Add a C ABI library option to the cppjswasm template - #221

Description

add_c_abi now also applies to the cppjswasm template and gives it the layout the cpp template got in #220:

• cpp/<name>/c_api.cpp plus the existing example.cpp build the generic library <module>/lib/lib<module>, declared in <module>/include/<module>/<module>.h.
• cpp/python/extension.cpp is the pybind11 extension, linked against the library through depends.
• cpp/CMakeLists.txt builds and installs the library without Python.
• The package gets the get_* helpers and the Windows DLL directory, cibuildwheel gets the repair exclusions, and Pyodide builds use HATCH_CPP_LIBRARY_SOURCE=direct.
• The JavaScript side is unchanged. It still compiles the C++ sources with Emscripten and does not use the C ABI.

Differences from the cpp template:

• This template binds the Example class, so the C ABI has an opaque <module>_example handle with _new, _free, _stuff, and _set_stuff. Functions that allocate catch exceptions and report failure through the return value. The extension wraps the handle in a small class, so the Python surface is the same with and without the option.
• The C API definitions in c_api.cpp are wrapped in // clang-format off and on. Each signature contains the module name twice, so clang-format would break the lines differently per project and a new project could fail make lint. I checked the generated C++ and Python formatting with a 3-character, a 30-character, and a 46-character module name.

Two changes also affect the output without add_c_abi:

• extension_cpp.pyi now declares Example. The extension already exported it, and ty rejected the import in the new test.
• js/build.mjs keeps the extension path in a constant. The path contains the module name and was repeated, so oxfmt wrapped two calls once the name passed 25 characters and pnpm lint failed for the cppjswasm-capi example.

The Windows consumer test and the delvewheel exclusion match #223.

Checked on macOS with the cppjswasm-capi example and SKIP_HATCH_JS=1: C++ and Python lint, make checks, and the five tests pass, and the package builds and imports in external mode against the CMake install and in direct mode. I did not run the JavaScript build, Windows, or Pyodide locally.

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)