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/examples.sx

@@ -32,26 +32,26 @@
     (p :class "text-stone-600 mb-6"
       "Complete reference for the sx client library.")
     (div :class "grid gap-4 sm:grid-cols-2"
-      (a :href "/geography/hypermedia/reference/attributes"
-        :sx-get "/geography/hypermedia/reference/attributes" :sx-target "#main-panel" :sx-select "#main-panel"
+      (a :href "/(geography.(hypermedia.(reference.attributes)))"
+        :sx-get "/(geography.(hypermedia.(reference.attributes)))" :sx-target "#main-panel" :sx-select "#main-panel"
         :sx-swap "outerHTML" :sx-push-url "true"
         :class "block p-5 rounded-lg border border-stone-200 hover:border-violet-300 hover:shadow-sm transition-all no-underline"
         (h3 :class "text-lg font-semibold text-violet-700 mb-1" "Attributes")
         (p :class "text-stone-600 text-sm" "All sx attributes — request verbs, behavior modifiers, and sx-unique features."))
-      (a :href "/geography/hypermedia/reference/headers"
-        :sx-get "/geography/hypermedia/reference/headers" :sx-target "#main-panel" :sx-select "#main-panel"
+      (a :href "/(geography.(hypermedia.(reference.headers)))"
+        :sx-get "/(geography.(hypermedia.(reference.headers)))" :sx-target "#main-panel" :sx-select "#main-panel"
         :sx-swap "outerHTML" :sx-push-url "true"
         :class "block p-5 rounded-lg border border-stone-200 hover:border-violet-300 hover:shadow-sm transition-all no-underline"
         (h3 :class "text-lg font-semibold text-violet-700 mb-1" "Headers")
         (p :class "text-stone-600 text-sm" "Custom HTTP headers used to coordinate between the sx client and server."))
-      (a :href "/geography/hypermedia/reference/events"
-        :sx-get "/geography/hypermedia/reference/events" :sx-target "#main-panel" :sx-select "#main-panel"
+      (a :href "/(geography.(hypermedia.(reference.events)))"
+        :sx-get "/(geography.(hypermedia.(reference.events)))" :sx-target "#main-panel" :sx-select "#main-panel"
         :sx-swap "outerHTML" :sx-push-url "true"
         :class "block p-5 rounded-lg border border-stone-200 hover:border-violet-300 hover:shadow-sm transition-all no-underline"
         (h3 :class "text-lg font-semibold text-violet-700 mb-1" "Events")
         (p :class "text-stone-600 text-sm" "DOM events fired during the sx request lifecycle."))
-      (a :href "/geography/hypermedia/reference/js-api"
-        :sx-get "/geography/hypermedia/reference/js-api" :sx-target "#main-panel" :sx-select "#main-panel"
+      (a :href "/(geography.(hypermedia.(reference.js-api)))"
+        :sx-get "/(geography.(hypermedia.(reference.js-api)))" :sx-target "#main-panel" :sx-select "#main-panel"
         :sx-swap "outerHTML" :sx-push-url "true"
         :class "block p-5 rounded-lg border border-stone-200 hover:border-violet-300 hover:shadow-sm transition-all no-underline"
         (h3 :class "text-lg font-semibold text-violet-700 mb-1" "JS API")