alice writes M1 content=v1-8f31ac72f3a5488e945456896b12255a on node-1
  M1 id=19a26650-ea90-4215-a971-cb14959ee0b4
settle 5s for initial replication
bob updates M1 content=v2-488aaa760a8c41b8af5d9d87abc46e7b 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="v2-488aaa760a8c41b8af5d9d87abc46e7b" agent_id="ai:alice"