Stop drawing each section's title twice on its page

Plan step 38 review: the frame's structural rule that kept a section's
own <h2> for screen readers (Tailwind's [&>section>h2]:sr-only before,
a showcase.css rule during the rewrite) was removed as dead weight
once every section had a real heading, so each page drew its title
twice, the <h1> and an identical headline under it. The <h2>s are
md-visually-hidden again, the frame's comment says why, and the
section page test asserts it. The Layout section's description said
"window size classes"; M3's term is breakpoints.

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

resources/views/showcase/sections/containment.blade.php

@@ -246,7 +246,7 @@
 @endphp
 
 <x-livewire-material::stack as="section" id="containment" gap="space300">
-    <h2 class="md-type-headline-md">Containment</h2>
+    <h2 class="md-visually-hidden">Containment</h2>
 
     <p class="md-type-body-md md-ink-variant">
         <code>&lt;x-card&gt;</code>, <code>&lt;x-list&gt;</code> and <code>&lt;x-list-item&gt;</code>, <code>&lt;x-divider&gt;</code>, <code>&lt;x-collapse&gt;</code>,