fix(website): stop the partner carousel scrolling the page, and turn the hero showcase into a real flipping carousel

Three faults, all found by watching the built site rather than reading it.

The partner carousel dragged the whole page down to itself. Its timer called
`el.scrollIntoView()` every few seconds, and scrollIntoView walks up and scrolls
*every* scrollable ancestor including the document — so a reader anywhere on the
page was hauled to the partners section on a loop. It now sets `scrollLeft` on
its own track, which cannot move anything but itself. Verified: twelve seconds
on the homepage, `window.scrollY` never leaves 0, while the track's own
scrollLeft still advances.

The hero showcase was manual when it should play itself. The reference plays
each design for a beat, turns it on its Y axis to show the back, holds again,
then hands over to the next — an Embla autoplay at `delay: 2500` with a
`duration-700` `rotateY(180deg)` flip and `backfaceVisibility`, read out of
their own bundle rather than guessed. Ours sat still until someone clicked a
swatch. It now runs that cycle: a genuine 3D flip of one object, not a crossfade
between two pictures, with both faces backface-hidden so they never show through
one another mid-turn. Hovering pauses it; reduced-motion skips it entirely.

The colour swatches were large filled circles under the garment, which read as
loud page furniture rather than a control. They are gone by default. A site that
wants them gets `showcase_controls`, and they render as the same discreet
progress dashes the other carousels use.

New fields: `showcase_autoplay` (default on), `showcase_interval` (default
2500ms, matching the reference), `showcase_controls` (default off).

Verified on a local replica carrying the full 11-page site: sampling the hero
across a cycle shows active=1 front → active=1 flipped → active=2 front, so the
flip and the hand-over both really happen; 22 page/locale combinations answer
200 with no logged block failures; 137 block/variant combinations render; suite
passes.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>