alice writes M1 content=v1-c790bf02fac441f28411374094bbcd39 on node-1
  M1 id=1cd37850-29ba-400c-ba88-a26ba72e1c64
settle 5s for initial replication
bob updates M1 content=v2-da72a13025cf41f0ad429620df9c77d9 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-da72a13025cf41f0ad429620df9c77d9" agent_id="ai:alice"