alice writes M1 on node-1
bob writes M2 on node-2
  M1=8f1b25be-a6f4-4c6e-b6d3-5e07994c6718 M2=9b3d2a7e-cd75-4e9f-9a02-5a7e868b1e3f
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)