[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/cudax/test/cuco/utility/test_capacity.cu

@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of CUDA Experimental 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) 2026 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#include <cuda/std/cstddef>
+
+#include <cuda/experimental/__cuco/capacity.cuh>
+#include <cuda/experimental/__cuco/hash_functions.cuh>
+#include <cuda/experimental/__cuco/probing_scheme.cuh>
+
+#include <testing.cuh>
+
+namespace cudax = cuda::experimental;
+
+C2H_TEST("cuco make_valid_capacity rounding and validation", "[capacity]")
+{
+  using probing                         = cudax::cuco::double_hashing<1, cudax::cuco::hash<int>>;
+  [[maybe_unused]] constexpr int bucket = 1;
+
+  static_assert(cudax::cuco::is_double_hashing_v<probing>, "scheme is double hashing");
+
+  // make_valid_capacity rounds up and is idempotent; is_valid_capacity is derived from it
+  constexpr auto valid = cudax::cuco::make_valid_capacity<probing, bucket>(::cuda::std::size_t{1000});
+  static_assert(valid >= 1000, "rounds up");
+  static_assert(cudax::cuco::is_valid_capacity<probing, bucket>(valid), "result is valid");
+  static_assert(cudax::cuco::make_valid_capacity<probing, bucket>(valid) == valid, "idempotent");
+
+  // 1000 is not a valid double-hashing capacity; it rounds up to a prime-cycle capacity
+  static_assert(!cudax::cuco::is_valid_capacity<probing, bucket>(::cuda::std::size_t{1000}), "1000 is not valid");
+
+  // equal-rounding requests produce the same valid capacity
+  static_assert(cudax::cuco::make_valid_capacity<probing, bucket>(::cuda::std::size_t{1000})
+                  == cudax::cuco::make_valid_capacity<probing, bucket>(::cuda::std::size_t{1008}),
+                "requests that round to the same capacity agree");
+
+  // cuCollections extent_test parity: double hashing, cg_size 2, bucket_size 4.
+  // 1234 rounds up to next_prime(ceil(1234 / 8) = 155) = 157, times the stride 8 -> 1256.
+  using dh4                              = cudax::cuco::double_hashing<2, cudax::cuco::hash<int>>;
+  [[maybe_unused]] constexpr int bucket4 = 4;
+  static_assert(cudax::cuco::make_valid_capacity<dh4, bucket4>(::cuda::std::size_t{1234}) == ::cuda::std::size_t{1256},
+                "compile-time valid capacity matches the cuCollections extent test");
+  REQUIRE(cudax::cuco::make_valid_capacity<dh4, bucket4>(::cuda::std::size_t{1234}) == ::cuda::std::size_t{1256});
+
+  // a desired load factor outside (0, 1] is rejected
+  using lp4   = cudax::cuco::linear_probing<2, cudax::cuco::hash<int>>;
+  auto bad_lf = [](double __lf) {
+    [[maybe_unused]] auto __r = cudax::cuco::make_valid_capacity<lp4, bucket4>(::cuda::std::size_t{1000}, __lf);
+  };
+  REQUIRE_THROWS(bad_lf(0.0));
+  REQUIRE_THROWS(bad_lf(-0.5));
+  REQUIRE_THROWS(bad_lf(1.5));
+}