Class UIComponentAbstract

Base class for every UI element. It provides:

1. A composite tree. A component owns children; three of the four lifecycle phases (afterMount / graphReady / destroy) recurse into them automatically. mount is the exception — it needs a per-child slot, so addChild mounts each child explicitly instead. A parent declares its children via addChild and their lifecycle is driven for it.
2. A disposable registry. Anything registered via track / listen (event unsubscribes, DOM listeners, timers) is torn down on destroy.

Subclasses override the on* hooks rather than the lifecycle methods themselves, so the recursion and teardown always run.