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>

sx/sxc/docs.sx

@@ -2,7 +2,6 @@
 
 (defcomp ~doc-page (&key title &rest children)
   (div :class "max-w-4xl mx-auto px-6 py-8"
-    (h1 :class "text-4xl font-bold text-stone-900 mb-8 text-center" title)
     (div :class "prose prose-stone max-w-none space-y-6" children)))
 
 (defcomp ~doc-section (&key title id &rest children)