Function candidates 

pub fn candidates(
    records: &[Record],
    frame: Box4,
    rotation: i32,
    images: &[Box4],
    visibility: &[Visibility],
    overlap: f64,
) -> Option<Candidates>

批量应用字符过滤、视觉裁剪与图像并集覆盖；未知可见性沿用参考的保守可见默认值。