alice writes M1 content=v1-d65b9d890a69456da21069af765780e9 on node-1
  M1 id=92624bee-e196-4c35-8b3e-3108ba269133
settle 5s for initial replication
bob updates M1 content=v2-a256b546781846dba97d140a50a9bcc0 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-a256b546781846dba97d140a50a9bcc0" agent_id="ai:alice"