Fix distance attenuation in lidar intensity

[Daniiiil1]

Summary

• apply inverse-square distance attenuation to simulated LiDAR intensity
• keep reflectivity distance-independent by matching the attenuation assumed by its existing calculation
• document the intensity model and add coverage for distance scaling and reflectivity

Root cause

The ray-tracing path scaled intensity by the incidence-angle cosine but did not account for geometric spreading. The reflectivity node already multiplied intensity by squared distance, assuming that intensity contained the reciprocal factor. As a result, intensity did not decay with range and derived reflectivity increased with distance.

Validation

• clang-format 14.0.6 --dry-run --Werror on all changed C++/CUDA files
• git diff --check
• analytical checks for inverse-square scaling and distance cancellation in reflectivity

The CUDA/OptiX test suite could not be executed locally because the development machine does not have an NVIDIA GPU or CUDA/OptiX toolchain. The added tests are intended to run in the repository's supported NVIDIA environment.

Fixes #367