alice writes M1 content=v1-8ab071a0d10d435aa391750202d3d103 on node-1
  M1 id=d41ee9f2-e284-4d40-bdb2-2a49d8801cd1
settle 5s for initial replication
bob updates M1 content=v2-195661c818064cfc871cc4bcf988d010 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="v2-195661c818064cfc871cc4bcf988d010" agent_id="ai:alice"