alice writes M1 content=v1-d26cf62d024e4491aac80330a8fa68e0 on node-1
  M1 id=73faba3f-3b27-407f-b16a-f9a5fd546e19
settle 5s for initial replication
bob updates M1 content=v2-b86560f6451945ccbbf1073a710dfd72 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-b86560f6451945ccbbf1073a710dfd72" agent_id="ai:alice"