Pin the carousel showcase's viewport and de-flake one assertScript

Found running the full browser suite for plan step 38's last batch,
unrelated to its content changes (confirmed by reverting every
touched file and reproducing both failures against the pre-batch
commit): CarouselTest never pinned a window size, so the multi-browse
row's keyline math only lined "scroll by 2 slots" up with the open
keyline at some widths — the host's own default (here, wider than the
carousel was ever exercised at) left a remainder large enough to fail
four assertions. carouselShowcase() now resizes to 1280x900, matching
the width other browser tests already treat as "large desktop".

One test still failed after that: it put a `.click()` inside the
script assertScript asserts, and assertScript retries its whole
expression on a false result (this file's own comment says so) — a
retry re-fires the click, drifting the carousel one item further each
time and never recovering, unlike every other test here, which clicks
through a separate call first. Split the click out the same way.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

tests/Browser/CarouselTest.php

@@ -70,7 +70,11 @@ const FIRST_ITEM = '#carousel [data-md-carousel-item] >> nth=0';
 
 function carouselShowcase(array $options = [])
 {
+    // Pinned rather than left to the host's own window: at a wide enough viewport the multi-browse
+    // row's 8 items (220px each) leave a remainder the keyline math does not reduce to zero the way
+    // it does here, so "scroll by exactly 2 slots" no longer lands item 2 on the open keyline.
     return visit('/material/carousel', $options)
+        ->resize(1280, 900)
         ->waitForEvent('networkidle')
         ->assertScript("typeof window.Alpine !== 'undefined'");
 }
@@ -164,12 +168,16 @@ it('brings a partly hidden item into focus when it is pressed, and leaves an ope
 });
 
 it('scrolls instantly and leaves every item unmasked under reduced motion', function () {
-    carouselShowcase(['reducedMotion' => 'reduce'])
+    // A click, then a separate read-only assertScript: assertScript retries its whole expression
-        ->assertScript(onCarousel(0, <<<'JS'
+    // on a false result (this file's own comment above), and a click inside it would fire again on
-            root.querySelector('[aria-label="Next"]').click()
+    // every retry, drifting the carousel one further item each time and never recovering.
-            const arrived = at(1)
+    $page = carouselShowcase(['reducedMotion' => 'reduce']);
+
+    $page->script(onCarousel(0, 'root.querySelector(\'[aria-label="Next"]\').click()'));
+
+    $page->assertScript(onCarousel(0, <<<'JS'
         await pause(50)
-            return arrived && items.every((_, i) => inset(i) === 0)
+        return at(1) && items.every((_, i) => inset(i) === 0)
             && items.every((item) => Math.abs(item.getBoundingClientRect().width - size) < 1.5)
         JS));
 });