title-only check approximates accessible description; build a proper accessibleDescription method #5212

Follow-up from review of #5170 (comment).

The title-only check (lib/checks/label/title-only-evaluate.js) decides whether an element's only labeling comes from title/aria-describedby:

function titleOnlyEvaluate(node, options, virtualNode) {
  const labelText = labelVirtual(virtualNode);
  const title = titleText(virtualNode);
  const ariaDescribedBy = virtualNode.attr('aria-describedby');

  return !labelText && !!(title || ariaDescribedBy);
}

This is buggy (pre-existing, not introduced by #5170): it treats the mere presence of an aria-describedby attribute as an accessible description, without resolving the referenced elements or checking whether they actually contribute any text. An aria-describedby pointing at a missing/empty element yields no description, but this check still counts it.

Rather than patch the heuristic, we should build a proper accessibleDescription method — analogous to the existing accessible-name computation (accessibleText) — per the accname description algorithm, and use it here.

Tasks

• Add an accessibleDescription helper (alongside accessible-name computation in commons/text) implementing the accessible description algorithm.
• Update title-only (and other consumers of the title/aria-describedby presence heuristic) to use it.
• Cover with unit tests, including empty/missing aria-describedby references.