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/pages.py

@@ -76,7 +76,7 @@ def register_page_helpers(service: str, helpers: dict[str, Any]) -> None:
     Then in .sx::
 
         (defpage docs-page
-          :path "/docs/<slug>"
+          :path "/language/docs/<slug>"
           :auth :public
           :content (docs-content slug))
     """