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/elevation.blade.php

@@ -1,5 +1,5 @@
 <x-livewire-material::stack as="section" id="elevation" gap="space300">
-    <h2 class="md-type-headline-md">Elevation</h2>
+    <h2 class="md-visually-hidden">Elevation</h2>
 
     <p class="md-type-body-md md-ink-variant">
         Five shadow levels as <code>--md-sys-elevation-*</code>, for what floats over content.