alice writes M1 content=v1-2fbccde5680547c1a16382427c37db9b on node-1
  M1 id=91381da1-6546-4b0e-85cd-1dc926a88844
settle 5s for initial replication
bob updates M1 content=v2-4db3ac014a744446a9e39350fbb80e12 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="v2-4db3ac014a744446a9e39350fbb80e12" agent_id="ai:alice"