alice writes M1 content=v1-60a055220c8d4ad599c1ddeea23bb537 on node-1
  M1 id=a12dd953-ee41-4209-beae-740179f9f26d
settle 5s for initial replication
bob updates M1 content=v2-990c94dda59a4337a17ac3490a387a7f 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="v2-990c94dda59a4337a17ac3490a387a7f" agent_id="ai:alice"