[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/src/app.c

@@ -57,6 +57,37 @@
   } \
   while (0)
 
+extern _Atomic(enum RunState) p_appState;
+
+enum RunState app_getState(void)
+{
+  return atomic_load_explicit(&p_appState, memory_order_acquire);
+}
+
+void app_setState(enum RunState state)
+{
+  if (state == APP_STATE_SHUTDOWN)
+  {
+    atomic_store_explicit(&p_appState, state, memory_order_release);
+    return;
+  }
+
+  enum RunState current = atomic_load_explicit(
+      &p_appState, memory_order_acquire);
+  while (current != APP_STATE_SHUTDOWN &&
+      !atomic_compare_exchange_weak_explicit(&p_appState, &current, state,
+        memory_order_acq_rel, memory_order_acquire));
+}
+
+bool app_transitionState(enum RunState from, enum RunState to)
+{
+  if (from == APP_STATE_SHUTDOWN)
+    return false;
+
+  return atomic_compare_exchange_strong_explicit(
+      &p_appState, &from, to, memory_order_acq_rel, memory_order_acquire);
+}
+
 bool app_isRunning(void)
 {
   const enum RunState state = app_getState();