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/page-helpers-demo.sx

@@ -78,13 +78,13 @@
 
             ;; Post-process sf-result: count forms per category
             (for-each (fn (k)
-              (let ((count (len (get sf-result k))))
-                (set! sf-cats (assoc sf-cats k count))
-                (set! sf-total (+ sf-total count))))
+              (dict-set! sf-cats k (len (get sf-result k))))
               (keys sf-result))
 
             (reset! results
-              {"sf-cats" sf-cats "sf-total" sf-total "sf-ms" sf-ms
+              {"sf-cats" sf-cats
+               "sf-total" (reduce (fn (acc k) (+ acc (get sf-cats k))) 0 (keys sf-cats))
+               "sf-ms" sf-ms
                "ref-sample" ref-sample "ref-ms" ref-ms
                "attr-result" attr-result "attr-ms" attr-ms
                "comp-result" comp-result "comp-ms" comp-ms