alice writes M1 on node-1
bob writes M2 on node-2
  M1=2e6897e6-aadb-4abb-a1ff-79642bfd00fd M2=4c4b2dea-fc7d-4b4f-b076-441c8ee6f14c
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)