Leave carousel items unmasked under reduced motion

Plan step 12, containment.md C-05. Only the parallax half of M3's rule
was honoured: the mask was still written every frame, so items kept
growing and shrinking between the keylines. Under reduced motion the
inset, the shift and the label fade are now all zero, so every item
stays at the strategy's large size and the keylines only decide where
the row snaps. `--material-carousel-pin` went with it: with no mask
there is nothing to pin the content to.

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

tests/Browser/CarouselTest.php

@@ -146,15 +146,14 @@ it('brings a partly hidden item into focus when it is pressed', function () {
     $page->assertScript(onCarousel(0, 'return inset(4) < 0.5 && scroller.scrollLeft > 0'));
 });
 
-it('scrolls instantly and keeps content pinned to its mask under reduced motion', function () {
+it('scrolls instantly and leaves every item unmasked under reduced motion', function () {
     carouselShowcase(['reducedMotion' => 'reduce'])
         ->assertScript(onCarousel(0, <<<'JS'
             root.querySelector('[aria-label="Next"]').click()
             const arrived = at(1)
             await pause(50)
-            const last = items.length - 1
-            return arrived && inset(last) > 0.5
-                && surface(last).style.getPropertyValue('--material-carousel-pin') === surface(last).style.getPropertyValue('--material-carousel-inset')
+            return arrived && items.every((_, i) => inset(i) === 0)
+                && items.every((item) => Math.abs(item.getBoundingClientRect().width - size) < 1.5)
             JS));
 });