feat: upgrade to C++20 standard

- CMakeLists.txt: CMAKE_CXX_STANDARD 17 → 20
- Add target_compile_features(cxx_std_20)
- Remove obsolete CUDA/ONNX/TensorRT options
- README: update C++ badges to C++20

Build verified: 0 errors with GCC 11.4 C++20

README.md

@@ -1,9 +1,9 @@
 # ViewDesignEngine — 高性能 CAD 几何引擎
 
-C++17 全栈 B-Rep 几何内核。对标 Parasolid/ACIS，覆盖曲面建模、网格处理、格式交换、CAM 制造、工程图、装配体。
+C++20 全栈 B-Rep 几何内核。对标 Parasolid/ACIS，覆盖曲面建模、网格处理、格式交换、CAM 制造、工程图、装配体。
 
 [![Version](https://img.shields.io/badge/version-1.0.0--beta.1-blue)](CHANGELOG.md)
-[![C++](https://img.shields.io/badge/C%2B%2B-17-blue)](CMakeLists.txt)
+[![C++](https://img.shields.io/badge/C%2B%2B-20-blue)](CMakeLists.txt)
 [![License](https://img.shields.io/badge/license-Apache%202.0-green)](LICENSE)
 [![Build](https://img.shields.io/badge/build-0%20errors-brightgreen)]()
 [![Tests](https://img.shields.io/badge/tests-1000%2B-brightgreen)](tests/)
@@ -20,7 +20,7 @@ docker run --rm -v $(pwd):/ws -w /ws vde-builder:latest \
 cd build && ctest --output-on-failure -E Benchmark
 ```
 
-**依赖**: CMake ≥ 3.16, C++17, Eigen 3, GoogleTest（自动下载）
+**依赖**: CMake ≥ 3.16, C++20, Eigen 3, GoogleTest（自动下载）
 
 ## 模块总览