alice writes M1 content=v1-8268a12d80fa42e99d723c41d03a8e3d on node-1
  M1 id=e9169b84-08b7-4e73-9afe-f819c6c0e3a4
settle 5s for initial replication
bob updates M1 content=v2-644d09c6424d4d90aba7280c8ba5e017 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="v2-644d09c6424d4d90aba7280c8ba5e017" agent_id="ai:alice"