Put the carousel's tab stop on its items, as M3 asks

Plan step 19, containment.md C-18. The row was the focusable `region`
and the items were not focusable at all, which is the thing M3's
accessibility page draws a Don't for: "use Tab to place initial focus on
the first carousel item", "avoid focusing on the carousel container".
Each item is now `tabindex="0"` with the focus ring drawn inside it, the
row is out of the tab order, and from a focused item the arrows move one
item (moving focus with them), Home and End go to the ends, and Space or
Enter opens one that is not fully in view.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/views/components/carousel-item.blade.php

@@ -11,8 +11,11 @@
      for under text on an image; the text itself is `inverse-on-surface`, a role a scheme and a
      contrast profile follow, rather than a literal white.
 
-     A `group` with `aria-roledescription="slide"`, named "n of m" by the carousel around it
-     (WAI-ARIA's carousel pattern). The item is laid out at the carousel's large size and masked:
+     A focusable `group` with `aria-roledescription="slide"`, named "n of m" by the carousel
+     around it. M3 puts the tab stop on the item, not on the row: Tab reaches the first item, the
+     arrow keys move between them and Space or Enter opens the focused one
+     (docs/reference/m3/components-actions-communication-containment.md § Carousel →
+     Accessibility). The item is laid out at the carousel's large size and masked:
      the surface inside is clipped by `--material-carousel-inset` from both sides with M3's
      extra-large corner (28px, CarouselDefaults' item shape) and moved by
      `--material-carousel-shift`, both written by resources/js/carousel.js. Without script the
@@ -24,13 +27,14 @@
 
 <div {{ $attributes
     ->class([
-        'relative h-full w-(--material-carousel-slot) max-w-full shrink-0 snap-start snap-always',
+        'focus-ring relative h-full w-(--material-carousel-slot) max-w-full shrink-0 snap-start snap-always rounded-corner-xl focus-visible:-outline-offset-3',
     ])
     ->merge([
         'role' => 'group',
         'aria-roledescription' => __('slide'),
         'aria-label' => '[material-carousel-position]',
         'data-material-carousel-item' => true,
+        'tabindex' => '0',
     ]) }}>
     <div
         data-material-carousel-surface