features2d: implement ORB image pyramid

[kalwalt]

Implement the image pyramid logic required for ORB keypoint detection.

• Add pure-Rust pyramid builder as a function, reusing or extending any existing pyramid functionality in imgproc if possible.
• Result should be Vec<Matrix<u8>> with correct scale factor and level count.
• Ensure scale metadata is available for correct keypoint scaling.

Starter API:

pub fn build_orb_pyramid(image: &Matrix<u8>, nlevels: usize, scale_factor: f32) -> Result<Vec<Matrix<u8>>> {
    // ...
}

• Add tests for correct number of pyramid levels and scaling.

References: https://docs.opencv.org/4.10.0/d1/d89/classcv_1_1ORB.html