Function dense_gemv 

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

2026-09-25: Dense BF16 GEMV for one row, C = A @ B^T: A [1, K], B [N, K], C [1, N]. Each 256-thread block computes 4 outputs, 64 threads per output (kernel dense_gemv_bf16(A, B, C, N, K)).