alice writes M1 content=v1-1add2edcee854573b4218c125a53d51d on node-1
  M1 id=7d713cb1-b837-42a7-8af9-c61b6fa027b5
settle 5s for initial replication
bob updates M1 content=v2-c5180a529c0b4e16ad712487fcad43c5 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-c5180a529c0b4e16ad712487fcad43c5" agent_id="ai:alice"