alice writes M1 on node-1
bob writes M2 on node-2
  M1=c97a6525-f59e-4284-92ae-a214a5186b95 M2=6580e4b3-4aa1-41e4-9826-e3eff78e8bc1
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)