Function std::conv2d

int conv2d(
    []const double img,
    unsigned long h,
    unsigned long w,
    []const double kernel,
    unsigned long kh,
    unsigned long kw,
    []double out,
)

'img' is hw row-major; 'kernel' is khkw row-major (kh, kw should be odd so the kernel has a well-defined center tap); 'out' must have room for h*w doubles (same size as 'img' — out-of-bounds kernel taps read as zero, i.e. zero-padded boundary handling).