fix: Implement proper skybox rendering with Perlin noise

- Added ObjectType::SKYBOX for dedicated skybox rendering.
- Created assets/final/shaders/skybox.wgsl for background rendering.
- Implemented a two-pass rendering strategy in Renderer3D::render:
    - First pass renders the skybox without depth writes.
    - Second pass renders scene objects with depth testing.
- Corrected GlobalUniforms struct in common_uniforms.wgsl and src/3d/renderer.h to include  and explicit padding for 112-byte alignment.
- Updated Renderer3D::update_uniforms to set the new  and zero-initialize padding.
- Reverted sky sampling logic in renderer_3d.wgsl to  for SDF misses, preventing background bleed-through.
- Updated test_3d_render.cc to include a SKYBOX object with Perlin noise.

handoff(Gemini): The skybox is now correctly rendered with Perlin noise as a dedicated background pass. Objects render correctly without transparency to the sky. All necessary C++ and WGSL shader changes are implemented and verified.

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