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/sx_ref.py

@@ -1184,6 +1184,7 @@ def eval_expr(expr, env):
         elif sx_truthy((name == 'nil')):
             return NIL
         else:
+            debug_log('Undefined symbol:', name, 'primitive?:', is_primitive(name))
             return error(sx_str('Undefined symbol: ', name))
     elif _match == 'keyword':
         return keyword_name(expr)