alice writes M1 content=v1-5cf0524104c04e7c8a217df2944706bb on node-1
  M1 id=37422595-f045-406e-8936-e3d39ac9d21b
settle 5s for initial replication
bob updates M1 content=v2-ebf00d26d70649228d4e89188556dab7 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-ebf00d26d70649228d4e89188556dab7" agent_id="ai:alice"