Fix SX nav morphing, retry error modal, and aria-selected CSS extraction

- Re-read verb URL from element attributes at execution time so morphed
  nav links navigate to the correct destination
- Reset retry backoff on fresh requests; skip error modal when sx-retry
  handles the failure
- Strip attribute selectors in CSS registry so aria-selected:* classes
  resolve correctly for on-demand CSS
- Add @css annotations for dynamic aria-selected variant classes
- Add SX docs integration test suite (102 tests)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/static/scripts/body.js

@@ -542,8 +542,10 @@ document.addEventListener('DOMContentLoaded', () => {
 document.body.addEventListener("sx:responseError", function (event) {
   var resp = event.detail.response;
   if (!resp) return;
-  var status = resp.status || 0;
+  // Don't show error modal when sx-retry will handle the failure
   var triggerEl = event.target;
+  if (triggerEl && triggerEl.getAttribute("sx-retry")) return;
+  var status = resp.status || 0;
   var form = triggerEl ? triggerEl.closest("form") : null;
 
   var title = "Something went wrong";