Function tac_bytes 

pub fn tac_bytes(
    data: &[u8],
    separator: u8,
    before: bool,
    out: &mut impl Write,
) -> Result<()>

Reverse records separated by a single byte. Uses backward SIMD scan (memrchr) — zero Vec allocation, single pass. Output uses write_vectored (writev) for zero-copy from mmap’d data.