style(website): add CSS for category-breadcrumb and assert absence on parent

Mirrors the .category-subtitle a underline style for visual cohesion in
the hero, and locks in the gating behavior with a negative assertion so
a regression that drops the page_kind guard would be caught.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

website/static/style.css

@@ -462,6 +462,23 @@ kbd {
   text-wrap: balance;
 }
 
+.category-breadcrumb {
+  margin: 0 0 1rem;
+  color: var(--hero-muted);
+  font-size: clamp(1rem, 1.5vw, 1.1rem);
+}
+
+.category-breadcrumb a {
+  color: var(--hero-text);
+  text-decoration: underline;
+  text-decoration-color: oklch(100% 0 0 / 0.32);
+  text-underline-offset: 0.2em;
+}
+
+.category-breadcrumb a:hover {
+  text-decoration-color: oklch(100% 0 0 / 0.7);
+}
+
 .category-subtitle {
   max-width: 68ch;
   margin-top: 1.1rem;

website/tests/test_build.py

@@ -604,6 +604,10 @@ class TestBuild:
         assert 'href="/categories/web-frameworks/"' in sync
         assert "Web Frameworks" in sync
         assert "<h1>Synchronous</h1>" in sync
+        assert "category-breadcrumb" in sync
+
+        parent = (site / "categories" / "web-frameworks" / "index.html").read_text(encoding="utf-8")
+        assert "category-breadcrumb" not in parent
 
     def test_build_creates_group_pages(self, tmp_path):
         readme = textwrap.dedent("""\