Support PEP 829

[henryiii]

PEP 829 replaces import lines in .pth.

🤖 Plan summary based on PEP 829

Problem

Implement PEP 829 support for scikit-build-core's editable install startup mechanism. Today redirect editable wheels write _<name>_editable.py plus _<name>_editable.pth containing an import _<name>_editable line; inplace editable writes only a path-only .pth. PEP 829 changes startup code execution to paired .start files on Python 3.15+, while keeping import lines in .pth for path extensions and for straddling older Pythons.

Current state

• Redirect editable startup artifacts are produced in src/scikit_build_core/build/wheel.py::_make_editable.
• The callable bootstrap implementation already exists in src/scikit_build_core/resources/_editable_redirect.py; today it is invoked through import _<name>_editable from the .pth.
• Redirect editable wheel coverage exists in tests/test_pyproject_pep660.py; low-level redirect content coverage exists in tests/test_editable_unit.py and tests/test_editable_redirect.py.
• Inplace editable mode only writes package paths into _<name>_editable.pth, so PEP 829 startup-entry changes should only affect redirect mode.
• Stable ABI / cross-version intent is represented through wheel.py-api (for example cp39, cp315t, py3) and tag computation in builder/wheel_tag.py.

Proposed approach

1. Add a small helper in the editable wheel-writing path that decides which startup artifacts to emit for redirect editable wheels:
   • always keep _<name>_editable.py
   • keep _<name>_editable.pth import statements for older Python compatibility and Cython path injection
   • add _<name>_editable.start pointing at a zero-argument callable inside _<name>_editable.py
   • gate .start emission so it is always written on Python 3.15+, and also written on older builders for non-version-specific tags (Stable ABI cp3x / cp3xxt and Pythonless py3 / py2.py3) because those wheels may later be installed on Python 3.15+
2. Refactor the generated editable bootstrap so the startup action can be referenced as a PEP 829 entry point such as _<name>_editable:install.
3. Keep inplace editable behavior unchanged unless investigation during implementation shows a compatibility reason to touch it.
4. Extend tests around editable wheel contents and generated bootstrap text to cover:
   • redirect wheels on Python 3.15+ producing .start
   • redirect wheels on Python < 3.15 keeping compatibility behavior
   • Stable ABI / non-version-specific cases that must still emit .start
   • inplace wheels continuing to avoid executable startup artifacts
5. Update user-facing docs if editable wheel artifact behavior or compatibility guidance is documented.

Notes / considerations

• PEP 829 requires .start files to contain pkg.mod:callable entries, so the generated redirect module likely needs a dedicated zero-argument entry-point wrapper instead of relying on side effects at import time.
• The existing .pth also adds package source paths for Cython support; the plan should preserve those path lines regardless of .start handling.
• Scope decision: treat Pythonless tags like Stable ABI tag