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/reactive-islands/event-bridge.sx

@@ -28,7 +28,7 @@
       (ul :class "space-y-2 text-stone-600 list-disc pl-5"
         (li (strong "Swap inside island: ") "Signals survive. The lake content is replaced but the island's signal closures are untouched. Effects re-bind to new DOM nodes if needed.")
         (li (strong "Swap outside island: ") "Signals survive. The island is not affected by swaps to other parts of the page.")
-        (li (strong "Swap replaces island: ") "Signals are " (em "lost") ". The island is disposed. This is where " (a :href "/geography/reactive/named-stores" :sx-get "/geography/reactive/named-stores" :sx-target "#main-panel" :sx-select "#main-panel" :sx-swap "outerHTML" :sx-push-url "true" :class "text-violet-700 underline" "named stores") " come in — they persist at page level, surviving island destruction.")))
+        (li (strong "Swap replaces island: ") "Signals are " (em "lost") ". The island is disposed. This is where " (a :href "/(geography.(reactive.named-stores))" :sx-get "/(geography.(reactive.named-stores))" :sx-target "#main-panel" :sx-select "#main-panel" :sx-swap "outerHTML" :sx-push-url "true" :class "text-violet-700 underline" "named stores") " come in — they persist at page level, surviving island destruction.")))
 
     (~doc-section :title "Spec" :id "spec"
       (p "The event bridge is spec'd in " (code "signals.sx") " (sections 12-13). Three functions:")