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>

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 {