Function hc_pre_lowrank 

pub fn hc_pre_lowrank(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    streams: DevicePtr,
    w: &HcLowRank,
    y_out: DevicePtr,
    inj_out: DevicePtr,
    scratch: DevicePtr,
    num_tokens: u32,
    hidden_size: u32,
    hc_mult: u32,
    norm_eps: f32,
    stream: u64,
) -> Result<()>

2026-09-25: Collapse the hc_mult streams to one, and emit the per-stream injection weights the matching hc_post_lowrank needs: streams [T, hc, H] -> y_out [T, H] BF16, inj_out [T, hc] FP32. Fails when w.inject_w is null (a site without an injection weight is the model-level mixer, which uses hc_head_lowrank).