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/showcase/sections/carousel.blade.php

@@ -78,7 +78,7 @@
 
     <p class="max-w-3xl type-body-md text-on-surface-variant">
         <code>&lt;x-carousel&gt;</code> and <code>&lt;x-carousel-item&gt;</code>: items that change size between M3's keylines as they scroll.
-        Swipe, scroll with Shift and the wheel, or focus a row and use the arrow keys.
+        Swipe, scroll with Shift and the wheel, or tab to an item and use the arrow keys.
     </p>
 
     @foreach ($examples as $title => $code)