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/sx/plans/self-hosting-bootstrapper.sx

@@ -17,7 +17,7 @@
         (p :class "text-green-700 text-sm"
           (code "py.sx") " is implemented and verified. G0 == G1: 128/128 defines match, "
           "1490 lines, 88,955 bytes — byte-for-byte identical. "
-          (a :href "/bootstrappers/self-hosting" :class "underline text-green-600 font-medium"
+          (a :href "/language/bootstrappers/self-hosting" :class "underline text-green-600 font-medium"
              "See live verification."))))
 
     ;; -----------------------------------------------------------------------