alice writes M1 on node-1
bob writes M2 on node-2
  M1=6a534cb7-adb5-4217-a617-74dd46ec4338 M2=26097f8c-9b1c-4df5-8f41-abc6f654e048
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)