alice writes M1 content=v1-5632a76dcd4b4e959367686da88498af on node-1
  M1 id=55ae4f6f-6c8a-4c0d-a85f-a40dc1589e03
settle 5s for initial replication
bob updates M1 content=v2-1cf078c3af9240a9b438d9ddf431c0f2 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="v2-1cf078c3af9240a9b438d9ddf431c0f2" agent_id="ai:alice"