Function moe_expert_gemv_silu_down 

pub fn moe_expert_gemv_silu_down(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    gate_out: DevicePtr,
    up_out: DevicePtr,
    packed_ptrs: DevicePtr,
    scale_ptrs: DevicePtr,
    scale2_vals: DevicePtr,
    output: DevicePtr,
    expert_indices: DevicePtr,
    n: u32,
    k: u32,
    top_k: u32,
    stream: u64,
) -> Result<()>

2026-09-25: Down expert GEMV whose activation silu(gate) * up is computed inline from gate_out and up_out.