The last of the deep-review deferred reuse items:
- extract the scroll-affordance machinery (watch the scroller AND its swappable/
lazily-growing children, diff two edge booleans) that useScrollFade (vertical) and
Carousel's useHScroll (horizontal) each hand-duplicated into one shared
lib/useScrollEdges(axis); both hooks now just map start/end to their own shape.
- migrate PlexInfo's two remaining hand-rolled `flex overflow-x-auto` strips — the
cast row and the collection strips — to the S1 <Carousel> primitive, so they get the
edge fade + hover/focus scroll buttons (only on the clipped side) for free, matching
the Discover rails / CastStrip / RelatedStrip.
Pure refactor, no behaviour change. Gate green (tsc, vitest 351, eslint, prettier,
knip). Browser-verified on a movie info page: cast row + collection strip render as
Carousels with the edge fade and the right-scroll button on the clipped cast row.
