Heroes are drawn at the prose measure

A page hero is capped at the width the prose is read in.

The hero of a page is written by be-best/layouts/_partials/images/featured-image.html into main, above the .prose block, so it filled main: on a maximised window a post showed a picture 1860 pixels wide over paragraphs 65ch wide, and the full-width sizes hint of image-variants.html told the browser to take the widest variant it offers. public/post/cryengine/on-building-a-world/index.html asks for the 1920w webp of its hero, 99 KB, for a box about 600 pixels wide.

Cards were already sized to their grid by card.html, which passes its own sizes and widths; only the page hero had no hint.

Heroes are drawn at the prose measure

• The measure is one token, --prose-measure: 65ch in tokens.css, read by .prose (in prose-overrides.css, the value the typography plugin already gave it) and by .featured-image in images.css.
• A page hero carries featured-image on the box image-variants.html writes, so it is capped at that measure and its w-full h-auto image scales inside it. The five page layouts (post, about, project, profile, recipe) ask for the hero with hero=true; a card does not.
• A hero is offered the variants its box selects: the hint is (min-width: 1024px) 65ch, calc(100vw - 60px) and the ladder is 320, 640, 960, 1280, which fills the measure at 2x density. 1600 and 1920 are off the hero path.
• A card’s media is untouched: it takes no featured-image class, keeps the three-breakpoint grid hint and the ladder to 960, and cards.css still sizes it through .card-media.

Acceptance Criteria

• Every <img> in a featured-image box is served from a srcset whose largest entry is at most 1280w: 20 hero pages, 0 pages offer 1600w or 1920w for a hero.
• Every hero page carries the one hero sizes hint: 20 pages, 1 distinct value.
• Card media carries no featured-image class and keeps the grid hint: 12 cards on public/post/index.html, all with the grid hint, 0 with the hero class.
• The prose and the hero read one measure: .prose and .featured-image declare max-width: var(--prose-measure) and nothing else caps a hero.
• pnpm run test passes.

Notes

• The hero payload on the measured page falls from the 99 KB 1920w variant to the 48 KB 1280w one, which is the widest a 2x window selects; at 1x the 960w variant, 34 KB, fills the measure.
• Images inside the prose are already in that column, and the img shortcode and Markdown render hook keep the 320-1920 ladder, so an in-body image declares a full-width hint while it is drawn at the measure. Widening that hint is a separate card.