alice writes M1 content=v1-861cd5fbd5c14e5b9ee30487d574e949 on node-1
  M1 id=aeea1e58-ddb4-4e2b-9e1d-79decd752bab
settle 5s for initial replication
bob updates M1 content=v2-0d3a5bad071f476ab05ed7748108c756 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="v2-0d3a5bad071f476ab05ed7748108c756" agent_id="ai:alice"