Function simd_sum_sq_diff_f64 

pub fn simd_sum_sq_diff_f64(data: &[f64], mean: f64) -> f64

SIMD-accelerated computation of sum((x - mean)²) for f64 slices.

Computes the sum of squared differences from the mean in a single pass without allocating an intermediate Vec. Uses 4 SIMD accumulators for ILP.