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:")

sx/sx/reactive-islands/index.sx

@@ -145,7 +145,7 @@
               (td :class "px-3 py-2 text-stone-700" "Phase 2 remaining")
               (td :class "px-3 py-2 text-stone-500 font-medium" "P2")
               (td :class "px-3 py-2 font-mono text-xs text-stone-500"
-                (a :href "/geography/reactive/phase2" :sx-get "/geography/reactive/phase2" :sx-target "#main-panel" :sx-select "#main-panel" :sx-swap "outerHTML" :sx-push-url "true" :class "text-violet-700 underline" "Error boundaries + resource + patterns")))
+                (a :href "/(geography.(reactive.phase2))" :sx-get "/(geography.(reactive.phase2))" :sx-target "#main-panel" :sx-select "#main-panel" :sx-swap "outerHTML" :sx-push-url "true" :class "text-violet-700 underline" "Error boundaries + resource + patterns")))
             (tr :class "border-b border-stone-100"
               (td :class "px-3 py-2 text-stone-700" "Error boundaries")
               (td :class "px-3 py-2 text-green-700 font-medium" "Done")

sx/sx/reactive-islands/plan.sx

@@ -46,7 +46,7 @@
 
     (~doc-section :title "htmx Lakes" :id "lakes"
       (p "An htmx lake is server-driven content " (em "inside") " a reactive island. The island provides the reactive boundary; the lake content is swapped via " (code "sx-get") "/" (code "sx-post") " like normal hypermedia.")
-      (p "This works because signals live in JavaScript closures, not in the DOM. When a swap replaces lake content, the island's signals are unaffected. The lake can communicate back to the island via the " (a :href "/geography/reactive/event-bridge" :sx-get "/geography/reactive/event-bridge" :sx-target "#main-panel" :sx-select "#main-panel" :sx-swap "outerHTML" :sx-push-url "true" :class "text-violet-700 underline" "event bridge") ".")
+      (p "This works because signals live in JavaScript closures, not in the DOM. When a swap replaces lake content, the island's signals are unaffected. The lake can communicate back to the island via the " (a :href "/(geography.(reactive.event-bridge))" :sx-get "/(geography.(reactive.event-bridge))" :sx-target "#main-panel" :sx-select "#main-panel" :sx-swap "outerHTML" :sx-push-url "true" :class "text-violet-700 underline" "event bridge") ".")
 
       (~doc-subsection :title "Navigation scenarios"
         (div :class "space-y-3"
@@ -155,7 +155,7 @@
         (li (strong "Spec-first.") " Signal semantics live in " (code "signals.sx") ". Bootstrapped to JS and Python. The same primitives will work in future hosts — Go, Rust, native.")
         (li (strong "No build step.") " Reactive bindings are created at runtime during DOM rendering. No JSX compilation, no Babel transforms, no Vite plugins."))
 
-      (p :class "mt-4" "The recommendation from the " (a :href "/etc/essays/client-reactivity" :class "text-violet-700 underline" "Client Reactivity") " essay was: \"Tier 4 probably never.\" This plan is what happens when the answer changes. The design avoids every footgun that essay warns about — no useState cascading to useEffect cascading to Context cascading to a state management library. Signals are one primitive. Islands are one boundary. The rest is composition."))))
+      (p :class "mt-4" "The recommendation from the " (a :href "/(etc.(essay.client-reactivity))" :class "text-violet-700 underline" "Client Reactivity") " essay was: \"Tier 4 probably never.\" This plan is what happens when the answer changes. The design avoids every footgun that essay warns about — no useState cascading to useEffect cascading to Context cascading to a state management library. Signals are one primitive. Islands are one boundary. The rest is composition."))))
 
 
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