alice writes M1 content=d-cf8fa5728f554753a3531864bd35ea5d on node-1
  created memory id=5ce7e171-10dd-45a5-8df0-4b8ab1906b54
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)