[CCCL] Add missing CCCL components: c2h, nvbench_helper, cmake, cudax, AGENTS.md

Added 863 files from NVIDIA/cccl sparse checkout:
- c2h/ (27 files): Catch2 test helpers — generators, validators, runner
- nvbench_helper/ (10 files): Benchmark harness utilities
- cmake/ (29 files): CMake presets and build helpers
- cudax/ (794 files): Experimental CUDA extensions
- AGENTS.md: NVIDIA's official AI agent instructions for CCCL
- CMakePresets.json: Standardized build configurations
- cccl-version.json: Version tracking

Also added CCCL_ASSET_MAP.md mapping all 4295 CCCL files to
competition value and PRD items.

cccl_upstream now covers 100% of competition-critical assets:
- 27 tuning headers (SM80/90/100 benchmark data)
- 32 dispatch headers (algorithm implementations)
- 60 Thrust examples (correctness verification)
- 217 CUB Catch2 tests (regression matrix)
- 153 CUB benchmarks (parameter space search)
- 18 CUB examples (API verification)
- 27 test helpers + benchmark harness
- 794 cudax experimental extensions

cccl_upstream/cudax/test/execution/test_conditional.cu

@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 "common/checked_receiver.cuh"
+#include "common/utility.cuh"
+#include "testing.cuh"
+
+namespace ex = cuda::experimental::execution;
+
+namespace
+{
+C2H_TEST("simple use of conditional runs exactly one of the two closures", "[adaptors][conditional]")
+{
+  for (int i = 42; i < 44; ++i)
+  {
+    bool even{false};
+    bool odd{false};
+
+    auto sndr1 =
+      ex::just(i)
+      | ex::conditional(
+        [](int i) {
+          return i % 2 == 0;
+        },
+        ex::then([&](int) {
+          even = true;
+        }),
+        ex::then([&](int) {
+          odd = true;
+        }));
+
+    check_value_types<types<>>(sndr1);
+    check_sends_stopped<false>(sndr1);
+#if _CCCL_HAS_EXCEPTIONS()
+    check_error_types<ex::exception_ptr>(sndr1);
+#else // ^^^ _CCCL_HAS_EXCEPTIONS() ^^^ / vvv !_CCCL_HAS_EXCEPTIONS() vvv
+    check_error_types<>(sndr1);
+#endif // !_CCCL_HAS_EXCEPTIONS()
+
+    auto op = ex::connect(std::move(sndr1), checked_value_receiver<>{});
+    ex::start(op);
+
+    CHECK(even == (i % 2 == 0));
+    CHECK(odd == (i % 2 == 1));
+  }
+}
+} // namespace