Function glm_hc_expand 

pub fn glm_hc_expand(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    hidden: DevicePtr,
    streams: DevicePtr,
    num_tokens: u32,
    hidden_size: u32,
    hc_mult: u32,
    stream: u64,
) -> Result<()>

2026-09-25: Broadcast each of num_tokens BF16 hidden rows into its hc_mult FP32 highway streams.