Fix spec-explorer-data: pass metadata from SX routing instead of env lookup

The helper was trying to look up all-spec-items from get_component_env(),
but that only contains defcomp/defmacro — not regular defines. Now the
SX routing layer calls find-spec and passes filename/title/desc directly.

Also adds boundary declaration for spec-explorer-data.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sx/boundary.sx

@@ -109,3 +109,8 @@
   :params ()
   :returns "dict"
   :service "sx")
+
+(define-page-helper "spec-explorer-data"
+  :params (filename title desc)
+  :returns "dict"
+  :service "sx")