Add Specs section, Reflexive Web essay, fix highlight and dev caching

- Fix highlight() returning SxExpr so syntax-highlighted code renders
  as DOM elements instead of leaking SX source text into the page
- Add Specs section that reads and displays canonical SX spec files
  from shared/sx/ref/ with syntax highlighting
- Add "The Reflexive Web" essay on SX becoming a complete LISP with
  AI as native participant
- Change logo from (<x>) to (<sx>) everywhere
- Unify all backgrounds to bg-stone-100, center code blocks
- Skip component/style cookie cache in dev mode so .sx edits are
  visible immediately on refresh without clearing localStorage

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

sx/sx/layouts.sx

@@ -11,7 +11,8 @@
            (dict :label "Reference" :href "/reference/")
            (dict :label "Protocols" :href "/protocols/wire-format")
            (dict :label "Examples" :href "/examples/click-to-load")
-           (dict :label "Essays" :href "/essays/sx-sucks"))))
+           (dict :label "Essays" :href "/essays/sx-sucks")
+           (dict :label "Specs" :href "/specs/core"))))
     (<> (map (lambda (item)
       (~nav-link
         :href (get item "href")