Lifecycle

Your app implements the guest interface from koba:app:

• start — build the initial UI tree
• on_event — handle inbound events (press, widget activated, …)
• pause / resume — host backgrounded or foregrounded your app
• stop — tear down before the host unloads you

The retained tree is process-local. If the host starts a new process for your app (for example after a crash), rebuild the tree in start.