alice writes M1 content=v1-ef9c602b1a7744148ae2b8aa33a4f1a4 on node-1
  M1 id=aab7d15f-2185-4853-998a-bf485edc8266
settle 5s for initial replication
bob updates M1 content=v2-570374eb157f4728bd9ea366f7e67c45 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="v2-570374eb157f4728bd9ea366f7e67c45" agent_id="ai:alice"