alice writes M1 on node-1
bob writes M2 on node-2
  M1=81d07697-2c88-4558-97ce-2086dd3d0d55 M2=25f678b9-eb14-440e-b8c4-a1060cb53f4d
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)