alice writes M1 on node-1
bob writes M2 on node-2
  M1=74421c8d-3d3d-4222-b8ee-83bf0eb129db M2=8bbed542-f605-489d-bbc4-b6cbf5ec5e01
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)