Function ios_value 

pub fn ios_value(a: &BoundingBox, b: &BoundingBox) -> f32

Intersection-over-Smaller value in [0, 1]. Mirrors ModelPack _match(metric='ios'): inter / max(min(area_a, area_b), 1e-9). Used by the tiled-detection merge: a seam-split object has low IoU but high IoS.