[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/cub/cub/block/block_topk.cuh

@@ -0,0 +1,89 @@
+// SPDX-FileCopyrightText: Copyright (c) 2025-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#pragma once
+
+#include <cub/config.cuh>
+
+#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 <cub/block/specializations/block_topk_air.cuh>
+#include <cub/device/dispatch/dispatch_common.cuh>
+#include <cub/util_type.cuh>
+
+CUB_NAMESPACE_BEGIN
+
+namespace detail
+{
+// TODO (elstehle): Add documentation
+template <typename KeyT, int BlockDimX, int ItemsPerThread, typename ValueT = NullType>
+class block_topk
+{
+private:
+  using internal_block_topk_t = block_topk_air<KeyT, BlockDimX, ItemsPerThread, ValueT>;
+
+public:
+  struct TempStorage
+  {
+    typename internal_block_topk_t::TempStorage topk_storage;
+  };
+
+private:
+  TempStorage& storage;
+
+public:
+  _CCCL_DEVICE_API _CCCL_FORCEINLINE block_topk(TempStorage& storage)
+      : storage(storage)
+  {}
+
+  template <bool IsFullTile>
+  _CCCL_DEVICE_API _CCCL_FORCEINLINE void max_pairs(
+    KeyT (&keys)[ItemsPerThread],
+    ValueT (&values)[ItemsPerThread],
+    int k,
+    int num_valid,
+    int begin_bit = 0,
+    int end_bit   = sizeof(KeyT) * 8)
+  {
+    internal_block_topk_t(storage.topk_storage)
+      .template select_pairs<detail::topk::select::max, IsFullTile>(keys, values, k, num_valid, begin_bit, end_bit);
+  }
+
+  template <bool IsFullTile>
+  _CCCL_DEVICE_API _CCCL_FORCEINLINE void
+  max_keys(KeyT (&keys)[ItemsPerThread], int k, int num_valid, int begin_bit = 0, int end_bit = sizeof(KeyT) * 8)
+  {
+    internal_block_topk_t(storage.topk_storage)
+      .template select_keys<detail::topk::select::max, IsFullTile>(keys, k, num_valid, begin_bit, end_bit);
+  }
+
+  template <bool IsFullTile>
+  _CCCL_DEVICE_API _CCCL_FORCEINLINE void min_pairs(
+    KeyT (&keys)[ItemsPerThread],
+    ValueT (&values)[ItemsPerThread],
+    int k,
+    int num_valid,
+    int begin_bit = 0,
+    int end_bit   = sizeof(KeyT) * 8)
+  {
+    internal_block_topk_t(storage.topk_storage)
+      .template select_pairs<detail::topk::select::min, IsFullTile>(keys, values, k, num_valid, begin_bit, end_bit);
+  }
+
+  template <bool IsFullTile>
+  _CCCL_DEVICE_API _CCCL_FORCEINLINE void
+  min_keys(KeyT (&keys)[ItemsPerThread], int k, int num_valid, int begin_bit = 0, int end_bit = sizeof(KeyT) * 8)
+  {
+    internal_block_topk_t(storage.topk_storage)
+      .template select_keys<detail::topk::select::min, IsFullTile>(keys, k, num_valid, begin_bit, end_bit);
+  }
+};
+} // namespace detail
+
+CUB_NAMESPACE_END