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/content-addressed-components.sx

@@ -280,7 +280,7 @@
       (~doc-subsection :title "CID References in Page Registry"
         (p "The page registry (shipped to the client as " (code "<script type=\"text/sx-pages\">") ") currently lists deps by name. Extend to include CIDs:")
         (~doc-code :code (highlight "{:name \"docs-page\" :path \"/language/docs/<slug>\"\n :auth \"public\" :has-data false\n :deps ({:name \"~essay-foo\" :cid \"bafy...essay\"}\n        {:name \"~doc-code\"  :cid \"bafy...doccode\"})\n :content \"(case slug ...)\" :closure {}}" "lisp"))
-        (p "The " (a :href "/etc/plans/predictive-prefetch" :class "text-violet-700 underline" "predictive prefetch system") " uses these CIDs to fetch components from the resolution cascade rather than only from the origin server's " (code "/sx/components") " endpoint."))
+        (p "The " (a :href "/(etc.(plan.predictive-prefetch))" :class "text-violet-700 underline" "predictive prefetch system") " uses these CIDs to fetch components from the resolution cascade rather than only from the origin server's " (code "/sx/components") " endpoint."))
 
       (~doc-subsection :title "SX Response Component Headers"
         (p "Currently, " (code "SX-Components") " header lists loaded component names. Extend to support CIDs:")
@@ -407,9 +407,9 @@
     (~doc-section :title "Relationships" :id "relationships"
       (p "This plan is the foundation for several other plans and roadmaps:")
       (ul :class "list-disc pl-5 text-stone-700 space-y-1"
-        (li (a :href "/etc/plans/sx-activity" :class "text-violet-700 underline" "SX-Activity") " Phase 2 (content-addressed components on IPFS) is a summary of this plan. This plan supersedes that section with full detail.")
-        (li (a :href "/etc/plans/predictive-prefetch" :class "text-violet-700 underline" "Predictive prefetching") " gains CID-based resolution — the " (code "/sx/components") " endpoint and IPFS gateway become alternative resolution paths in the prefetch cascade.")
-        (li (a :href "/etc/plans/isomorphic-architecture" :class "text-violet-700 underline" "Isomorphic architecture") " Phase 1 (component distribution) is enhanced — CIDs make per-page bundles verifiable and cross-server shareable.")
+        (li (a :href "/(etc.(plan.sx-activity))" :class "text-violet-700 underline" "SX-Activity") " Phase 2 (content-addressed components on IPFS) is a summary of this plan. This plan supersedes that section with full detail.")
+        (li (a :href "/(etc.(plan.predictive-prefetch))" :class "text-violet-700 underline" "Predictive prefetching") " gains CID-based resolution — the " (code "/sx/components") " endpoint and IPFS gateway become alternative resolution paths in the prefetch cascade.")
+        (li (a :href "/(etc.(plan.isomorphic-architecture))" :class "text-violet-700 underline" "Isomorphic architecture") " Phase 1 (component distribution) is enhanced — CIDs make per-page bundles verifiable and cross-server shareable.")
         (li "The SX-Activity vision of " (strong "serverless applications on IPFS") " depends entirely on this plan. Without content-addressed components, applications can't be pinned to IPFS as self-contained artifacts."))
       (div :class "rounded border border-amber-200 bg-amber-50 p-3 mt-2"
         (p :class "text-amber-800 text-sm" (strong "Depends on: ") "deps.sx (complete), boundary enforcement (complete), IPFS infrastructure (exists in artdag, needs wiring to web platform)."))))))