POST /v2/runs/{run}/resume. The body is optional and every field defaults, so a resume of an ordinary interruption is a bodyless POST.
confirm is a person deciding, never evidence. §7.6's last row - no evidence from a host that did not restart - proves that nothing is running and nothing can start under the retired key, and leaves earlier work possibly done and unobserved. Such a run is never resumed on its own; a person is told that and asks for it deliberately, which is what this flag carries. It changes nothing about the fence: the key is retired at the host before the next generation opens either way, and a key found live still refuses with attempt_live. Ignored for a run whose prior attempt is known to have ended.