alice writes M1 content=v1-4836bce082bc42e8bacb63bb0e71c165 on node-1
  M1 id=37a540a6-7cdf-402f-80b5-ac0250d44f1c
settle 5s for initial replication
bob updates M1 content=v2-bf917f0673ea442dbd3acb6bf68690db 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-bf917f0673ea442dbd3acb6bf68690db" agent_id="ai:alice"