[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/examples/thrust_flexible_device_system/README.md

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+# Thrust Flexible Device System Example
+
+This example illustrates best practices for writing generic CMake that supports user-configuration of the Thrust device system via the `CCCL_THRUST_DEVICE_SYSTEM` CMake option.
+
+Valid values for this option are:
+
+- `CUDA`
+- `OMP` (OpenMP)
+- `TBB` (Intel Thread Building Blocks)
+- `CPP` (Serial C++ backend)
+
+The CMakeLists.txt file for this example is annotated to show how to achieve a generic build system that supports any of device system.
+
+## How To Use This Example
+
+Configure and build this example as follows:
+
+```
+# Checkout example and prepare build directory:
+git clone https://github.com/NVIDIA/cccl.git
+cd cccl/thrust_flexible_device_system
+mkdir build
+cd build
+
+# Configure:
+cmake .. -DCCCL_THRUST_DEVICE_SYSTEM=CUDA # or TBB, OMP, CPP
+
+# Build:
+cmake --build .
+
+# Run:
+ctest -V
+```
+
+## Advanced Thrust Usecases
+
+For more control over the Thrust configuration, see the Thrust CMake package's [README.md](../../lib/cmake/thrust/README.md).
+This details how to use the `thrust_create_target` function to generate Thrust interface targets in CMake.
+
+If using `thrust_create_target` directly, you may also want to set the CMake option `CCCL_ENABLE_DEFAULT_THRUST_TARGET=OFF` to prevent the default `CCCL::Thrust` target from being initialized.
+This will avoid checking for any dependencies required for the default target that may be unnecessary for your project.
+
+## Further Reading About CCCL + CMake
+
+The `basic` example's [README.md](../basic/README.md) has additional information that you may find useful for using CCCL with CPM and CMake.