A real accessibility tree

The engine now computes a genuine accessibility tree: the ARIA role and accessible name of every element, the exact data a screen reader or an accessibility audit consumes. Roles come from the standard HTML-to-ARIA mappings (a button is a button, a link with an href is a link, a heading is a heading, an image carries its alt text as its name), an explicit role= overrides them, aria-hidden subtrees are pruned, and disabled / checked / expanded states are reflected. It is exposed through the same DevTools Protocol (Accessibility.getFullAXTree) that the reference browser uses. The accessibility sub-score moves for the real tree.