GraphSX URL routing: s-expression URLs for sx-docs

Replace path-based URLs with nested s-expression URLs across the sx app.
URLs like /language/docs/introduction become /(language.(doc.introduction)),
making the URL simultaneously a query, render instruction, and address.

- Add sx_router.py: catch-all route evaluator with dot→space conversion,
  auto-quoting slugs, two-phase eval, streaming detection, 301 redirects
- Add page-functions.sx: section + page functions for URL dispatch
- Rewrite nav-data.sx: ~200 hrefs to SX expression format, tree-descent
  nav matching via has-descendant-href? (replaces prefix heuristics)
- Convert ~120 old-style hrefs across 26 .sx content files
- Add SX Protocol proposal (etc/plans/sx-protocol)
- Wire catch-all route in app.py with before_request redirect handler

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 "/etc/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.(essay.tail-call-optimization))" :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 ()