SX docs: configurable shell, SX-native event handlers, nav fixes

- Configurable page shell (~sx-page-shell kwargs + SX_SHELL app config)
  so each app controls its own assets — sx docs loads only sx-browser.js
- SX-evaluated sx-on:* handlers (eval-expr instead of new Function)
  with DOM primitives registered in PRIMITIVES table
- data-init boot mode for pure SX initialization scripts
- Jiggle animation on links while fetching
- Nav: 3-column grid for centered alignment, is-leaf sizing,
  fix map-indexed param order (index, item), guard mod-by-zero
- Async route eval failure now falls back to server fetch
  instead of silently rendering nothing
- Remove duplicate h1 title from ~doc-page
- Re-bootstrap sx-ref.js + sx-browser.js

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

shared/static/styles/basics.css

@@ -49,3 +49,13 @@
 .sx-loading-btn .sx-spinner {
     display: none;
 }
+
+/* Subtle jiggle on links while fetching */
+@keyframes sxJiggle {
+    0%, 100% { transform: translateX(0); }
+    25% { transform: translateX(-0.5px); }
+    75% { transform: translateX(0.5px); }
+}
+a.sx-request {
+    animation: sxJiggle 0.3s ease-in-out infinite;
+}