alice writes M1 content=v1-08cd735ad98f43f28674743ecb3926db on node-1
  M1 id=fe211d50-a068-4bb4-8efd-041803709ef8
settle 5s for initial replication
bob updates M1 content=v2-d1b232a49c20490da5ba752ac623c532 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-d1b232a49c20490da5ba752ac623c532" agent_id="ai:alice"