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/docs-content.sx

@@ -79,7 +79,7 @@
       (p :class "text-stone-600"
         "Special forms are syntactic constructs whose arguments are NOT evaluated before dispatch. Each form has its own evaluation rules — unlike primitives, which receive pre-evaluated values. Together with primitives, special forms define the complete language surface.")
       (p :class "text-stone-600"
-        "Forms marked with a tail position enable " (a :href "/essays/tco" :class "text-violet-600 hover:underline" "tail-call optimization") " — recursive calls in tail position use constant stack space.")
+        "Forms marked with a tail position enable " (a :href "/etc/essays/tco" :class "text-violet-600 hover:underline" "tail-call optimization") " — recursive calls in tail position use constant stack space.")
       (div :class "space-y-10" forms))))
 
 (defcomp ~docs-server-rendering-content ()