Function expand_kv 

pub fn expand_kv(
    kv_c: &[f32],
    w_kv_b: &[f32],
    dims: DsaDims,
    seq: usize,
) -> (Vec<f32>, Vec<f32>)

Expand the compressed latent into per-head K and V.

🪤 NoPE: k_rot is zero-width, so the key_states[..., nope:] copy HF performs is a no-op. Reproducing it as a padded RoPE section would change qk_head_dim and the scale with it.