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/boot.sx

@@ -211,6 +211,13 @@
                 (or (nil? text) (empty? (trim text)))
                   nil
 
+                ;; Init scripts — evaluate SX for side effects (event listeners etc.)
+                (dom-has-attr? s "data-init")
+                  (let ((exprs (sx-parse text)))
+                    (for-each
+                      (fn (expr) (eval-expr expr (env-extend (dict))))
+                      exprs))
+
                 ;; Mount directive
                 (dom-has-attr? s "data-mount")
                   (let ((mount-sel (dom-get-attr s "data-mount"))