Function fetch_token_embedding 

pub fn fetch_token_embedding<'a>(
    mmap: &'a [u8],
    tensor_data_start: u64,
    tensor: &GgufTensorInfo,
    token_id: u32,
) -> Result<&'a [u8], ExecutionError>

Return a zero-copy &[u8] slice of the packed embedding row for token_id.