Function paste_parallel_stream 

pub fn paste_parallel_stream(
    file_data: &[&[u8]],
    config: &PasteConfig,
) -> Result<()>

Streaming paste for the parallel (normal) mode. Scans each file line-by-line with memchr on-the-fly — no pre-split offset arrays. Uses a single 2MB output buffer with raw fd writes.