Add Testing as top-level docs section with per-module specs

New /testing/ section with 6 pages: overview (all specs), evaluator,
parser, router, renderer, and runners. Each page runs tests server-side
(Python) and offers a browser "Run tests" button (JS). Modular browser
runner (sxRunModularTests) loads framework + per-spec sources from DOM.

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

sx/sx/boundary.sx

@@ -64,3 +64,8 @@
   :params ()
   :returns "dict"
   :service "sx")
+
+(define-page-helper "run-modular-tests"
+  :params (spec-name)
+  :returns "dict"
+  :service "sx")