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/analyzer.sx

@@ -12,7 +12,7 @@
       "Each bar shows how many of the "
       (strong (str total-components))
       " total components a page actually needs, computed by the "
-      (a :href "/language/specs/deps" :class "text-violet-700 underline" "deps.sx")
+      (a :href "/(language.(spec.deps))" :class "text-violet-700 underline" "deps.sx")
       " transitive closure algorithm. "
       "Click a page to see its component tree; expand a component to see its SX source.")