Function pbrs_simd 

pub fn pbrs_simd(
    rewards: &[f64],
    phi_current: &[f64],
    phi_next: &[f64],
    gamma: f64,
    dones: &[f64],
) -> Vec<f64>

SIMD-friendly PBRS: result[i] = rewards[i] + gamma * phi_next[i] - phi_current[i]

At episode boundaries (dones[i] == 1.0), returns the raw reward. Processes 4 f64s at a time (256-bit AVX2 = 4 x f64).

Panics

Panics if all slices are not the same length.