alice writes M1 content=v1-78e40dfee5e94a8fac7dd9c5dd292c3d on node-1
  M1 id=59becf2c-e0b2-4202-9013-cd4f92cc0d5c
settle 5s for initial replication
bob updates M1 content=v2-368777c06120465c99ec8b157e6da17c 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="v2-368777c06120465c99ec8b157e6da17c" agent_id="ai:alice"