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/plans/predictive-prefetch.sx

@@ -248,7 +248,7 @@
 
     (~doc-section :title "Relationship to Isomorphic Roadmap" :id "relationship"
       (p "This plan sits between Phase 3 (client-side routing) and Phase 4 (client async & IO bridge) of the "
-        (a :href "/etc/plans/isomorphic-architecture" :class "text-violet-700 underline" "isomorphic architecture roadmap")
+        (a :href "/(etc.(plan.isomorphic-architecture))" :class "text-violet-700 underline" "isomorphic architecture roadmap")
         ". It extends Phase 3 by making more navigations go client-side without needing any IO bridge — purely by ensuring component definitions are available before they're needed.")
       (div :class "rounded border border-amber-200 bg-amber-50 p-3 mt-2"
         (p :class "text-amber-800 text-sm" (strong "Depends on: ") "Phase 3 (client-side routing with deps checking). No dependency on Phase 4.")))))