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/sx.js

@@ -1685,9 +1685,17 @@
     // ---- Request executor -------------------------------------------------
 
     function executeRequest(el, verbInfo, extraParams) {
+      // Re-read verb from element in case attributes were morphed since binding
+      var currentVerb = getVerb(el);
+      if (currentVerb) verbInfo = currentVerb;
       var method = verbInfo.method;
       var url = verbInfo.url;
 
+      // Reset retry backoff on fresh (non-retry) requests
+      if (!el.classList.contains("sx-error")) {
+        el.removeAttribute("data-sx-retry-ms");
+      }
+
       // sx-media: skip if media query doesn't match
       var media = el.getAttribute("sx-media");
       if (media && !window.matchMedia(media).matches) return Promise.resolve();