alice writes M1 content=v1-66a5c0664c6349c3adbc4f7938111b1e on node-1
  M1 id=f04c3b14-5a27-4ed3-981b-37f5cdffdb67
settle 5s for initial replication
bob updates M1 content=v2-a2b539f898eb4006b831de93b4cf2fee 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-a2b539f898eb4006b831de93b4cf2fee" agent_id="ai:alice"