Function generate_code 

pub fn generate_code<R: RandomSource>(
    policy: &CodePolicy,
    rng: &mut R,
) -> Result<PlainCode, RandomError>

Generate a fresh plaintext code under policy, drawing randomness from rng.

The algorithm (RFC-003 §4): for each position, read one random byte; accept it only if below the alphabet’s unbiased ceiling, then map alphabet[byte % len]; otherwise discard and redraw.

Errors

Returns RandomError if the RNG fails at any point. On error no code is produced; there is no fallback (INV-3).