alice writes M1 on node-1
bob writes M2 on node-2
  M1=cfdbae33-198c-4e85-8f5a-67ad1b4a8968 M2=2465a19b-3d19-48ea-abcb-2ef0d3a6e726
settle 5s for pre-link replication
alice links M1 -> M2 with relation=related_to
  link POST returned HTTP 201
settle 8s for link fanout
charlie queries links of M1 on node-3
  charlie sees M1->M2 link: 1 (expected >=1)