[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/cudax/test/stf/dot/sections_stackable.cu

@@ -1,61 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of CUDASTF in CUDA C++ Core Libraries,
-// under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-// SPDX-FileCopyrightText: Copyright (c) 2022-2024 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-/**
- * @file
- * @brief This test makes sure we can generate a dot file with sections and stackable contexts
- */
-
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-int main()
-{
-// TODO (miscco): Make it work for windows
-#if !_CCCL_COMPILER(MSVC)
-  // Test configuration constants
-  constexpr size_t buffer_size    = 64;
-  constexpr size_t num_iterations = 3;
-
-  stackable_ctx ctx;
-
-  // Create logical data for the computation pipeline
-  auto lA = ctx.logical_data(shape_of<slice<char>>(buffer_size));
-  auto lB = ctx.logical_data(shape_of<slice<char>>(buffer_size));
-  auto lC = ctx.logical_data(shape_of<slice<char>>(buffer_size));
-
-  // Initialize all logical data in a dedicated DOT section
-  auto r_init = ctx.dot_section("init");
-  ctx.task(lA.write()).set_symbol("initA")->*[](cudaStream_t, auto) {};
-  ctx.task(lB.write()).set_symbol("initB")->*[](cudaStream_t, auto) {};
-  ctx.task(lC.write()).set_symbol("initC")->*[](cudaStream_t, auto) {};
-  r_init.end();
-
-  // Test nested DOT sections with stackable contexts
-  // This creates a hierarchical structure to verify DOT graph generation
-  for (size_t j = 0; j < num_iterations; j++)
-  {
-    auto r0 = ctx.dot_section("lvl0");
-    ctx.task(lA.rw()).set_symbol("f1")->*[](cudaStream_t, auto) {};
-
-    ctx.push();
-    {
-      auto r1 = ctx.dot_section("lvl1");
-      ctx.task(lA.read(), lB.rw()).set_symbol("f2")->*[](cudaStream_t, auto, auto) {};
-      ctx.task(lA.read(), lC.rw()).set_symbol("f2")->*[](cudaStream_t, auto, auto) {};
-      ctx.task(lB.read(), lC.read(), lA.rw()).set_symbol("f3")->*[](cudaStream_t, auto, auto, auto) {};
-    }
-    ctx.pop();
-  }
-  ctx.finalize();
-
-#endif // !_CCCL_COMPILER(MSVC)
-}