[CCCL] 瘦身 + 补全: 移除 cudax/python/libcudacxx-tests 冗余文件, 新增 c2h 测试助手 + cmake 构建系统 + 8 个 CUDA thrust examples

变更摘要:
- 删除: cudax/ (783 files, 7.2M) — 实验性组件，竞赛不需要
- 删除: python/ (226 files, 2.0M) — Python 绑定，竞赛不需要
- 删除: libcudacxx/{test,benchmarks,codegen,cmake,share} (4432 files, 31M)
  保留: libcudacxx/include/ (1463 headers, cuda::std 编译依赖)
- 新增: c2h/ (27 files) — CUB Catch2 测试辅助头文件，编译 243 个测试必需
- 新增: cmake/ (29 files) — CCCL 原生 CMake 构建系统
- 新增: thrust/examples/cuda/ (7 files) + cpp_integration/ (1 file)
  async_reduce, custom_temporary_allocation, explicit_cuda_stream,
  global_device_vector, range_view, unwrap_pointer, wrap_pointer, device

结果: cccl_upstream 从 74M→35M (瘦身 53%), 核心内容 100% 保留:
  27/27 tuning headers, 78 benchmarks, 243 tests,
  60 thrust examples, 18 CUB examples, 全部编译头文件

cccl_upstream/python/cuda_cccl/README.md

@@ -1,49 +0,0 @@
-# CUDA CCCL Python Package
-
-[`cuda.cccl`](https://nvidia.github.io/cccl/python)
-provides a Pythonic interface to the
-[CUDA Core Compute Libraries](https://nvidia.github.io/cccl/cpp.html#cccl-cpp-libraries).
-It provides the following modules:
-
-- **`cuda.compute`** - Device-level parallel algorithms (reduce, scan, sort, etc.) and iterators
-- **`cuda.cccl.headers`** - Programmatic access to CCCL headers
-
-## Installation
-
-Install from PyPI:
-
-```bash
-pip install cuda-cccl[cu13]  # For CUDA 13.x (pip-installed cuda-toolkit)
-pip install cuda-cccl[cu12]  # For CUDA 12.x (pip-installed cuda-toolkit)
-```
-
-If you already have a CUDA toolkit on your system and do not want pip to
-install it, use the `sysctk` variants:
-
-```bash
-pip install cuda-cccl[sysctk13]  # For CUDA 13.x (system CUDA toolkit)
-pip install cuda-cccl[sysctk12]  # For CUDA 12.x (system CUDA toolkit)
-```
-
-For a minimal install without Numba (useful when supplying pre-compiled operators):
-
-```bash
-pip install cuda-cccl[minimal-cu13]      # pip-installed cuda-toolkit
-pip install cuda-cccl[minimal-sysctk13]  # system CUDA toolkit
-```
-
-Install from conda-forge:
-
-```bash
-conda install -c conda-forge cccl-python
-```
-
-**Requirements:** Python 3.10+, CUDA Toolkit 12.x or 13.x, NVIDIA GPU with Compute Capability 7.5+
-
-## Documentation
-
-For complete documentation, examples, and API reference, visit:
-
-- **Full Documentation**: [nvidia.github.io/cccl/python](https://nvidia.github.io/cccl/python)
-- **Repository**: [github.com/NVIDIA/cccl](https://github.com/NVIDIA/cccl)
-- **Examples**: [github.com/NVIDIA/cccl/tree/main/python/cuda_cccl/tests/compute/examples](https://github.com/NVIDIA/cccl/tree/main/python/cuda_cccl/tests/compute/examples)