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/self-hosting-bootstrapper.sx

@@ -17,7 +17,7 @@
         (p :class "text-green-700 text-sm"
           (code "py.sx") " is implemented and verified. G0 == G1: 128/128 defines match, "
           "1490 lines, 88,955 bytes — byte-for-byte identical. "
-          (a :href "/language/bootstrappers/self-hosting" :class "underline text-green-600 font-medium"
+          (a :href "/(language.(bootstrapper.self-hosting))" :class "underline text-green-600 font-medium"
              "See live verification."))))
 
     ;; -----------------------------------------------------------------------