alice writes M1 content=v1-1e3ee78ed1a349b8b5b8882381c61e43 on node-1
  M1 id=ddda2e95-a72d-4741-87b2-8fd58478b422
settle 5s for initial replication
bob updates M1 content=v2-874c2dad1fe64f5fb71e74f6e3fa7ea9 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="v2-874c2dad1fe64f5fb71e74f6e3fa7ea9" agent_id="ai:alice"