Function linear_bf16 

pub fn linear_bf16(
    x: &[f32],
    rows: usize,
    in_f: usize,
    w: &[f32],
    out_f: usize,
) -> Vec<f32>

2026-09-25: bf16 linear: f32 accumulate, bf16 output. x: [rows, in], w: [out, in].