Function validate_bullwhip_effect 

pub fn validate_bullwhip_effect(
    demand_variance: f64,
    order_variance: f64,
    lead_time: f64,
    review_period: f64,
    tolerance: f64,
) -> Result<TpsTestResult, String>

Validates Bullwhip Effect amplification.

TC-4: Variance amplifies upstream in supply chain

Errors

This function always returns Ok. The Result type is for consistency.