alice writes M1 content=v1-df8df9c593a14e4687e766ad022d5bad on node-1
  M1 id=8a29c9b5-d6bf-4e0e-b526-e9514808de9a
settle 5s for initial replication
bob updates M1 content=v2-65d9bac4050744b2b3bf88803fb48e74 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="v2-65d9bac4050744b2b3bf88803fb48e74" agent_id="ai:alice"