[Toolkit][Shadcn] Add marker recipe

[ker0x]

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

Adds the marker recipe to the Shadcn kit, porting Shadcn UI's Marker.

Marker displays an inline status, system note, bordered row, or labeled separator in a conversation — it composes well with the bubble recipe in a chat thread.

Components

Marker
├── Marker:Icon
└── Marker:Content

<twig:Marker variant="default">
    <twig:Marker:Icon>
        <twig:ux:icon name="lucide:check" />
    </twig:Marker:Icon>
    <twig:Marker:Content>Explored 4 files</twig:Marker:Content>
</twig:Marker>

Three variants are available via the variant prop:

Variant	Description
default	An inline marker for status, notes, and actions.
border	A default marker with a bottom border under the row.
separator	A centered label with divider lines on each side.

The root is polymorphic through the as prop (upstream's asChild), so a marker can render as a real a or button when it is interactive. Marker:Icon is decorative and carries aria-hidden="true", so Marker:Content holds the accessible name. RTL is supported through ltr:/rtl: class variants.

All upstream examples are ported as live-preview blocks in the recipe README.md (Variants, Status, Shimmer, Separator, Border, With Icon, Links and Buttons), plus the usual RTL section.

Notes for reviewers

• Shimmer. Upstream's shimmer example relies on the shimmer utility class, which ships in the shadcn npm package's tailwind.css (background-clip: text plus a tw-shimmer keyframe). The Toolkit ships no CSS, and Tailwind cannot declare keyframes in an arbitrary value, so the example uses the built-in animate-pulse to convey the same in-progress cue. The README states this explicitly. Happy to drop the section instead if preferred.
• Upstream source paths. apps/v4/styles/radix-nova/ui/ and its ui-rtl/ counterpart no longer exist upstream — that directory is now gitignored and generated by the registry build. Canonical Tailwind classes were taken from apps/v4/registry/new-york-v4/ui/marker.tsx (the registry/bases/radix variant only carries cn-marker CSS hooks). With no ui-rtl file left to diff, the RTL variants for the three directional classes (text-left, before:mr-1, after:ml-1) were derived by hand following the kit's existing ltr:<physical> rtl:<logical> idiom.
• New icon fixtures. lucide:file-text, lucide:book-open-check and lucide:rotate-ccw were added to tests/Fixtures/icons/lucide/ for the examples.
• Snapshots regenerated and committed; bin/ux-toolkit-kit-lint --fail-on-warning kits/shadcn and twig-cs-fixer are clean.

A companion PR on symfony/ux.symfony.com will follow to rebuild the Tailwind CSS — the recipe introduces classes new to the kit (group/marker, min-h-4, before:flex-1, before:bg-border, rtl:before:me-1, rtl:after:ms-1). No Stimulus controller is shipped.