Add server-side test runner to /specs/testing page

Python evaluator runs test.sx at page load, results shown alongside
the browser runner. Both hosts prove the same 81 tests from the same
spec file — server on render, client on click.

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

sx/sx/boundary.sx

@@ -59,3 +59,8 @@
   :params ()
   :returns "dict"
   :service "sx")
+
+(define-page-helper "run-spec-tests"
+  :params ()
+  :returns "dict"
+  :service "sx")