feat(cccl): integrate missing CCCL directories — python/, ci/, .agent/, docs/, test/

Sparse-checkout from NVIDIA/cccl main branch to complete cccl_upstream:

Added:
- python/cuda_cccl/ (226 files) — Python bindings for device-level algorithms
  Critical for muh toolchain: cuda.compute.reduce_into, scan, radix_sort, etc.
  Includes 204 .py files with full test coverage for all 27 algorithms
- ci/ (163 files) — Build/test infrastructure
  build_cub.sh, test_cub.sh, build_and_test_targets.sh, matrix.yaml
  Directly maps to our [INFRA-CI] and [INFRA-BUILD] items
- .agent/skills/ (7 files) — NVIDIA's own agent skills for CCCL
  cccl-style/SKILL.md, cccl-test/SKILL.md, sass-diff/SKILL.md
- docs/ (491 files) — Official CCCL documentation
  CI references, CMake guides, Python compute docs, libcudacxx PTX docs
- test/ (12 files) — Top-level integration tests (cuda_smoke, stdpar)
- Root configs: .clang-format, .clang-tidy, CONTRIBUTING.md, pyproject.toml
- CLAUDE.md symlink → AGENTS.md (NVIDIA's standard)

cccl_upstream now mirrors full NVIDIA/cccl structure:
  Before: 42M (cub + thrust + libcudacxx + cudax + c + examples + benchmarks)
  After:  53M (+python +ci +docs +.agent +test +configs)

This completes the CCCL base needed for:
- [muh-bench] items: ci/util/build_and_test_targets.sh for targeted builds
- [CCCL-verify] items: python/cuda_cccl/tests/ as reference implementations
- [CCCL-test] items: ci/test_cub.sh, ci/test_thrust.sh
- Agent workflow: .agent/skills/ for consistent style and test patterns

cccl_upstream/docs/libcudacxx/standard_api/c_library/cstring.rst

@@ -0,0 +1,47 @@
+.. _libcudacxx-standard-api-cstring:
+
+``<cuda/std/cstring>``
+======================
+
+``cuda::std::memset``
+---------------------
+
+.. code:: cuda
+
+   __host__ __device__
+   inline void* memset(void* dest, int ch, size_t count) noexcept;
+
+See `std::memset <https://en.cppreference.com/w/cpp/string/byte/memset.html>`_ for the full documentation.
+
+**Preconditions**
+
+The following preconditions are only enabled with CCCL 3.2 or later:
+
+    - ``dest`` is a valid pointer.
+    - ``dest + count`` is a valid pointer.
+
+A valid pointer is one that is not NULL and within the correct range if it belongs to the shared memory address space.
+
+----
+
+``cuda::std::memcpy``
+---------------------
+
+.. code:: cuda
+
+   __host__ __device__
+   inline void* memcpy(void* dest, const void* src, size_t count) noexcept;
+
+See `std::memcpy <https://en.cppreference.com/w/cpp/string/byte/memcpy.html>`_  for the full documentation.
+
+**Preconditions**
+
+The following preconditions are only enabled with CCCL 3.2 or later:
+
+    - ``src`` is a valid pointer.
+    - ``src + count`` is a valid pointer.
+    - ``dest`` is a valid pointer.
+    - ``dest + count`` is a valid pointer.
+    - ``src`` and ``dest`` don't overlap.
+
+A valid pointer is one that is not NULL and within the correct range if it belongs to the shared memory address space.