[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/examples/stf/heat.cu

@@ -1,126 +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 An example solving heat equation with finite differences using the
- * parallel_for construct.
- *
- * A multi-gpu version is shown in the heat_mgpu.cu example.
- *
- * This example also illustrate how to annotate resources with set_symbol
- */
-
-#include <cuda/experimental/stf.cuh>
-
-using namespace cuda::experimental::stf;
-
-void dump_iter(slice<const double, 2> sUn, int iter)
-{
-  /* Create a binary file in the PPM format */
-  char name[64];
-  snprintf(name, 64, "heat_%06d.ppm", iter);
-  FILE* f = fopen(name, "wb");
-  fprintf(f, "P6\n%zu %zu\n255\n", sUn.extent(0), sUn.extent(1));
-  for (size_t j = 0; j < sUn.extent(1); j++)
-  {
-    for (size_t i = 0; i < sUn.extent(0); i++)
-    {
-      int v = (int) (255.0 * sUn(i, j) / 100.0);
-      // we assume values between 0.0 and 100.0 : max value is in red,
-      // min is in blue
-      unsigned char color[3];
-      color[0] = static_cast<char>(v); /* red */
-      color[1] = static_cast<char>(0); /* green */
-      color[2] = static_cast<char>(255 - v); /* blue */
-      fwrite(color, 1, 3, f);
-    }
-  }
-  fclose(f);
-}
-
-int main()
-{
-  context ctx;
-
-  const size_t N = 800;
-
-  auto lU  = ctx.logical_data(shape_of<slice<double, 2>>(N, N));
-  auto lU1 = ctx.logical_data(lU.shape());
-
-  // Initialize the Un field with boundary conditions, and a disk at a lower
-  // temperature in the middle.
-  ctx.parallel_for(lU.shape(), lU.write())->*[=] _CCCL_DEVICE(size_t i, size_t j, auto U) {
-    double rad = U.extent(0) / 8.0;
-    double dx  = (double) i - U.extent(0) / 2;
-    double dy  = (double) j - U.extent(1) / 2;
-
-    U(i, j) = (dx * dx + dy * dy < rad * rad) ? 100.0 : 0.0;
-
-    /* Set up boundary conditions */
-    if (j == 0.0)
-    {
-      U(i, j) = 100.0;
-    }
-    if (j == U.extent(1) - 1)
-    {
-      U(i, j) = 0.0;
-    }
-    if (i == 0.0)
-    {
-      U(i, j) = 0.0;
-    }
-    if (i == U.extent(0) - 1)
-    {
-      U(i, j) = 0.0;
-    }
-  };
-
-  // diffusion constant
-  double a = 0.5;
-
-  double dx  = 0.1;
-  double dy  = 0.1;
-  double dx2 = dx * dx;
-  double dy2 = dy * dy;
-
-  // time step
-  double dt = dx2 * dy2 / (2.0 * a * (dx2 + dy2));
-
-  double c = a * dt;
-
-  int nsteps     = 1000;
-  int image_freq = -1;
-
-  for (int iter = 0; iter < nsteps; iter++)
-  {
-    if (image_freq > 0 && iter % image_freq == 0)
-    {
-      // Dump Un in a PPM file
-      ctx.host_launch(lU.read())->*[=](auto U) {
-        dump_iter(U, iter);
-      };
-    }
-
-    // Update Un using Un1 value with a finite difference scheme
-    ctx.parallel_for(inner<1>(lU.shape()), lU.read(), lU1.write())
-        ->*[=]
-      _CCCL_DEVICE(size_t i, size_t j, auto U, auto U1) {
-        U1(i, j) =
-          U(i, j)
-          + c * ((U(i - 1, j) - 2 * U(i, j) + U(i + 1, j)) / dx2 + (U(i, j - 1) - 2 * U(i, j) + U(i, j + 1)) / dy2);
-      };
-
-    std::swap(lU, lU1);
-  }
-
-  ctx.finalize();
-}