[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/tests/headers/test_cuda_cccl.py

@@ -1,64 +0,0 @@
-import pytest
-
-import cuda.cccl as cccl
-
-
-def test_version():
-    v = cccl.__version__
-    assert isinstance(v, str)
-
-
-@pytest.fixture
-def inc_paths():
-    return cccl.get_include_paths()
-
-
-def test_headers_has_cuda(inc_paths):
-    assert hasattr(inc_paths, "cuda")
-
-
-def test_headers_has_cub(inc_paths):
-    assert hasattr(inc_paths, "cub")
-
-
-def test_headers_has_cudacxx(inc_paths):
-    assert hasattr(inc_paths, "libcudacxx")
-
-
-def test_headers_has_thrust(inc_paths):
-    assert hasattr(inc_paths, "thrust")
-
-
-def test_headers_as_tuple(inc_paths):
-    tpl = inc_paths.as_tuple()
-    assert len(tpl) == 4
-
-    thrust_, cub_, cudacxx_, cuda_ = tpl
-    assert cuda_ == inc_paths.cuda
-    assert cub_ == inc_paths.cub
-    assert cudacxx_ == inc_paths.libcudacxx
-    assert thrust_ == inc_paths.thrust
-
-
-def test_cub_version(inc_paths):
-    cub_dir = inc_paths.cub / "cub"
-    cub_version = cub_dir / "version.cuh"
-    assert cub_version.exists()
-
-
-def test_thrust_version(inc_paths):
-    thrust_dir = inc_paths.thrust / "thrust"
-    thrust_version = thrust_dir / "version.h"
-    assert thrust_version.exists()
-
-
-def test_cudacxx_version(inc_paths):
-    cudacxx_dir = inc_paths.libcudacxx / "cuda"
-    cudacxx_version = cudacxx_dir / "version"
-    assert cudacxx_version.exists()
-
-
-def test_nv_target(inc_paths):
-    nv_dir = inc_paths.libcudacxx / "nv"
-    nv_target = nv_dir / "target"
-    assert nv_target.exists()