Function render_gaussians_cpu 

pub fn render_gaussians_cpu(
    scene: &GaussianScene,
    camera: &GaussianCamera,
) -> SceneResult<GaussianFramebuffer>

Renders a Gaussian scene with a CPU alpha soft-splat (front-to-back).

Each primitive is projected with the pinhole camera, sorted by depth, and composited with a screen-space isotropic Gaussian whose radius tracks the projected max scale. This is a portable reference path; a GPU rasterizer can later share the same GaussianScene container.