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/sx/docs-content.sx

@@ -2,7 +2,7 @@
 
 (defcomp ~sx-home-content ()
   (div :id "main-content" :class "max-w-3xl mx-auto px-4 py-6"
-    (highlight "(defcomp ~sx-header ()
+    (~doc-code :code (highlight "(defcomp ~sx-header ()
   (a :href \"/\"
      :sx-get \"/\" :sx-target \"#main-panel\"
      :sx-select \"#main-panel\"
@@ -13,7 +13,7 @@
     (p :class \"text-lg text-stone-500 mb-1\"
       \"Framework free reactive hypermedia\")
     (p :class \"text-xs text-stone-400\"
-      \"© Giles Bradshaw 2026\")))" "lisp")))
+      \"© Giles Bradshaw 2026\")))" "lisp"))))
 
 (defcomp ~docs-introduction-content ()
   (~doc-page :title "Introduction"