preflight gating

docs/ARCHITECTURE-BLUEPRINT.md

@@ -310,6 +310,9 @@ Resolves an assessment profile into an executable run plan:
 - isolation and timeout policy,
 - artifact retention policy.
 
+At execution time, a failing preflight blocks downstream check groups for the
+same extension so expensive or misleading harness steps are not invoked.
+
 ### Runner Bridge
 
 Executes or coordinates extension checks.