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

tests/Feature/Components/CarouselTest.php

@@ -1,6 +1,6 @@
 <?php
 
-it('is a focusable carousel region of slides named n of m', function () {
+it('is a carousel region of focusable slides named n of m', function () {
     $html = (string) $this->blade(<<<'BLADE'
         <x-carousel label="Recent uploads">
             <x-carousel-item><img src="/a.jpg" alt="A lake" /></x-carousel-item>
@@ -17,14 +17,16 @@ it('is a focusable carousel region of slides named n of m', function () {
         ->toContain('role="region"')
         ->toContain('aria-roledescription="carousel"')
         ->toContain('aria-label="Recent uploads"')
-        ->toContain('tabindex="0"')
         ->toContain('aria-roledescription="slide"')
         ->toContain('aria-label="1 of 3"')
         ->toContain('aria-label="2 of 3"')
         ->toContain('aria-label="3 of 3"')
         ->toContain('<img src="/a.jpg" alt="A lake" />')
         ->not->toContain('[material-carousel-position]')
-        ->and(substr_count($html, 'role="group"'))->toBe(3);
+        ->and(substr_count($html, 'role="group"'))->toBe(3)
+        // M3 puts the tab stop on each item and tells you not to focus the container.
+        ->and(substr_count($html, 'tabindex="0"'))->toBe(3)
+        ->and($html)->toMatch('/<div\s+x-ref="scroller"(?:(?!tabindex)[^>])*>/');
 });
 
 it('counts the slides of a carousel inside a slide on their own', function () {