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/extended_api/math/pow2.rst

@@ -0,0 +1,70 @@
+.. _libcudacxx-extended-api-math-pow2:
+
+Power of Two Utilities
+======================
+
+Defined in the ``<cuda/cmath>`` header.
+
+.. code:: cuda
+
+   namespace cuda {
+
+   template <typename T>
+   [[nodiscard]] __host__ __device__ constexpr
+   bool is_power_of_two(T value) noexcept;
+
+   template <typename T>
+   [[nodiscard]] __host__ __device__ constexpr
+   T next_power_of_two(T value) noexcept;
+
+   template <typename T>
+   [[nodiscard]] __host__ __device__ constexpr
+   T prev_power_of_two(T value) noexcept;
+
+   } // namespace cuda
+
+The functions provide utilities to determine if an integer value is a power of two, and to compute the next and previous power of two.
+
+**Parameters**
+
+- ``value``: The input value.
+
+**Return value**
+
+- ``is_power_of_two``: Return ``true`` if ``value`` is a power of two, ``false`` otherwise.
+- ``next_power_of_two``: Return the smallest power of two greater than or equal to ``value``.
+- ``prev_power_of_two``: Return the largest power of two less than or equal to ``value``.
+
+**Constraints**
+
+- ``T`` is an integer types. Contrary to ``cuda::std::has_single_bit``, ``cuda::std::bit_floor``, and ``cuda::std::bit_ceil``, ``T`` can be both signed and unsigned.
+
+**Preconditions**
+
+- ``value > 0``
+
+**Performance considerations**
+
+See :ref:`\<cuda/std/bit\> performance considerations <libcudacxx-standard-api-numerics-bit>`
+
+Example
+-------
+
+.. code:: cuda
+
+    #include <cuda/cmath>
+    #include <cuda/std/cassert>
+
+    __global__ void pow2_kernel() {
+        assert(!cuda::is_power_of_two(20));
+        assert(cuda::next_power_of_two(20) == 32);
+        assert(cuda::prev_power_of_two(20) == 16);
+    }
+
+    int main() {
+        pow2_kernel<<<1, 1>>>();
+        cudaDeviceSynchronize();
+        return 0;
+    }
+
+`See it on Godbolt 🔗 <https://godbolt.org/z/896Yx3vf8>`__