AJAX on main thread, fix double-push in click delegation

- AJAX requests (SX-Request: true) now render on main thread instead
  of queueing behind slow full-page renders in worker pool
- Remove pushState from click handler — handle-history does it after
  swap succeeds, preventing double-push that triggered popstate handler

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -2238,12 +2238,29 @@ let http_mode port =
           match Hashtbl.find_opt response_cache cache_key with
           | Some cached -> write_response fd cached; true
           | None ->
-            let headers = if is_ajax then parse_http_headers data else [] in
-            Mutex.lock render_mutex;
-            render_queue := !render_queue @ [(fd, path, headers)];
-            Condition.signal render_cond;
-            Mutex.unlock render_mutex;
-            false
+            if is_ajax then begin
+              (* AJAX: render on main thread — aser only, fast, no SSR.
+                 Avoids queueing behind slow full-page renders. *)
+              let headers = parse_http_headers data in
+              let response =
+                try match http_render_page env path headers with
+                  | Some body ->
+                    let resp = http_response ~content_type:"text/sx; charset=utf-8" body in
+                    Hashtbl.replace response_cache cache_key resp; resp
+                  | None -> http_response ~status:404 "nil"
+                with e ->
+                  Printf.eprintf "[ajax] Error for %s: %s\n%!" path (Printexc.to_string e);
+                  http_response ~status:500 "nil"
+              in
+              write_response fd response; true
+            end else begin
+              (* Full page: queue to render worker *)
+              Mutex.lock render_mutex;
+              render_queue := !render_queue @ [(fd, path, [])];
+              Condition.signal render_cond;
+              Mutex.unlock render_mutex;
+              false
+            end
         end else if String.length path > 8 && String.sub path 0 8 = "/static/" then begin
           write_response fd (serve_static_file static_dir path); true
         end else begin

shared/static/wasm/sx-platform.js

@@ -464,10 +464,8 @@
           if (e.metaKey || e.ctrlKey || e.shiftKey || e.altKey) return;
           e.preventDefault();
           var url = el.getAttribute("href") || el.getAttribute("sx-get");
-          // Push URL first (execute-request doesn't handle URL updates)
-          if (el.getAttribute("sx-push-url")) {
-            history.pushState({}, "", url);
-          }
+          // Don't push URL here — execute-request's handle-history does it.
+          // Double-push causes popstate handler to clobber the SX swap.
           // Store the element reference for SX to pick up
           window.__sxClickEl = el;
           try {