Project structure and bug fix

[Atraxus]

Sorry @valentinstoecker for the larger rework. I tried to really make this stable for the future after I encountered a bug. Here is a summary of the changes, following online documentation:

Commit overview

76602b7 — src-layout + private _core + abi3 wheels

• Package to src/rayx/, C++ to src/rayx_ext.cpp, one top-level CMake. [1][3]
• Compiled module now private _core; __init__.py re-exports it. Fixes the unimportable-wheel bug. [1]
• STABLE_ABI → one cp312-abi3 wheel; needs Development.SABIModule (CMake ≥3.26) + wheel.py-api="cp312". [2][4][6]
• __init__.py cleanup: dynamic __all__, drop __name__ hack, no leaked Path.

61b7dc5 — public typing

• py.typed marker. [7]
• Hand-written __init__.pyi (native types come from generated _core.pyi). [2][7]
• .gitignore !__init__.pyi so the stub ships (scikit-build-core is gitignore-aware). [5]

afe69fc — tests

• New-layout tests + __all__-completeness guard.
• test_stubs.py: fail if a helper is missing from the stub (stubtest too fragile with editable + generated stubs).

Sources

[1] https://nanobind.readthedocs.io/en/latest/packaging.html
[2] https://nanobind.readthedocs.io/en/latest/api_cmake.html
[3] https://github.com/wjakob/nanobind_example
[4] https://scikit-build-core.readthedocs.io/en/latest/configuration/index.html
[5] scikit-build-core build/_file_processor.py (each_unignored_file)
[6] https://cmake.org/cmake/help/latest/module/FindPython.html
[7] https://peps.python.org/pep-0561/