Function confidence_interval 

pub fn confidence_interval(
    mean: f64,
    stddev: f64,
    confidence_level: f64,
) -> (f64, f64)

Calculate confidence interval bounds

Returns (lower_bound, upper_bound) for the given confidence level. Common confidence levels:

• 0.90 = 90% confidence (z = 1.645 one-tailed, 1.96 two-tailed)
• 0.95 = 95% confidence (z = 1.645 one-tailed, 1.96 two-tailed)
• 0.99 = 99% confidence (z = 2.326 one-tailed, 2.576 two-tailed)