[Toolkit][Shadcn] Add carousel recipe

[ker0x]

Q	A
Bug fix?	no
New feature?	yes
Deprecations?	no
Documentation?	yes
Issues	Part of #3233
License	MIT

Adds the carousel recipe to the Shadcn kit.

<twig:Carousel orientation="horizontal | vertical" align="start | center | end" loop>
    <twig:Carousel:Content>
        <twig:Carousel:Item>...</twig:Carousel:Item>
        <twig:Carousel:Item>...</twig:Carousel:Item>
        <twig:Carousel:Item>...</twig:Carousel:Item>
    </twig:Carousel:Content>
    <twig:Carousel:Previous />
    <twig:Carousel:Next />
</twig:Carousel>

Embla

Upstream shadcn builds Carousel on Embla Carousel, an npm dependency. No Toolkit recipe ships one — every controller in the kit imports only @hotwired/stimulus — so the behaviour is reimplemented as a self-contained carousel controller: a flex track translated with a CSS transform, whose snap positions are measured from the rendered slides rather than configured, so any basis-* utility on a Carousel:Item works with no extra wiring, and they are re-measured on resize.

Consequences for the public API:

• Embla's opts become props on Carousel: align and loop.
• Embla's Autoplay plugin becomes an autoplay prop (delay in ms). The upstream "Plugins" section is therefore documented as Autoplay.
• Embla's setApi becomes a carousel:select event carrying { index, count }. The recipe ships a small carousel-display controller that renders the current position from it, mirroring what slider does with slider-display.

Notes

• Positions are expressed along the logical axis, so the same math drives horizontal, vertical and RTL carousels; only the sign of the applied transform changes. Directional classes are logical (-ms-4, ps-4, -start-12, -end-12), which also keeps the "Spacing" example overridable in both directions.
• Carousel:Previous renders disabled when the carousel does not loop, so the first slide has no enabled → disabled flash on load; the controller then syncs both buttons on every change.
• Arrow keys follow the carousel axis and the text direction (in RTL, ← advances).
• A click that terminates a drag is swallowed, so dragging over a link or a button inside a slide does not activate it.

Testing

Snapshots regenerated; bin/ux-toolkit-kit-lint --fail-on-warning kits/shadcn, twig-cs-fixer, oxfmt and oxlint are clean.

Behaviour driven in headless Chromium against a real Tailwind v4 build of the rendered examples, with no console errors:

	Result
LTR / vertical / RTL stepping	correct transforms and snap counts per orientation
prev/next disabled sync	disabled at both ends, both enabled in between
arrow keys	follow the axis and the text direction
pointer drag	works on all three axes, with rubber-band and momentum snap
click after drag	swallowed
resize	re-measures and stays on the same slide
loop + autoplay	wraps, pauses on hover/focus, resets after interaction
disconnect/reconnect	no leaked timers or listeners

Companion PR

Needed, since the recipe ships Stimulus controllers and new Tailwind classes: register carousel / carousel-display in assets/toolkit-shadcn.js and rebuild the CSS on symfony/ux.symfony.com. Will link it here.