alice writes M1 content=v1-2ee0e2752d40470d9731a2b4c1afd1d0 on node-1
  M1 id=f70d5e1e-303d-4c89-bf8e-d26dad5d85f7
settle 5s for initial replication
bob updates M1 content=v2-eb6feea472d94cd2b3375e4cc37f6d77 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-eb6feea472d94cd2b3375e4cc37f6d77" agent_id="ai:alice"