fix(TabsItem): let renderless items supply a focus target #912

Surfaced by @paper/bulma in #760.

TabsItem in renderless mode (as={null}) registers Atom's null element, so focusSelectedTab / focusAdjacent are no-ops. ArrowLeft/ArrowRight still update selection, panels, and aria-selected, but the visible focus ring and screen-reader announcement stay on the previously focused node.

BuTab has to render li > a with is-active on the li (Bulma's fixture). Emerald uses a real <button> tab and never hits this.

This is an APG tabs deviation the axe sweep cannot catch.

Expected

Let TabsItem accept an element input at registration so a renderless consumer can point focus-follow at the real control (the <a> inside the <li>).