Function quantile_sorted 

pub fn quantile_sorted(sorted_data: &[f64], p: f64) -> Option<f64>

Computes the p-th quantile on pre-sorted data (R-7 method).

This avoids the O(n log n) sort when calling multiple quantiles on the same dataset. The caller must guarantee that sorted_data is sorted in non-decreasing order.

Returns

• None if sorted_data is empty or p is outside [0, 1].