[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_on.cu

@@ -0,0 +1,125 @@
+//===----------------------------------------------------------------------===//
+//
+// 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) 2024 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+#include <cuda/experimental/execution.cuh>
+
+#include "testing.cuh"
+
+namespace ex = cudax::execution;
+
+__host__ __device__ bool _on_device() noexcept
+{
+  NV_IF_ELSE_TARGET(NV_IS_HOST, //
+                    ({ return false; }),
+                    ({ return true; }));
+}
+
+auto const main_thread_id = ::std::this_thread::get_id();
+
+void simple_start_on_thread_test()
+{
+  ex::thread_context ctx;
+  auto sch  = ctx.get_scheduler();
+  auto sndr = ex::on(sch, ex::just() | ex::then([] {
+                            CHECK(::std::this_thread::get_id() != main_thread_id);
+                          }))
+            | ex::then([]() -> int {
+                CHECK(::std::this_thread::get_id() == main_thread_id);
+                return 42;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+void simple_continue_on_thread_test()
+{
+  ex::thread_context ctx;
+  auto sch  = ctx.get_scheduler();
+  auto sndr = ex::just() | ex::on(sch, ex::then([] {
+                                    CHECK(::std::this_thread::get_id() != main_thread_id);
+                                  }))
+            | ex::then([]() -> int {
+                CHECK(::std::this_thread::get_id() == main_thread_id);
+                return 42;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+void simple_start_on_stream_test()
+{
+  cudax::stream str{cuda::device_ref(0)};
+  auto sch  = cudax::stream_ref{str};
+  auto sndr = ex::on(sch, ex::just(42) | ex::then([] __host__ __device__(int i) noexcept -> int {
+                            return _on_device() ? i : -i;
+                          }))
+            | ex::then([] __host__ __device__(int i) noexcept -> int {
+                return _on_device() ? -1 : i;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+void simple_continue_on_stream_test()
+{
+  cudax::stream str{cuda::device_ref(0)};
+  auto sch  = cudax::stream_ref{str};
+  auto sndr = ex::just(42) | ex::on(sch, ex::then([] __host__ __device__(int i) noexcept -> int {
+                                      return _on_device() ? i : -i;
+                                    }))
+            | ex::then([] __host__ __device__(int i) noexcept -> int {
+                return _on_device() ? -1 : i;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+void test_continues_on_updates_env()
+{
+  ex::thread_context ctx;
+  auto sch  = ctx.get_scheduler();
+  auto sndr = ex::just() | ex::on(sch, ex::let_value([] {
+                                    return ex::read_env(ex::get_scheduler);
+                                  }))
+            | ex::then([](auto sch2) -> int {
+                STATIC_REQUIRE(cuda::std::same_as<decltype(sch2), decltype(sch)>);
+                return 42;
+              });
+  auto [result] = ex::sync_wait(std::move(sndr)).value();
+  CHECK(result == 42);
+}
+
+namespace
+{
+C2H_TEST("simple on(sch, sndr) thread test", "[on]")
+{
+  simple_start_on_thread_test();
+}
+
+C2H_TEST("simple on(sndr, sch, closure) thread test", "[on]")
+{
+  simple_continue_on_thread_test();
+}
+
+C2H_TEST("simple on(sch, sndr) stream test", "[on][stream]")
+{
+  simple_start_on_stream_test();
+}
+
+C2H_TEST("simple on(sndr, sch, closure) stream test", "[on][stream]")
+{
+  simple_continue_on_stream_test();
+}
+
+C2H_TEST("test that on(sndr, sch, closure) updates the env for closure", "[on][stream]")
+{
+  test_continues_on_updates_env();
+}
+} // namespace