Function integral_image 

pub fn integral_image(input: &Tensor) -> Result<Tensor, ImgProcError>

Compute integral image (summed area table). Input: [H, W]. Computes the integral (summed-area table) image from a [H, W, 1] tensor.

Uses a two-pass approach:

1. Horizontal prefix sums per row (parallelized for large images)
2. Vertical accumulation using SIMD