link wraps blocks, and card and bento-cell href render through it

Acceptance Criteria

• {% link %} accepts block content and renders a real <a> around it when it holds no links, buttons or form controls
• The choice between wrap and stretch is made in the identity transform, so links added later (entity auto-links, resolved {% ref %}) are seen; a test asserts that a link around content that gains an auto-link in postProcess renders the stretched form, with no nested <a>
• When the content is interactive, link renders a stretched <a data-name="link"> inside the wrapped rune's root, focusable and labelled by aria-label when given, else by the content's title slot
• link emits no data-rune, and a test asserts that a schema property inside a link still publishes
• An empty href renders the content unchanged
• card's and bento-cell's href render through link and are not deprecated
• The changeset says that linked cards and bento cells are now reachable by keyboard and screen readers, one tab stop per card
• Contracts are regenerated on both copies and the diff reviewed