Function hc_pre_site 

pub fn hc_pre_site(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    streams: DevicePtr,
    site: &HcSiteWeights,
    hc: &HcWeights,
    y_out: DevicePtr,
    post_out: DevicePtr,
    comb_out: DevicePtr,
    scratch: DevicePtr,
    num_tokens: u32,
    hidden_size: u32,
    norm_eps: f32,
    stream: u64,
) -> Result<()>

2026-09-25: Collapse the streams to one and emit what the matching hc_post needs: post and comb for Sinkhorn, the injection vector in post_out for low-rank.