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/sx/css_registry.py

@@ -252,6 +252,8 @@ def _css_selector_to_class(selector: str) -> str:
             i += 2
         elif name[i] == ':':
             break  # pseudo-class — stop here
+        elif name[i] == '[':
+            break  # attribute selector — stop here
         else:
             result.append(name[i])
             i += 1