alice writes M1 content=v1-15e35d2489f447ebb78774b4ea835e7d on node-1
  M1 id=054907c2-a079-4f52-9560-012fd21c068c
settle 5s for initial replication
bob updates M1 content=v2-307767e96fb544a78dd6c570a01108fe 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="v2-307767e96fb544a78dd6c570a01108fe" agent_id="ai:alice"