alice writes M1 content=v1-8076a31b5f6845bdaf9190ad7f138899 on node-1
  M1 id=00cf0479-2322-4643-92ce-a9bc092f10cc
settle 5s for initial replication
bob updates M1 content=v2-3c708791c9f14e019cf281fba429222e on node-2 via PUT
  PUT returned HTTP 200
settle 8s for update fanout
charlie reads M1 on node-3 and checks content + provenance
  charlie sees content="v2-3c708791c9f14e019cf281fba429222e" agent_id="ai:alice"