alice writes M1 content=v1-cb63e600458b4ed7900268a28417df26 on node-1
  M1 id=ef664272-5439-47c2-ae3b-d78d3b5ab28d
settle 5s for initial replication
bob updates M1 content=v2-7ca004c62f1a4a3a8e7a7da6d87e3879 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="v2-7ca004c62f1a4a3a8e7a7da6d87e3879" agent_id="ai:alice"