Constant MERSENNE31_POSEIDON2_HALF_FULL_ROUNDS 

pub const MERSENNE31_POSEIDON2_HALF_FULL_ROUNDS: usize = 4;

Number of full rounds per half for Mersenne31 Poseidon2 (RF / 2).

The total number of full rounds is RF = 8 (4 beginning + 4 ending). Follows the Poseidon2 paper’s security analysis with a +2 RF margin.