alice writes M1 content=v1-1c4055e491374a5a8a1138a10f88cdc6 on node-1
  M1 id=0b8e0103-c7c5-4686-a6e2-bb1fb3af97f0
settle 5s for initial replication
bob updates M1 content=v2-d8b1969f865041698d524713a7b3edcf 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-d8b1969f865041698d524713a7b3edcf" agent_id="ai:alice"