alice writes M1 on node-1
bob writes M2 on node-2
  M1=e07a7888-7bb3-423c-9951-7b80de13445a M2=8c9da4a3-044f-4b31-9e6c-0bd00084cd06
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)