Function tac_string_separator 

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

Reverse records using a multi-byte string separator. Uses SIMD-accelerated memmem + write_all output.

For single-byte separators, delegates to tac_bytes which uses memchr (faster).