Perf: Serve homepage and assets without invoking Functions

Add _routes.json so /, /robots.txt and /assets/* skip the worker and
don't count against the Functions request quota. rate-limit.js moves to
assets/ so the homepage reads it client-side instead of via HTMLRewriter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

index.html

@@ -127,5 +127,9 @@ Use images to complement your responses, powered by Brave.</code></pre>
             },()=>getSelection().selectAllChildren(p));
         };
     </script>
+    <script type="module">
+        import * as L from '/assets/rate-limit.js';
+        for(const e of document.querySelectorAll('[data-limit]'))e.textContent=L[e.dataset.limit]??e.textContent;
+    </script>
 </body>
 </html>