Prefix all SX URLs with /sx/ for WhatsApp-safe sharing

All routes moved under /sx/ prefix:
- / redirects to /sx/
- /sx/ serves home page
- /sx/<path:expr> is the catch-all for SX expression URLs
- Bare /(...) and /~... redirect to /sx/(...) and /sx/~...
- All ~600 hrefs, sx-get attrs, defhandler paths, redirect
  targets, and blueprint routes updated across 44 files

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 "/sx/(geography.(reactive.named-stores))" :sx-get "/sx/(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:")