alice writes M1 on node-1
bob writes M2 on node-2
  M1=a94aafe1-46c0-4ac8-b294-c2c891c320cb M2=0b55a1dd-5d2a-4705-898b-3a0a5c035c09
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)