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>

shared/sx/ref/eval.sx

@@ -91,7 +91,8 @@
             (= name "true")        true
             (= name "false")       false
             (= name "nil")         nil
-            :else                   (error (str "Undefined symbol: " name))))
+            :else                   (do (debug-log "Undefined symbol:" name "primitive?:" (primitive? name))
+                                      (error (str "Undefined symbol: " name)))))
 
       ;; --- keyword → its string name ---
       "keyword" (keyword-name expr)