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/ipow.rst

@@ -0,0 +1,60 @@
+.. _libcudacxx-extended-api-math-ipow:
+
+``cuda::ipow``
+====================================
+
+Defined in the ``<cuda/cmath>`` header.
+
+.. code:: cuda
+
+   namespace cuda {
+
+   template <typename T, typename E>
+   [[nodiscard]] __host__ __device__ constexpr
+   T ipow(T base, E exp) noexcept;
+
+   } // namespace cuda
+
+The function computes the integer ``base`` raised to the power of ``exp``.
+
+**Parameters**
+
+- ``base``: The base value.
+- ``exp``: The exponent value.
+
+**Return value**
+
+- The result of raising ``base`` to the power of ``exp``. If ``exp`` is negative, the result is 0.
+
+**Constraints**
+
+- ``T`` is an integer type.
+- ``E`` is an integer type.
+
+**Preconditions**
+
+- if ``base`` is 0, then ``exp`` must be non-negative.
+
+Example
+-------
+
+.. code:: cuda
+
+    #include <cuda/cmath>
+    #include <cuda/std/cassert>
+
+    __global__ void ipow_kernel() {
+        assert(cuda::ipow(0, 0) == 1);
+        assert(cuda::ipow(2, 2) == 4);
+        assert(cuda::ipow(99, 1) == 99);
+        assert(cuda::ipow(4, 7) == 16384);
+        assert(cuda::ipow(-1, 3) == -1);
+        assert(cuda::ipow(23, -1) == 0);
+    }
+
+    int main() {
+        ipow_kernel<<<1, 1>>>();
+        cudaDeviceSynchronize();
+    }
+
+`See it on Godbolt 🔗 <https://godbolt.org/z/TMacWvz8v>`__