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/sxc/pages/docs.sx

@@ -239,4 +239,5 @@
     "sx-manifesto" (~essay-sx-manifesto)
     "tail-call-optimization" (~essay-tail-call-optimization)
     "continuations" (~essay-continuations)
+    "godel-escher-bach" (~essay-godel-escher-bach)
     :else (~essay-sx-sucks)))