Function hc_pre 

pub fn hc_pre(
    x: &[f32],
    pre: &[f32],
    tokens: usize,
    hc: usize,
    dim: usize,
) -> Vec<f32>

2026-09-25: Collapse the copies, y[d] = sum_c pre[c] * x[c][d], back to bf16.