feat(11.1-02): add the documentation theme, chroma highlighting and docs:serve

- WinterCMS-style shell: header with search and theme toggle, grouped
  sidebar, on-page TOC, pager, page actions, callouts, heading
  permalinks, footer and a 404 page
- fenced code highlighted at build time by chroma/v2 into tok-* classes,
  with a copy button; no inline script, style or handler
- vendored DM Sans/DM Mono fonts and Lucide icons with their licences
- client-side search over search-index.json built with textContent only
- summer docs:serve builds into a temp dir, serves on loopback by default,
  returns 404.html with status 404 and rebuilds on change

internal/docsite/docsite_test.go

@@ -129,7 +129,7 @@ func TestReadmeIngestion(t *testing.T) {
 		return string(b)
 	}
 	html := read("api/alpha.html")
-	for _, want := range []string{`href="/api/delta.html#api"`, `href="/setup/start.html"`, `<h2 id="usage">Usage</h2>`} {
+	for _, want := range []string{`href="/api/delta.html#api"`, `href="/setup/start.html"`, `<h2 id="usage">Usage<a class="heading-anchor" href="#usage" aria-label="Link to section: Usage">#</a></h2>`} {
 		if !strings.Contains(html, want) {
 			t.Errorf("api/alpha.html missing %s", want)
 		}