alice writes M1 on node-1
bob writes M2 on node-2
  M1=0e404a7a-5cb1-4a20-85d4-a89430dd9c36 M2=d998d4fb-cc22-40a6-9bba-f0ff2e8b6acc
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)