Fix wheel ABI: produce .abi3.so to match cp312-abi3 wheel tag

[marevol]

Summary

scikit-build-core tags wheels as cp312-abi3 via wheel.py-api = "cp312" in pyproject.toml, but nanobind_add_module(...) without STABLE_ABI produces version-specific .cpython-3X-<platform>.so files. The resulting wheel claims abi3 compatibility in its filename but fails to load on Python 3.13/3.14 because the inner .so files use a CPython-3.12-only extension suffix:

>>> from irspack.evaluation import EvaluatorCore
ModuleNotFoundError: No module named 'irspack.evaluation._core_evaluator'

Root cause

nanobind_add_module(... STABLE_ABI ...) silently turns STABLE_ABI off when Python::SABIModule is not a CMake target (see nanobind-config.cmake L368-L373). scikit-build-core already sets SKBUILD_SABI_COMPONENT=Development.SABIModule, but the find_package(Python ...) call in CMakeLists.txt was not forwarding it, so the SABI target was never created.

Changes

• Pass ${SKBUILD_SABI_COMPONENT} to find_package(Python ...) so the Python::SABIModule target exists for SABI builds.
• Add STABLE_ABI to each nanobind_add_module call so extensions are compiled against the limited API and named .abi3.so.

Test plan

• Local build on CPython 3.13.3 produces _core_evaluator.abi3.so etc.
• Local build on CPython 3.14.5 produces _core_evaluator.abi3.so etc.
• import irspack and from irspack.evaluation import EvaluatorCore succeed on both 3.13 and 3.14 from the cp312-abi3 wheel.
• CI green on all supported Python versions.

[tohtsky]

LGTM!