alice writes M1 content=v1-4a563ab8db6b43f0b27bd227f9528efe on node-1
  M1 id=e90c4fd2-6cd5-4c29-bbc8-09b7553c8c11
settle 5s for initial replication
bob updates M1 content=v2-cd692076e4924636b413bbd8cd110127 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-cd692076e4924636b413bbd8cd110127" agent_id="ai:alice"