alice writes M1 content=v1-174e19fba5cd4ef3a4002e3ba6a26760 on node-1
  M1 id=7a60fee2-9736-4d9b-9cde-ee68eb269973
settle 5s for initial replication
bob updates M1 content=v2-71df055e1f4b4e399926bd42d1b04161 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="v2-71df055e1f4b4e399926bd42d1b04161" agent_id="ai:alice"