[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/execution/test_visit.cu

@@ -1,81 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of CUDA Experimental 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) 2024 NVIDIA CORPORATION & AFFILIATES.
-//
-//===----------------------------------------------------------------------===//
-
-#include <cuda/std/__algorithm/max.h>
-
-#include <cuda/experimental/execution.cuh>
-
-#include "testing.cuh" // IWYU pragma: keep
-
-namespace
-{
-struct S0
-{};
-struct S1
-{
-  int a;
-};
-struct S2
-{
-  int a, b;
-};
-static_assert(cudax_async::structured_binding_size<S0> == 0);
-static_assert(cudax_async::structured_binding_size<S1> == 1);
-static_assert(cudax_async::structured_binding_size<S2> == 2);
-
-template <class Fn>
-struct recursive_lambda
-{
-  template <class... Args>
-  auto operator()(Args&&... args)
-  {
-    return fn(*this, cuda::std::forward<Args>(args)...);
-  }
-
-  Fn fn;
-};
-
-template <class Fn>
-recursive_lambda(Fn) -> recursive_lambda<Fn>;
-
-C2H_TEST("sender visitation API works", "[visit]")
-{
-  int leaves = 0;
-  int depth  = 0;
-
-  auto snd = cudax_async::when_all(
-    cudax_async::just(3), //
-    cudax_async::just(0.1415),
-    cudax_async::then(cudax_async::just(0.1415), [](double f) {
-      return f;
-    }));
-  auto snd1 = std::move(snd) | cudax_async::then([](int x, double y, double z) {
-                return x + y + z;
-              });
-
-  auto count_leaves = recursive_lambda{[](auto& self, int& leaves, auto, auto&, auto&... child) {
-    leaves += (sizeof...(child) == 0);
-    ((cudax_async::visit(self, child, leaves)), ...);
-  }};
-
-  cudax_async::visit(count_leaves, snd1, leaves);
-  CHECK(leaves == 3);
-
-  auto max_depth = recursive_lambda{[i = 0](auto& self, int& depth, auto, auto&, auto&... child) mutable {
-    ++i;
-    depth = cuda::std::max(depth, i);
-    ((cudax_async::visit(self, child, depth)), ...);
-    --i;
-  }};
-
-  cudax_async::visit(max_depth, snd1, depth);
-  CHECK(depth == 4);
-}
-} // namespace