[zfergus]

Description

Replaces the hand-written setup.py (setuptools plus a custom CMake build_ext) with scikit-build-core, touching only the Python packaging, its docs, and the CI path filters.

• [tool.scikit-build] in pyproject.toml drives CMake: a Release build of only the ipctk target, in a persistent build/<wheel tag>/ folder.
• python/CMakeLists.txt installs the module into a python component under if(SKBUILD), and the wheel holds only that component.
• setup.py and MANIFEST.in are removed. The sdist is the git-tracked files minus .github/, docs/ (except PYPI_README.md), notebooks/, and tests/.

Impacts:

• Wheels drop from 297 files to 5 (3.2 MB → 2.6 MB on macOS arm64). On main, setuptools' package discovery treats src/ as a src-layout and ships 291 C++ sources as a top-level ipc/ package in site-packages.
• Third-party install rules (oneTBB's headers and static libraries, ...) stay out of the wheel.
• Reinstalls are incremental: a repeat in-tree pip install . takes 30 s instead of 111 s.
• The minimum Python goes from 3.6 to 3.8, the floor of both pybind11 3 and scikit-build-core.

Build options

• IPCTK_WITH_SIMD and CMAKE_ARGS work as before, so the CI workflows only change their path filters.
• CMake options can also be passed with pip install . -C cmake.define.<NAME>=<VALUE>.
• CMake and Ninja are fetched from PyPI only when the system lacks suitable versions.

Breaking changes

• DEBUG=1 pip install . no longer builds in Debug; use SKBUILD_CMAKE_BUILD_TYPE=Debug (or -C cmake.build-type=Debug).
• An empty IPCTK_WITH_SIMD= now disables SIMD, since CMake treats an empty value as false. Before, it kept SIMD on.
• python setup.py ... commands are gone; use pip install . or python -m build.

Type of change

• Enhancement (non-breaking change which improves existing functionality)
• This change requires a documentation update

How Has This Been Tested?

• Built the sdist, then built a wheel from that sdist, which checks the sdist is complete.
• Installed the wheel in a clean venv and ran nose2 in python/tests: 107 tests pass.
• Compared its contents against a wheel built from main with setup.py.
• IPCTK_WITH_SIMD=0 pip install . gives IPC_TOOLKIT_WITH_SIMD:BOOL=0 in the CMake cache.
• The Linux, Windows, and CUDA wheels are covered by pypi.yml, which runs on this PR.

Test Configuration:

• OS and Version: macOS 27.2 (arm64)
• Compiler and Version: AppleClang 21.0, Python 3.13

Checklist

• I have followed the project style guide
• My code follows the clang-format style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules