alice writes M1 content=v1-53fc6966d92548fcb4832f71f9098c87 on node-1
  M1 id=20ae5cad-2c1d-416c-b990-62755d659334
settle 5s for initial replication
bob updates M1 content=v2-86e3b5cb14c54ba7a1b48e7ffff78a80 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="v2-86e3b5cb14c54ba7a1b48e7ffff78a80" agent_id="ai:alice"