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/cub/determinism.rst

@@ -0,0 +1,76 @@
+.. _cub-determinism:
+
+Determinism
+===============
+
+Several ``cub`` device algorithms let you request a reproducibility guarantee for a call. The concepts
+behind the three guarantees — ``not_guaranteed``, ``run_to_run``, and ``gpu_to_gpu`` — and the meaning
+of *reproducibility* are described in the :ref:`CCCL determinism overview <cccl-determinism>`. This
+page documents how to request a guarantee for a CUB algorithm and which algorithms support which
+guarantees.
+
+Requesting a guarantee
+----------------------
+
+A determinism guarantee is passed to a device algorithm through its execution environment using
+``cuda::execution::require``. The example below requests run-to-run reproducibility for
+``cub::DeviceReduce::Sum``:
+
+.. literalinclude:: ../../cub/test/catch2_test_device_reduce_env_api.cu
+   :language: c++
+   :dedent:
+   :start-after: example-begin sum-env-determinism
+   :end-before: example-end sum-env-determinism
+
+The general rules for requesting a guarantee are described in the
+:ref:`CCCL determinism overview <cccl-determinism>`.
+
+Each CUB algorithm has its own default guarantee, applied when none is requested, and its own type and
+operator constraints for each guarantee, summarized below. Requesting a guarantee that an algorithm
+does not support is rejected at compile time.
+
+Support matrix
+--------------
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 18 18 18 16
+
+   * - Algorithm
+     - ``not_guaranteed``
+     - ``run_to_run``
+     - ``gpu_to_gpu``
+     - Default
+   * - ``cub::DeviceReduce`` (``Reduce``, ``Sum``, ``Min``, ``Max``, ``TransformReduce``, ...)
+     - Yes
+     - Yes
+     - Yes (partial)
+     - ``run_to_run``
+   * - ``cub::DeviceScan`` (``ExclusiveSum``, ``ExclusiveScan``, ``InclusiveSum``, ``InclusiveScan``, ...)
+     - Yes
+     - Yes (partial)
+     - Yes (partial)
+     - ``not_guaranteed``
+   * - ``cub::DeviceSegmentedReduce``
+     - Yes
+     - Yes
+     - No
+     - ``run_to_run``
+
+.. note::
+
+   The set of algorithms that accept determinism requirements, and the type/operator constraints for
+   each guarantee, are expanding over time. The matrix above reflects the current implementation.
+
+Algorithm-specific determinism models
+--------------------------------------
+
+The three guarantees describe the *scope* of reproducibility and fit most algorithms, where a
+reproducible result means a *bitwise-identical* output. A few algorithms still use the same three
+levels but extend the model with additional, algorithm-specific controls, documented on their own
+pages:
+
+- :ref:`cub::DeviceTopK <cub-topk-requirements>` — determinism applies to *set membership* (which
+  *K* items are selected) rather than a bitwise-identical buffer, and it adds tie-breaking
+  (``cuda::execution::tie_break``) and output-ordering (``cuda::execution::output_ordering``)
+  controls.