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/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.(spec.deps))" :class "text-violet-700 underline" "deps.sx")
+      (a :href "/sx/(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.")