Function find_randomness_for_wots_encoding 

pub fn find_randomness_for_wots_encoding(
    message: &Message,
    epoch: u32,
    public_param: &PublicParam,
    rng: &mut impl CryptoRng,
) -> (Randomness, [u8; 42])

Draws randomness until it encodes validly.

The encoding is valid when both leftover bits are zero and the digits hit the target sum, so grinding takes fewer than 2^15 attempts on average.