Function anomaly_ratio 

pub fn anomaly_ratio(distance: f64, scale: f64) -> f64

Compute a non-negative anomaly score ratio with stable zero-denominator semantics.

When scale == 0.0:

• returns 0.0 if distance == 0.0 (on-center value),
• returns +inf if distance > 0.0 (off-center value under zero spread).