[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/libcudacxx/test/utils/nvidia/nvrtc/README.md

@@ -0,0 +1,55 @@
+# NVRTCC
+
+## How to use:
+
+Configure libcudacxx to test with NVRTC and `cmake --build` the project before executing `lit`.
+
+```sh
+cmake ... -DLIBCUDACXX_TEST_WITH_NVRTC=ON
+cmake --build $BUILD_DIR
+lit ... $TEST_DIR
+```
+
+## How it works
+
+`nvrtcc` processes incoming arguments matching for flags that modify its behavior, and passes the rest to NVRTC.
+It will hopefully filter any that don't apply (gcc warnings and such).
+
+The input file is processed to be compatible with NVRTC similarly to the `nvrtc.sh` scripts during compilation.
+The resulting file is then compiled with NVRTC and stored as a fatbin. This is, in effect, a compilation pass for NVRTC.
+`.fail.cpp` tests will be analyzable by lit and should work properly.
+
+For execution, the fatbin file is provided to nvrtcc again, but will instead launch the precompiled test unit on the GPU.
+
+```mermaid
+flowchart TD;
+    compile{nvrtcc};
+    runfat{nvrtcc};
+
+    start[Execute lit]
+    build(Execute nvrtcc)
+    pass(Pass: Continue)
+    fail(Fail: Compile time failure)
+    save[Save fatbin]
+
+    load[Load fatbin]
+    testpass[Pass: No failures]
+    testfail[Fail: Report driver error]
+
+    save --> load;
+
+    subgraph Build Pass
+    start   --  lit -sv -Denable_nvrtc=true    --> build;
+    build   --  nvrtcc -x cu test.pass.cpp ... --> compile;
+    compile --> fail;
+    compile --> pass;
+    pass    --> save;
+    end
+
+
+    subgraph Test Pass
+    load -- nvrtcc test.pass.cpp.fatbin --> runfat;
+    runfat --> testpass
+    runfat --> testfail
+    end
+```