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/nav-data.sx

@@ -64,7 +64,15 @@
   (dict :label "The SX Manifesto" :href "/essays/sx-manifesto")
   (dict :label "Tail-Call Optimization" :href "/essays/tail-call-optimization")
   (dict :label "Continuations" :href "/essays/continuations")
-  (dict :label "Godel, Escher, Bach" :href "/essays/godel-escher-bach")))
+  (dict :label "Godel, Escher, Bach" :href "/essays/godel-escher-bach")
+  (dict :label "The Reflexive Web" :href "/essays/reflexive-web")))
+
+(define specs-nav-items (list
+  (dict :label "Core" :href "/specs/core")
+  (dict :label "Parser" :href "/specs/parser")
+  (dict :label "Evaluator" :href "/specs/evaluator")
+  (dict :label "Primitives" :href "/specs/primitives")
+  (dict :label "Renderer" :href "/specs/renderer")))
 
 ;; Find the current nav label for a slug by matching href suffix.
 ;; Returns the label string or nil if no match.