Fix aria-labelledby referenced accessible names

[AKzar1el]

What this covers

• regression coverage for aria-labelledby references whose target has a non-empty aria-label but no text content
• package/root and hosted parity across images, semantic image roles, SVG image roles, image submit buttons, iframes, and image-map areas

Evidence

• current implementation derives referenced names only from target text content, so the new root regression is red before the fix
• W3C Accessible Name and Description Computation 1.2 computes each aria-labelledby reference through the text-alternative algorithm, where a non-empty aria-label precedes name-from-content

This PR is intentionally test-only first to preserve red-before-fix evidence; implementation will follow on this same branch after protected CI records the regression.