feat(exporting): add native glTF 2.0 and GLB export

[JarrettSJohnson]

Summary

Implements the glTF/GLB portion of #497 by adding native glTF 2.0 and binary GLB export without the external COLLADA2GLTF conversion step. The exporter preserves geometry scale, vertex colors, normals, and transparency for sticks, spheres, cartoons, and surfaces.

Export is opt-in at build time with --json=true and requires nlohmann-json. Text labels and ellipsoids remain unsupported in this first pass.

Testing

• Added coverage for GLB containers, self-contained glTF, representations, colors, normals, transparency, and empty scenes.
• Validated exported files with the Khronos glTF Validator and three.js.
• Verified builds with and without --json=true.
• All CI checks pass.