Function classify_regions 

pub fn classify_regions(
    regions: &mut [Region],
    baseline: &RgbaImage,
    candidate: &RgbaImage,
    diff_mask: &[bool],
    width: u32,
    height: u32,
)

Classify each region by analyzing pixels in the baseline and candidate images.

Labels:

• “added”: region area is mostly uniform/empty in baseline
• “removed”: region area is mostly uniform/empty in candidate
• “color-change”: structure (edges) matches but colors differ
• “content-change”: structure differs (fallback)