Function validate_schedule_policy_target 

pub fn validate_schedule_policy_target(
    spec: &SnapshotScheduleSpec,
) -> ValidationResult

Exactly one of policyRef / policySelector is set on a SnapshotSchedule (ADR-0005 §10). Neither ⇒ MissingRequiredField; both ⇒ MutuallyExclusive. Pure so the XOR decision is unit-tested directly.