[client] transport: stop after guest sessions end

Track unexpected loss of an admitted fallback independently from normal
stop and UUID revocation. Terminate once both primary and fallback
sessions have disappeared, regardless of notification order.

Make shutdown terminal so concurrent disconnect handling cannot replace
it with restart or running state.

client/tests/keybind_test.c

@@ -81,6 +81,16 @@ struct CursorState g_cursor;
 struct AppParams   g_params;
 _Atomic(enum RunState) p_appState;
 
+enum RunState app_getState(void)
+{
+  return atomic_load(&p_appState);
+}
+
+void app_setState(enum RunState state)
+{
+  atomic_store(&p_appState, state);
+}
+
 static const int fKeys[] =
 {
   KEY_F1 , KEY_F2 , KEY_F3 , KEY_F4,