preflight gating

docs/EXTENSION-SDK.md

@@ -182,6 +182,11 @@ package `artifact_manifest`.
 If a Python runner raises an exception, the core converts that failure into
 `infrastructure_error` evidence so the assessment package remains complete.
 
+Preflight runners are gates. If an extension preflight returns `fail`, `blocked`,
+or `infrastructure_error`, downstream check groups for that extension are not
+executed; they receive `blocked` evidence with `blocked_reason:
+preflight_failed`.
+
 ## Result Statuses
 
 Initial statuses: