pub fn skewness(values: &[f64]) -> Option<f64>
Sample skewness (Fisher–Pearson, g1 = m₃ / m₂^{3/2}), the standardised third moment. None if empty; Some(0.0) for a constant series (zero spread).
g1 = m₃ / m₂^{3/2}
None
Some(0.0)