[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/thrust/testing/unittest/exceptions.h

@@ -0,0 +1,61 @@
+#pragma once
+
+#include <iostream>
+#include <sstream>
+#include <string>
+#include <utility>
+
+namespace unittest
+{
+class UnitTestException
+{
+public:
+  std::string message;
+
+  UnitTestException() = default;
+  UnitTestException(std::string msg)
+      : message(std::move(msg))
+  {}
+
+  friend std::ostream& operator<<(std::ostream& os, const UnitTestException& e)
+  {
+    return os << e.message;
+  }
+
+  template <typename T>
+  UnitTestException& operator<<(const T& t)
+  {
+    std::ostringstream oss;
+    oss << t;
+    message += oss.str();
+    return *this;
+  }
+};
+
+class UnitTestError : public UnitTestException
+{
+public:
+  UnitTestError() = default;
+  UnitTestError(const std::string& msg)
+      : UnitTestException(msg)
+  {}
+};
+
+class UnitTestFailure : public UnitTestException
+{
+public:
+  UnitTestFailure() = default;
+  UnitTestFailure(const std::string& msg)
+      : UnitTestException(msg)
+  {}
+};
+
+class UnitTestKnownFailure : public UnitTestException
+{
+public:
+  UnitTestKnownFailure() = default;
+  UnitTestKnownFailure(const std::string& msg)
+      : UnitTestException(msg)
+  {}
+};
+} // end namespace unittest