[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/test_sequence.cu

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// 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) 2025 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+// Include this first
+#include <cuda/experimental/execution.cuh>
+
+// Then include the test helpers
+#include <nv/target>
+
+#include "common/checked_receiver.cuh"
+#include "common/utility.cuh"
+#include "testing.cuh"
+
+namespace ex = cuda::experimental::execution;
+
+namespace
+{
+C2H_TEST("simple use of sequence executes both child operations", "[adaptors][sequence]")
+{
+  bool flag1{false};
+  bool flag2{false};
+
+  auto sndr1 = ex::sequence(
+    ex::just() | ex::then([&] {
+      flag1 = true;
+    }),
+    ex::just() | ex::then([&] {
+      flag2 = true;
+    }));
+
+  check_value_types<types<>>(sndr1);
+  check_sends_stopped<false>(sndr1);
+  check_error_types<ex::exception_ptr>(sndr1);
+
+  auto op = ex::connect(std::move(sndr1), checked_value_receiver<>{});
+  ex::start(op);
+
+  CHECK(flag1);
+  CHECK(flag2);
+}
+} // namespace