Function rej_bounded_poly 

pub fn rej_bounded_poly(
    rho_prime: &[u8; 64],
    nonce: u16,
    eta: usize,
) -> [i32; 256]

Generate a polynomial with small coefficients via rejection sampling.

Implements Algorithm 31 of FIPS 204 (RejBoundedPoly). Samples coefficients in [-eta, eta] by reading half-bytes from a SHAKE256 stream seeded with rho_prime || nonce. Invalid half-byte values are rejected.

• rho_prime: 64-byte secret seed.
• nonce: 16-bit counter distinguishing different polynomials.
• eta: coefficient bound (2 or 4 depending on parameter set).

Returns a polynomial with coefficients in [-eta, eta].