alice writes M1 on node-1
bob writes M2 on node-2
  M1=94dac2d2-7cbc-4e8b-a9a9-087f91ceafb8 M2=1b0c9c5d-398e-43a4-a980-0b80a72cd939
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)