alice writes M1 content=v1-586762cb8daa42efa0208c5a31d31062 on node-1
  M1 id=f1943759-1f91-4fd6-8491-00436195f1c1
settle 5s for initial replication
bob updates M1 content=v2-2ceb73b07e5f4fb195a63f26fba11633 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="v2-2ceb73b07e5f4fb195a63f26fba11633" agent_id="ai:alice"