Function standard_normal_quantile 

pub fn standard_normal_quantile(p: f64) -> Result<f64, String>

Standard normal quantile Φ⁻¹(p) using Acklam’s rational approximation. Implementation lives in gam-math; re-exported to keep crate::probability::standard_normal_quantile resolving for all existing callers. Standard normal quantile Φ⁻¹(p) using Acklam’s rational approximation.