alice writes M1 on node-1
bob writes M2 on node-2
  M1=401122cf-e639-4f47-b3ca-291396aa04c1 M2=28d70bd7-c857-4203-b285-f6b7b4c46735
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)