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 using backward memrchr scanning. Scans backward from end, writes each record as found. No Vec allocation. Uses a backward-fill buffer: records are copied backward into an 8MB buffer, then flushed as a single write_all when full.