Detect evidence dependence and conflict

Implement evidence_matrix(records, claims). Records contain claim, origin, and stance (support or oppose). For each requested claim, return sorted unique support and oppose origins and a state: contested if both sets are nonempty; corroborated if at least two support origins and no opposition; single_origin for exactly one support origin; opposed for opposition only; unsupported for neither. Ignore unrequested claims. Input records are already relevant and temporally eligible. This is an evidence bookkeeping algorithm, not automated fact verification.