alice writes M1 on node-1
bob writes M2 on node-2
  M1=081e79a0-2691-4860-8fff-5634c75b0fb4 M2=e1711d53-9f93-4b1e-a47e-dac71a8d46ed
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)