alice writes M1 content=v1-53e7b96d32434b60a9248631f2cad593 on node-1
  M1 id=6b5e6263-021b-47bf-aff2-c5b27dd373ff
settle 5s for initial replication
bob updates M1 content=v2-26c23c1ec08647d58e4a45a825405e1a 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="v2-26c23c1ec08647d58e4a45a825405e1a" agent_id="ai:alice"