Function process_cut_reader 

pub fn process_cut_reader<R: BufRead>(
    reader: R,
    cfg: &CutConfig<'_>,
    out: &mut impl Write,
) -> Result<()>

Process input from a reader (for stdin). Uses batch reading: reads large chunks (4MB), then processes them in batch using the fast mmap-based paths, avoiding per-line read_until syscall overhead.