[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/cpp/read_const.cu

@@ -1,93 +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 ensures that read() can be called on const logical_data
- *        (typed and untyped).
- */
-
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-struct foo
-{
-  // Intentionally choose an odd (actually prime) size
-  foo(context& ctx)
-  {
-    l = ctx.logical_data(shape_of<slice<int>>(50867));
-  }
-
-  void set(context& ctx, int val)
-  {
-    ctx.parallel_for(l.shape(), l.write())->*[=] _CCCL_DEVICE(size_t i, auto dl) {
-      dl(i) = val;
-    };
-  }
-
-  void copy_from(context& ctx, const foo& other)
-  {
-    ctx.parallel_for(l.shape(), l.write(), other.l.read())->*[=] _CCCL_DEVICE(size_t i, auto dl, auto dotherl) {
-      dl(i) = dotherl(i);
-    };
-  }
-
-  void ensure(context& ctx, int val)
-  {
-    std::ignore = val;
-    ctx.parallel_for(l.shape(), l.read())->*[=] _CCCL_DEVICE(size_t i, auto dl) {
-      assert(dl(i) == val);
-    };
-  }
-
-  auto& get_l() const
-  {
-    return l;
-  }
-
-  logical_data<slice<int>> l;
-};
-
-void read_only_access(context& ctx, const foo& f)
-{
-  ctx.parallel_for(f.l.shape(), f.l.read())->*[] _CCCL_DEVICE(size_t i, auto dl) {
-    // no-op
-  };
-
-  ctx.parallel_for(f.get_l().shape(), f.get_l().read())->*[] _CCCL_DEVICE(size_t i, auto dl) {
-    // no-op
-  };
-}
-
-void read_only_access_untyped(const logical_data_untyped& ld)
-{
-  auto dep = ld.read();
-  (void) dep;
-}
-
-int main()
-{
-  context ctx;
-
-  foo A(ctx);
-  A.set(ctx, 42);
-  A.ensure(ctx, 42);
-  foo B(ctx);
-  B.copy_from(ctx, A);
-  B.ensure(ctx, 42);
-
-  read_only_access(ctx, A);
-
-  const logical_data_untyped& ld_untyped = A.l;
-  read_only_access_untyped(ld_untyped);
-
-  ctx.finalize();
-}