alice writes M1 on node-1
bob writes M2 on node-2
  M1=e4873ae2-400f-4934-ad9a-3c61b5090c7a M2=c234b557-a42a-4526-9f02-bfa44eb6c657
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)