alice writes M1 content=v1-e1a72cfb3ed646469eea1caa40679145 on node-1
  M1 id=a9284cc9-84a7-4be7-b2a0-1ccd0d103f46
settle 5s for initial replication
bob updates M1 content=v2-81e3693fb5704a778d9ac14825516bb0 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="v2-81e3693fb5704a778d9ac14825516bb0" agent_id="ai:alice"