alice writes M1 on node-1
bob writes M2 on node-2
  M1=aa2ad1f3-af57-439b-89ea-42ab1d1cc3b8 M2=3f2db917-9c2e-4acd-aed6-569397da8955
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)