[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/host/test_host_pytest_benchmark.py

@@ -1,95 +0,0 @@
-# Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. ALL RIGHTS RESERVED.
-#
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-from __future__ import annotations
-
-import pytest
-from host_benchmark_cases import (
-    CALL_CASES,
-    CASES,
-    HostBenchmarkCase,
-    patch_wrapper_to_skip_native_compute,
-    synchronize,
-)
-
-import cuda.compute as cc
-
-pytest.importorskip("pytest_benchmark")
-
-BUILD_TIME_ROUNDS = 10
-ONESHOT_ROUNDS = 20
-ONESHOT_ITERATIONS = 100
-TWOSHOT_ROUNDS = 20
-TWOSHOT_ITERATIONS = 1000
-
-
-def _case_params(cases: list[HostBenchmarkCase]) -> list[pytest.ParameterSet]:
-    params = []
-    for case in cases:
-        marks = []
-        if case.skip_reason is not None:
-            marks.append(pytest.mark.skip(reason=case.skip_reason))
-        params.append(pytest.param(case, id=case.name, marks=marks))
-    return params
-
-
-@pytest.mark.benchmark(group="cuda.compute.host.build_time")
-@pytest.mark.parametrize("case", _case_params(CASES))
-def test_build_time(benchmark, case: HostBenchmarkCase):
-    state = case.setup()
-    synchronize()
-
-    def setup() -> None:
-        cc.clear_all_caches()
-
-    def build():
-        return case.make_wrapper(state)
-
-    benchmark.pedantic(
-        build,
-        setup=setup,
-        rounds=BUILD_TIME_ROUNDS,
-        iterations=1,
-        warmup_rounds=0,
-    )
-
-
-@pytest.mark.benchmark(group="cuda.compute.host.oneshot_cached")
-@pytest.mark.parametrize("case", _case_params(CALL_CASES))
-def test_oneshot_cached_host_overhead(benchmark, case: HostBenchmarkCase):
-    cc.clear_all_caches()
-    state = case.setup()
-    wrapper = case.make_wrapper(state)
-    patch_wrapper_to_skip_native_compute(wrapper, case.noop_return_kind)
-    synchronize()
-
-    def call() -> None:
-        case.oneshot(state)
-
-    benchmark.pedantic(
-        call,
-        rounds=ONESHOT_ROUNDS,
-        iterations=ONESHOT_ITERATIONS,
-        warmup_rounds=0,
-    )
-
-
-@pytest.mark.benchmark(group="cuda.compute.host.twoshot_call")
-@pytest.mark.parametrize("case", _case_params(CALL_CASES))
-def test_twoshot_call_host_overhead(benchmark, case: HostBenchmarkCase):
-    cc.clear_all_caches()
-    state = case.setup()
-    wrapper = case.make_wrapper(state)
-    patch_wrapper_to_skip_native_compute(wrapper, case.noop_return_kind)
-    synchronize()
-
-    def call() -> None:
-        case.twoshot(state, wrapper)
-
-    benchmark.pedantic(
-        call,
-        rounds=TWOSHOT_ROUNDS,
-        iterations=TWOSHOT_ITERATIONS,
-        warmup_rounds=0,
-    )