alice writes M1 content=v1-48a4f58125b542cdbb1d392d72dbb502 on node-1
  M1 id=247af775-b33a-4d64-b56b-d4171f650c74
settle 5s for initial replication
bob updates M1 content=v2-a2e4f78b04a144659c5e495860dbc0f4 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-a2e4f78b04a144659c5e495860dbc0f4" agent_id="ai:alice"