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/sx/ref/py.sx

@@ -235,6 +235,7 @@
   "scan-io-refs-walk" "scan_io_refs_walk"
   "transitive-io-refs" "transitive_io_refs"
   "compute-all-io-refs" "compute_all_io_refs"
+  "component-io-refs-cached" "component_io_refs_cached"
   "component-pure?" "component_pure_p"
   "render-target" "render_target"
   "page-render-plan" "page_render_plan"