alice writes M1 content=d-441a63eeaec34377a152abd7eaeef5bc on node-1
  created memory id=76b5cbfa-d390-4dc3-8bce-46b6679fef8d
settle 8s for pre-delete fanout
pre-delete: verifying M1 is visible on all peers
  pre-delete node-2 sees 1
  pre-delete node-3 sees 1
  pre-delete node-4 sees 1
alice deletes M1 on node-1
  DELETE returned HTTP 200
settle 15s for tombstone propagation
post-delete: verifying M1 is GONE from all peers
  post-delete node-2 sees 0 (expected 0)
  post-delete node-3 sees 0 (expected 0)
  post-delete node-4 sees 0 (expected 0)