[INFRA] Import NVIDIA/CCCL upstream as optimization reference library

CCCL (CUDA C++ Core Libraries) provides:
- CUB: device/block/warp-level GPU primitives (reduce, scan, sort, topk)
- Thrust: high-level parallel algorithms (transform_reduce, sort, scan)
- libcudacxx: CUDA C++ standard library (atomics, barriers, memory)
- cudax: experimental features (memory resources, allocators)
- Tuning policies: per-SM hardware-specific algorithm parameters

Competition optimization vectors mapped to CCCL:
- Output TPS (83% weight): warp_reduce, block_reduce, device_topk
- Input TPS (14% weight): device_scan, block_load, prefetch
- Cache TPS (3% weight): prefix caching strategy patterns
- Memory (0.9 util): pooled/cached/buddy allocators

Source: https://github.com/NVIDIA/cccl (shallow clone, HEAD only)
License: Apache-2.0

cccl_upstream/libcudacxx/include/cuda/__semaphore/counting_semaphore.h

@@ -0,0 +1,53 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, 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) 2023 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _CUDA___SEMAPHORE_COUNTING_SEMAPHORE_H
+#define _CUDA___SEMAPHORE_COUNTING_SEMAPHORE_H
+
+#include <cuda/std/detail/__config>
+
+#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
+#  pragma GCC system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
+#  pragma clang system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
+#  pragma system_header
+#endif // no system header
+
+#include <cuda/std/__semaphore/atomic_semaphore.h>
+#include <cuda/std/cstdint>
+
+#include <cuda/std/__cccl/prologue.h>
+
+_CCCL_BEGIN_NAMESPACE_CUDA
+
+template <thread_scope _Sco, ptrdiff_t __least_max_value = INT_MAX>
+class counting_semaphore : public ::cuda::std::__atomic_semaphore<_Sco, __least_max_value>
+{
+  static_assert(__least_max_value <= ::cuda::std::__atomic_semaphore<_Sco, __least_max_value>::max());
+
+public:
+  _CCCL_HOST_DEVICE_API constexpr counting_semaphore(ptrdiff_t __count = 0)
+      : ::cuda::std::__atomic_semaphore<_Sco, __least_max_value>(__count)
+  {}
+  _CCCL_HIDE_FROM_ABI ~counting_semaphore() = default;
+
+  counting_semaphore(const counting_semaphore&)            = delete;
+  counting_semaphore& operator=(const counting_semaphore&) = delete;
+};
+
+template <thread_scope _Sco>
+using binary_semaphore = counting_semaphore<_Sco, 1>;
+
+_CCCL_END_NAMESPACE_CUDA
+
+#include <cuda/std/__cccl/epilogue.h>
+
+#endif // _CUDA___SEMAPHORE_COUNTING_SEMAPHORE_H