Function w4a16_gemv_sw 

pub fn w4a16_gemv_sw(
    gpu: &dyn GpuBackend,
    kernel: KernelHandle,
    input: DevicePtr,
    weight: &QuantizedWeight,
    output: DevicePtr,
    n: u32,
    k: u32,
    stream: u64,
) -> Result<()>

2026-09-25: Single-warp-per-output W4A16 GEMV for M=1, grid ceil(N/8).