Function pack_vertices_from_xy 

pub fn pack_vertices_from_xy(
    device: &Arc<Device>,
    queue: &Arc<Queue>,
    inputs: &StairsGpuInputs,
    params: &StairsGpuParams,
) -> Result<GpuVertexBuffer, String>

Builds a GPU-resident vertex buffer for stairs/step plots directly from provider-owned XY arrays.