alice writes M1 on node-1
bob writes M2 on node-2
  M1=7dc893a6-9d88-4907-a78a-276bde70e840 M2=5ffbb9f1-20f3-47a7-bfa2-bad777c1c007
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)