Load web/page-helpers.sx in HTTP server core files

build-reference-data, build-attr-detail, etc. were undefined because
page-helpers.sx wasn't in the explicit core_files list.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -2113,6 +2113,7 @@ let http_mode port =
     web_base ^ "/adapter-html.sx"; web_base ^ "/adapter-sx.sx";
     web_base ^ "/web-forms.sx"; web_base ^ "/engine.sx";
     web_base ^ "/request-handler.sx";
+    web_base ^ "/page-helpers.sx";
   ] in
   http_load_files env core_files;
   (* Libraries *)