Function generate_permutation 

pub fn generate_permutation(
    perm: &mut [u16],
    rng: &mut impl CryptoRng,
) -> Result<(), MlKemError>

Generate a uniform random permutation of 0..n using Fisher-Yates.

Iterates from the last index down to 1, swapping each element with a uniformly chosen earlier (or equal) element. Uses rejection sampling on 16-bit random values to avoid modular bias.

Arguments

• perm - Output slice of length n, initialized to [0, 1, ..., n-1] then shuffled in place.
• rng - A cryptographic RNG for generating swap indices.

Errors

Returns MlKemError::RngFailure if the RNG fails.