Bento cell adopts card's zone contract; bento becomes a grid primitive

Acceptance Criteria

• Bento is a grid primitive, not a page-section. Remove the sections preamble (eyebrow/title/blurb) + pageSectionAutoLabel from the Bento config; bento keeps width/spacing/inset block dimensions but is no longer a contentSection. With no preamble, every heading is a cell (no top heading consumed).
• A bento-cell's content splits on a top-level --- into media / body / footer zones using data-section, matching card.
• A visual rune (chart/map/gallery/diagram/embed/sandbox) in the media zone is sized by the WORK-339 media-zone selector, with no bento-specific per-guest CSS.
• The media zone is a clipping container (overflow: hidden) so a showcase guest can bleed to a partial view; the existing showcase--in-bento-cell modifier + showcase.css clipping work end-to-end.
• The cell background is tint-deferrable (the cell does not hard-set a background that would defeat tint), so a per-cell {% tint %} / tint-mode paints background + text.
• Cell titles render as a uniform, auto-detected heading level with uniform visual size (the headline zone typography) — not a bare <span>, not the input heading level — contributing to the document outline.
• The cell carries data-media-position (default top) on the same contract as card, ready for WORK-348 to drive placement.
• The existing icon-as-visual path keeps working; the docstring's image/emoji-as-visual claim is reconciled with the code under the new zone model.
• Tests cover: bento with no preamble, a cell with a --- media split, a guest sized in the media zone, a showcase bleed clipped to a cell, a tinted cell, and a uniform-level title.