Home hero becomes a 450px flex row with a pinned 240x450 category rail
(ShellCategoryMenu `pinned` mode) left of the 960px center-cropped
carousel; hovering a top-level category expands the mega-menu over the
carousel. The header no longer collapses on scroll, and its category
dropdown is now hover-only on non-home pages.
OpenSpec change: openspec/changes/pin-home-category-menu
BREAKING CHANGE: the header logo/search/cart bar no longer auto-hides
past 200px of scroll, and the home page no longer shows the header
category dropdown (it is pinned in the hero row instead).
james
ee2f421709