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/page-functions.sx

@@ -193,7 +193,7 @@
     (map (fn (item)
       (dict :title (get item "title") :desc (get item "desc")
             :prose (get item "prose")
-            :filename (get item "filename") :href (str "/(language.(spec." (get item "slug") "))")
+            :filename (get item "filename") :href (str "/sx/(language.(spec." (get item "slug") "))")
             :source (read-spec-file (get item "filename"))))
       items)))