Add a C ABI library option to the cpp template - #220

Description

add_c_abi now also applies to the cpp template. With it, the generated project has:

• cpp/<name>/: C++ code plus c_api.cpp, built by hatch-cpp as the generic library <module>/lib/lib<module> with hidden visibility. hatch-cpp checks its exports against the header and the <MODULE>_ABI_VERSION string.
• <module>/include/<module>/<module>.h: the C header with the export macro. It is checked in inside the package, because hatch-cpp does not copy headers and an editable install has to find it in the same place as a wheel does.
• cpp/python/extension.cpp: the pybind11 extension, linked against the library through depends.
• cpp/CMakeLists.txt: builds and installs the library without Python, with an exported target, a relocatable package config, a pkg-config file, an @rpath install name, and VERSION/SOVERSION. bump-my-version updates its version.
• get_prefix, get_include, get_library_dirs, and get_libraries in the package, and os.add_dll_directory for the bundled lib on Windows.
• auditwheel and delocate repair commands that leave the bundled library in place, and HATCH_CPP_LIBRARY_SOURCE=direct for Pyodide.
• Tests that load the library, compile a C program against the installed package, and build a find_package consumer against the CMake install.

The option requires hatch-cpp>=0.6.0. Without add_c_abi the generated project is unchanged apart from the new answer in .copier-answers.yaml.

Checked on macOS with the cpp-capi example: make lint, make lint-docs, make checks, and make test pass. A wheel repaired with the delocate command passes the tests in a clean environment, and the package builds and imports with HATCH_CPP_LIBRARY_SOURCE set to external (against the CMake install) and direct. The Windows and Pyodide parts have not been run.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated (if applicable)
• Changelog / version bump (if applicable)