alice writes M1 content=v1-74488aa7ab1b447999947b26ec7644a2 on node-1
  M1 id=9d4d5192-1450-4633-9366-54ae2b3c4531
settle 5s for initial replication
bob updates M1 content=v2-51e75e4c30e74de785f176b696439dc1 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="v2-51e75e4c30e74de785f176b696439dc1" agent_id="ai:alice"