feat(cli): keep every icon a site declares and headline the bare mark (#3727)

A brand band shows logo artwork on a transparency checker, where a badge (a
mark knocked out of a full-bleed disc or square) reads as a solid blob. The
capture kept only the first icon that downloaded, so which of a site's icons
survived was decided by ranking alone, and nothing recorded what the survivor
actually looked like.

Keep them all. Each declared icon is written as assets/icon-<rel>-<sizes>.<ext>
and classified as bare-mark, badge, or unknown, then the best one is copied onto
the historical assets/favicon.<ext> stem so a stem match still finds it.

Classification is deterministic and cheap. SVG is read from markup, not
rendered: these files routinely fill via CSS custom properties behind a
prefers-color-scheme query, and librsvg resolves neither, rasterising one real
favicon to a fully transparent image while Chrome draws it correctly. Raster
icons are sampled at the midpoint of each edge rather than the corners, because
a full-bleed disc has four transparent corners and would otherwise read as a
bare mark.

The headline prefers a bare mark, then falls back to the existing ranking.
Preference is binary on purpose: an "unknown" is not promoted above a badge,
or an undecodable .ico would outrank a good SVG purely for being unexaminable.

extracted/icons-manifest.json records every icon, its shape and the evidence
for that shape, plus which file became the headline, its rank, and why. A
choice whose losers are invisible is indistinguishable from having had no
choice, which is how a substituted icon went unnoticed in the first place.

Worth noting for consumers: sites often declare no bare mark at all. An
apple-touch-icon is composited onto an opaque tile per Apple's spec, so it is
a badge, not a mark. The manifest says so rather than implying the preference
was satisfied.