alice writes M1 on node-1
bob writes M2 on node-2
  M1=aa7d1f80-94d9-4b6d-9679-8fb596b803e9 M2=8d05cde5-e952-49c7-bfe3-def179a4a56a
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)