Scroll the full-screen carousel down the page, as M3's does

Plan step 19, containment.md C-11. The full-screen layout was another
horizontal row with a 28px corner and a mask, where M3's "shows one
edge-to-edge large item at a time and scrolls vertically". It is now a
vertical scroll-snap column: items fill the row with no corner and no
mask, 16dp apart, no end padding, the previous/next buttons point up and
down and the arrow keys are Up and Down. The row is capped at the 840px
medium window, which with the portrait rule in the header comment is as
far as CSS can hold M3's "compact and medium, portrait only". The ported
FullScreenCarouselStrategy keylines go with it: nothing measures them
any more.

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

@@ -55,8 +55,8 @@
                 @endforeach
             </x-carousel>
             BLADE,
-        'Full-screen' => <<<'BLADE'
-            <x-carousel layout="full-screen" label="Wallpapers" height="240" :controls="true">
+        'Full-screen: one item at a time, scrolled down' => <<<'BLADE'
+            <x-carousel layout="full-screen" label="Wallpapers" height="320" :controls="true">
                 @foreach ([
                     ['Morning', 'very-sunny', 'bg-linear-to-br from-primary-container to-tertiary-container text-on-primary-container'],
                     ['Noon', 'clam-shell', 'bg-linear-to-br from-secondary-container to-primary-container text-on-secondary-container'],