Function hc_post_lowrank 

pub fn hc_post_lowrank(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    block_out: DevicePtr,
    residual: DevicePtr,
    inj: DevicePtr,
    out: DevicePtr,
    num_tokens: u32,
    hidden_size: u32,
    hc_mult: u32,
    stream: u64,
) -> Result<()>

2026-09-25: Inject the block output back into every stream: out[t, s*H + d] = residual[t, s*H + d] + block_out[t, d] * inj[t, s]. There is no comb argument: this family scales each stream by one scalar.