alice writes M1 content=v1-4d9e5084f7064679ad0e0ca2489eacd3 on node-1
  M1 id=e8082c23-f79c-4e34-973b-fa3f3463c607
settle 5s for initial replication
bob updates M1 content=v2-a6818fa2ee5942609b14ab0a927b3452 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-a6818fa2ee5942609b14ab0a927b3452" agent_id="ai:alice"