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/reader-macros.sx

@@ -28,7 +28,7 @@
         (~doc-code :code (highlight "#'my-function  →  (quote my-function)" "lisp")))
 
       (~doc-subsection :title "Extensible dispatch: #name"
-        (p "User-defined reader macros via " (code "#name expr") ". The parser reads an identifier after " (code "#") ", looks up a handler in the reader macro registry, and calls it with the next parsed expression. See the " (a :href "/plans/reader-macro-demo" :class "text-violet-600 hover:underline" "#z3 demo") " for a working example that translates SX spec declarations to SMT-LIB.")))
+        (p "User-defined reader macros via " (code "#name expr") ". The parser reads an identifier after " (code "#") ", looks up a handler in the reader macro registry, and calls it with the next parsed expression. See the " (a :href "/etc/plans/reader-macro-demo" :class "text-violet-600 hover:underline" "#z3 demo") " for a working example that translates SX spec declarations to SMT-LIB.")))
 
 
     ;; -----------------------------------------------------------------------