actually changes. `last_counts`
  is reset to `None` on a successful reconnect so the freshly
  dialled session re-emits its initial counts.
* `chanora_bridge::api::BridgeEvent::SnapshotChanged` is the
  cross-bridge mirror.
* Flutter routes the event through `_onEvent`, which calls
  `_onRefresh()` to repopulate the snapshot view.

The probe-driven detection has known limits — pure within-channel
client moves do not change the count and so are not surfaced. That
gap will close when the supervisor tracks a content hash in
addition to the count; the count-only signal is sufficient for the
common "someone joined / someone left" case observed on cn.teamspeak.app.

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

@@ -211,6 +211,17 @@ sealed class BridgeEvent with _$BridgeEvent {
 
   /// Audio engine stopped.
   const factory BridgeEvent.audioStopped() = BridgeEvent_AudioStopped;
+
+  /// Snapshot probe observed a change in channel/client counts.
+  /// UI uses this to drive an auto-refresh without active
+  /// polling.
+  const factory BridgeEvent.snapshotChanged({
+    /// Latest channel count.
+    required int channels,
+
+    /// Latest client count.
+    required int clients,
+  }) = BridgeEvent_SnapshotChanged;
 }
 
 /// Coarse OS-reported network state. Mirrors