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

cmd/summer/docs_test.go

@@ -59,6 +59,12 @@ func TestDocsBuildRealTree(t *testing.T) {
 		"setup/installation.html", "setup/installation.md",
 		"llms.txt", "llms-full.txt", "search-index.json",
 		"assets/site.css", docsite.MarkerFile,
+		"404.html", "assets/site.js", "assets/search.js", "assets/theme-init.js",
+		"assets/LICENSE-lucide.txt", "assets/fonts/LICENSE-dm-sans.txt", "assets/fonts/LICENSE-dm-mono.txt",
+		"assets/fonts/dm-sans-latin-400-normal.woff2", "assets/fonts/dm-sans-latin-600-normal.woff2",
+		"assets/fonts/dm-sans-latin-ext-400-normal.woff2", "assets/fonts/dm-sans-latin-ext-600-normal.woff2",
+		"assets/fonts/dm-sans-latin-400-italic.woff2", "assets/fonts/dm-sans-latin-ext-400-italic.woff2",
+		"assets/fonts/dm-mono-latin-400-normal.woff2", "assets/fonts/dm-mono-latin-ext-400-normal.woff2",
 	} {
 		if _, err := os.Stat(filepath.Join(out, name)); err != nil {
 			t.Errorf("missing %s: %v", name, err)
@@ -139,6 +145,9 @@ func TestDocsAIOutputsInSync(t *testing.T) {
 		if strings.HasPrefix(rel, "assets/") {
 			return nil
 		}
+		if rel == "404.html" {
+			return nil
+		}
 		switch filepath.Ext(rel) {
 		case ".html":
 			htmlFiles = append(htmlFiles, strings.TrimSuffix(rel, ".html"))