alice writes M1 content=v1-9df48b69bd9049d39773cc0bfe36ad49 on node-1
  M1 id=c001fff2-2b58-4e3c-8335-0c59d3d6e8a6
settle 5s for initial replication
bob updates M1 content=v2-4c0d1732816349039b8d94deee5bf1eb 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="v2-4c0d1732816349039b8d94deee5bf1eb" agent_id="ai:alice"