pub fn percentile(values: &[f64], p: u32) -> Option<f64>
2026-09-26: p-th percentile (0–100) of values, nearest rank: idx = min(int(n*p/100 + 0.5), n-1) over the sorted finite values.
p
values
idx = min(int(n*p/100 + 0.5), n-1)