[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/benchmarks/compute/pixi.toml

@@ -1,89 +0,0 @@
-# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
-#
-# SPDX-License-Identifier: Apache-2.0
-
-[workspace]
-channels = ["conda-forge"]
-platforms = ["linux-64"]
-channel-priority = "disabled"
-
-# CUDA 13.1 system requirement
-[feature.cu13.system-requirements]
-cuda = "13"
-
-[feature.cu13.dependencies]
-cuda-version = "13.1.*"
-
-# Python benchmark dependencies
-[feature.bench.dependencies]
-python = "3.13.*"
-numpy = "*"
-cupy = "*"
-pytest-benchmark = "*"
-pyyaml = "*"
-pre-commit = "*"
-
-[feature.bench.pypi-dependencies]
-cuda-bench = ">=0.2.0"
-
-# C++ benchmark build dependencies
-[feature.cpp-bench.dependencies]
-cmake = "*"
-ninja = "*"
-cxx-compiler = "*"
-fmt = "*"
-cuda-cudart-dev = "*"
-
-[feature.cpp-bench.target.linux-64.dependencies]
-cuda-crt-dev_linux-64 = "*"
-cuda-driver-dev_linux-64 = "*"
-
-[feature.cpp-bench.target.linux-64.activation.env]
-CUDA_HOME = "$CONDA_PREFIX/targets/x86_64-linux"
-
-# Important: cuda-cccl installation variants
-
-# Released version from PyPI
-[feature.cccl-wheel.pypi-dependencies]
-cuda-cccl = { version = ">=0.1.0", extras = ["cu13"] }
-
-# Local source build (editable install from repo)
-# Needs nvcc, compilers, and CUDA dev libraries for scikit-build-core to build cuda-cccl
-[feature.cccl-source.dependencies]
-cuda-nvcc = "*"
-cuda-nvrtc-dev = "*"
-libnvjitlink-dev = "*"
-cuda-cudart-dev = "*"
-cuda-driver-dev = "*"
-c-compiler = "*"
-cxx-compiler = "*"
-
-[feature.cccl-source.pypi-dependencies]
-cuda-cccl = { path = "../..", editable = true, extras = ["cu13"] }
-
-# Environments
-[environments]
-wheel = { features = ["cu13", "bench", "cpp-bench", "cccl-wheel"] }
-source = { features = ["cu13", "bench", "cpp-bench", "cccl-source"] }
-
-# Tasks
-[tasks.bench]
-cmd = ["python", "run_benchmarks.py", "--py"]
-description = "Run Python cuda.compute benchmarks"
-
-[tasks.bench-quick]
-cmd = ["python", "run_benchmarks.py", "--py", "--quick"]
-description = "Run Python benchmarks with reduced parameter set"
-
-[tasks.bench-cpp]
-cmd = ["python", "run_benchmarks.py", "--cpp"]
-description = "Run C++ CUB benchmarks"
-
-[tasks.bench-all]
-cmd = ["python", "run_benchmarks.py"]
-description = "Run both Python and C++ benchmarks"
-
-[tasks.pre-commit]
-cmd = ["pre-commit", "run", "--all-files"]
-cwd = "../../../.."
-description = "Run pre-commit checks on the entire repo"