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

scripts/check-phase11.1.sh

@@ -83,6 +83,15 @@ run_deps() {
 		*) refuse "go.mod added $dep, which no phase decision approves" ;;
 		esac
 	done
+	# D-15: chroma/v2 highlights code at build time, as a direct requirement.
+	(cd "$ROOT" && go mod edit -json) | python3 -c '
+import json, sys
+direct = [r for r in json.load(sys.stdin).get("Require") or [] if r["Path"] == "github.com/alecthomas/chroma/v2" and not r.get("Indirect")]
+sys.exit(0 if direct else 1)
+' || refuse "go.mod does not require github.com/alecthomas/chroma/v2 directly"
+	if grep -q 'goldmark-highlighting' "$ROOT/go.mod"; then
+		refuse "go.mod requires goldmark-highlighting (D-15 rejects it)"
+	fi
 	echo "phase11.1 deps passed"
 }