Restructure SX docs nav into 4 top-level sections with nested routing

New hierarchy: Geography (Reactive Islands, Hypermedia Lakes, Marshes,
Isomorphism), Language (Docs, Specs, Bootstrappers, Testing),
Applications (CSSX, Protocols), Etc (Essays, Philosophy, Plans).

All routes updated to match: /reactive/* → /geography/reactive/*,
/docs/* → /language/docs/*, /essays/* → /etc/essays/*, etc.
Updates nav-data.sx, all defpage routes, API endpoints, internal links
across 43 files. Enhanced find-nav-match for nested group resolution.

Also includes: page-helpers-demo sf-total fix (reduce instead of set!),
rebootstrapped sx-browser.js and sx_ref.py, defensive slice/rest guards.

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

sx/sxc/handlers/examples.sx

@@ -89,7 +89,7 @@
         (range start (+ start 5)))
       (if (<= next 6)
         (div :id "scroll-sentinel"
-          :sx-get (str "/hypermedia/examples/api/scroll?page=" next)
+          :sx-get (str "/geography/hypermedia/examples/api/scroll?page=" next)
           :sx-trigger "intersect once"
           :sx-target "#scroll-items"
           :sx-swap "beforeend"