[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/execution/common/stopped_scheduler.cuh

@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+//
+// 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) 2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+
+#include <cuda/__utility/immovable.h>
+
+#include <cuda/experimental/execution.cuh>
+
+#include "testing.cuh" // IWYU pragma: keep
+
+namespace
+{
+struct _stopped_scheduler_attrs_t
+{
+  template <class _Env>
+  _CCCL_HOST_DEVICE auto
+  query(cudax_async::get_completion_scheduler_t<cudax_async::set_value_t>, const _Env& env) const noexcept
+    -> decltype(cudax_async::get_completion_scheduler<cudax_async::set_value_t>(env, env))
+  {
+    return cudax_async::get_completion_scheduler<cudax_async::set_value_t>(env, env);
+  }
+
+  template <class _Env>
+  _CCCL_HOST_DEVICE auto
+  query(cudax_async::get_completion_scheduler_t<cudax_async::set_stopped_t>, const _Env& env) const noexcept
+    -> decltype(cudax_async::get_completion_scheduler<cudax_async::set_stopped_t>(env, env))
+  {
+    return cudax_async::get_completion_scheduler<cudax_async::set_stopped_t>(env, env);
+  }
+
+  template <class _Env>
+  _CCCL_HOST_DEVICE auto
+  query(cudax_async::get_completion_domain_t<cudax_async::set_value_t>, const _Env& env) const noexcept
+    -> decltype(cudax_async::get_completion_domain<cudax_async::set_value_t>(env, env))
+  {
+    return cudax_async::get_completion_domain<cudax_async::set_value_t>(env, env);
+  }
+
+  template <class _Env>
+  _CCCL_HOST_DEVICE auto
+  query(cudax_async::get_completion_domain_t<cudax_async::set_stopped_t>, const _Env& env) const noexcept
+    -> decltype(cudax_async::get_completion_domain<cudax_async::set_stopped_t>(env, env))
+  {
+    return cudax_async::get_completion_domain<cudax_async::set_stopped_t>(env, env);
+  }
+
+  _CCCL_HOST_DEVICE static constexpr auto query(cudax_async::get_completion_behavior_t) noexcept
+  {
+    return cudax_async::completion_behavior::inline_completion;
+  }
+};
+
+//! Scheduler that returns a sender that always completes with stopped.
+struct stopped_scheduler : _stopped_scheduler_attrs_t
+{
+private:
+  template <class Rcvr>
+  struct _opstate_t : cuda::__immovable
+  {
+    using operation_state_concept = cudax_async::operation_state_t;
+
+    Rcvr _rcvr;
+
+    _CCCL_HOST_DEVICE void start() noexcept
+    {
+      cudax_async::set_stopped(static_cast<Rcvr&&>(_rcvr));
+    }
+  };
+
+  struct _sndr_t
+  {
+    using sender_concept = cudax_async::sender_t;
+
+    template <class Self>
+    _CCCL_HOST_DEVICE static constexpr auto get_completion_signatures()
+    {
+      return cudax_async::completion_signatures<cudax_async::set_value_t(), cudax_async::set_stopped_t()>();
+    }
+
+    template <class Rcvr>
+    _CCCL_HOST_DEVICE auto connect(Rcvr rcvr) const noexcept -> _opstate_t<Rcvr>
+    {
+      return {{}, static_cast<Rcvr&&>(rcvr)};
+    }
+
+    _CCCL_HOST_DEVICE auto get_env() const noexcept -> _stopped_scheduler_attrs_t
+    {
+      return {};
+    }
+  };
+
+public:
+  using scheduler_concept = cudax_async::scheduler_t;
+
+  stopped_scheduler() = default;
+
+  _CCCL_HOST_DEVICE auto schedule() const noexcept -> _sndr_t
+  {
+    return {};
+  }
+
+  _CCCL_HOST_DEVICE friend constexpr bool operator==(stopped_scheduler, stopped_scheduler) noexcept
+  {
+    return true;
+  }
+
+  _CCCL_HOST_DEVICE friend constexpr bool operator!=(stopped_scheduler, stopped_scheduler) noexcept
+  {
+    return false;
+  }
+};
+} // namespace