Gate server-side component expansion with contextvar, fix nth arg order, add GEB essay and manifesto links

- Add _expand_components contextvar so _aser only expands components
  during page slot evaluation (fixes highlight on examples, avoids
  breaking fragment responses)
- Fix nth arg order (nth coll n) in docs.sx, examples.sx (delete-row,
  edit-row, bulk-update)
- Add "Godel, Escher, Bach and SX" essay with Wikipedia links
- Update SX Manifesto: new authors, Wikipedia links throughout,
  remove Marx/Engels link

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

sx/sx/nav-data.sx

@@ -63,7 +63,8 @@
   (dict :label "SX Native" :href "/essays/sx-native")
   (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 "Continuations" :href "/essays/continuations")
+  (dict :label "Godel, Escher, Bach" :href "/essays/godel-escher-bach")))
 
 ;; Find the current nav label for a slug by matching href suffix.
 ;; Returns the label string or nil if no match.