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

tests/Feature/ShowcaseTest.php

@@ -35,11 +35,18 @@ it('gives every section a page of its own, linked from the overview and the rail
     foreach (Sections::all() as $key => $section) {
         $overview->assertSee(route('livewire-material.section', $key), false);
 
-        $this->withoutVite()
+        $html = $this->withoutVite()
             ->get("/material/{$key}")
             ->assertOk()
             ->assertSee("<title>{$section['title']} · Livewire Material</title>", false)
-            ->assertSee('id="'.$key.'"', false);
+            ->assertSee('id="'.$key.'"', false)
+            ->getContent();
+
+        // The page's <h1> names the section; the section's own <h2> is for a screen reader only,
+        // so the title is not drawn twice.
+        preg_match_all('/<h2\b[^>]*>/', $html, $headings);
+
+        expect($headings[0][0] ?? null)->toBe('<h2 class="md-visually-hidden">', "{$key}'s own <h2> is drawn");
     }
 
     $this->get('/material/not-a-section')->assertNotFound();