[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/dummy_scheduler.cuh

@@ -0,0 +1,107 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 ex = cuda::experimental::execution;
+
+namespace
+{
+namespace _dummy
+{
+template <class Domain>
+struct _attrs_t
+{
+  _CCCL_HOST_DEVICE constexpr auto query(ex::get_completion_scheduler_t<ex::set_value_t>) const noexcept;
+
+  _CCCL_HOST_DEVICE constexpr auto query(ex::get_completion_domain_t<ex::set_value_t>) const noexcept
+  {
+    return Domain{};
+  }
+};
+
+template <class Rcvr>
+struct _opstate_t : cuda::__immovable
+{
+  using operation_state_concept = ex::operation_state_t;
+
+  _CCCL_HOST_DEVICE constexpr _opstate_t(Rcvr rcvr) noexcept
+      : _rcvr(static_cast<Rcvr&&>(rcvr))
+  {}
+
+  _CCCL_HOST_DEVICE constexpr void start() noexcept
+  {
+    ex::set_value(static_cast<Rcvr&&>(_rcvr));
+  }
+
+  Rcvr _rcvr;
+};
+
+template <class Domain>
+struct _sndr_t
+{
+  using sender_concept = ex::sender_t;
+
+  template <class Self>
+  _CCCL_HOST_DEVICE static _CCCL_CONSTEVAL auto get_completion_signatures() noexcept
+  {
+    return ex::completion_signatures<ex::set_value_t()>();
+  }
+
+  template <class Rcvr>
+  _CCCL_HOST_DEVICE constexpr auto connect(Rcvr rcvr) const noexcept -> _opstate_t<Rcvr>
+  {
+    return _opstate_t<Rcvr>(static_cast<Rcvr&&>(rcvr));
+  }
+
+  [[nodiscard]] _CCCL_HOST_DEVICE constexpr auto get_env() const noexcept
+  {
+    return _attrs_t<Domain>{};
+  }
+};
+} // namespace _dummy
+
+//! Scheduler that returns a sender that always completes inline (successfully).
+template <class Domain = ex::default_domain>
+struct dummy_scheduler : _dummy::_attrs_t<Domain>
+{
+  using scheduler_concept = ex::scheduler_t;
+
+  _CCCL_HOST_DEVICE static constexpr auto schedule() noexcept -> _dummy::_sndr_t<Domain>
+  {
+    return {};
+  }
+
+  _CCCL_HOST_DEVICE friend constexpr bool operator==(dummy_scheduler, dummy_scheduler) noexcept
+  {
+    return true;
+  }
+
+  _CCCL_HOST_DEVICE friend constexpr bool operator!=(dummy_scheduler, dummy_scheduler) noexcept
+  {
+    return false;
+  }
+};
+
+namespace _dummy
+{
+template <class Domain>
+_CCCL_HOST_DEVICE constexpr auto _attrs_t<Domain>::query(ex::get_completion_scheduler_t<ex::set_value_t>) const noexcept
+{
+  return dummy_scheduler<Domain>{};
+}
+} // namespace _dummy
+} // namespace