alice writes claim: "sky-color-ab837f7d is blue" on node-1
bob writes contradicting claim: "sky-color-ab837f7d is red" on node-2
  alice.id=0d1168d8-b96f-4a99-846b-96ae197adea4 bob.id=d91010f0-e6e1-4a9f-81ee-7cfcec840448
settle 10s for quorum fanout + contradiction indexing
charlie queries /api/v1/contradictions on node-3
  HTTP 200
  sees both memories: True; sees contradicts link: True