alice writes M1 content=v1-d76e02386598470cb83993b2571dfd3f on node-1
  M1 id=b14e8f56-ca5c-428b-b928-380b1afda09f
settle 5s for initial replication
bob updates M1 content=v2-ade4b5e158f14a0c8a807c6b261f5d9d 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-ade4b5e158f14a0c8a807c6b261f5d9d" agent_id="ai:alice"