Function kv_sliding_view 

pub fn kv_sliding_view<'a>(
    k_cache: &'a [f32],
    v_cache: &'a [f32],
    kv_dim: usize,
    window: Option<usize>,
) -> Result<(&'a [f32], &'a [f32]), EngineError>

Restrict KV to the last window tokens (HF sliding-window). None is a no-op.