alice writes M1 content=v1-4a1e8354202a4664853bf35334197a2a on node-1
  M1 id=dc8f9b47-96ef-428e-90f2-07441bf24d34
settle 5s for initial replication
bob updates M1 content=v2-38981504ada9499494b3e03ea9682809 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="v2-38981504ada9499494b3e03ea9682809" agent_id="ai:alice"