build: modernize build system, dependencies and CI

[oberbichler]

Summary

The project no longer built on current toolchains (CMake 4, Python 3.11+). This PR modernizes the entire build, dependency and release setup — +280/−252,550 lines, most of it removed vendored libraries.

Build & packaging

• setup.py/distutils (removed in Python 3.12) replaced with pyproject.toml + scikit-build-core
• Version derived from git tags via setuptools-scm (replaces pumpit)
• CMake modernized: 3.24+, target-based includes/definitions

Dependencies

• Eigen 3.4, fmt 11, spdlog 1.15 and robin-map fetched via FetchContent instead of vendoring (~11 MB removed from the repo); only the frozen hyperjet copy stays vendored
• sparsehash dropped — dense_hash_map replaced by tsl::robin_map
• pybind11 comes from PyPI as a build requirement; code builds against pybind11 3.x and Python 3.12–3.14
• OpenMP is now optional (build works on macOS without libomp)
• Log.h/format use compile-time checked format strings (required by fmt 11)

CI/CD

• CI: current actions, matrix Python 3.12–3.14 × Linux/Windows/macOS
• Publish rewritten with cibuildwheel: current manylinux, first-time Linux-aarch64 and Apple-Silicon wheels, sdist, and PyPI Trusted Publishing (OIDC) instead of password secrets

Tests

• Adapted to the current hyperjet API (hj.variables(...), .hm()); skipped via importorskip when hyperjet is unavailable
• hyperjet is installed from GitHub master in CI — the PyPI release (1.5.0) no longer builds on modern Python

Verification

All 43 tests pass on Python 3.13 (macOS/arm64), both against a direct CMake build and against an install from the built sdist.

Notes

• ⚠️ Before the next release: configure Trusted Publishing on PyPI (project → Publishing → add GitHub oberbichler/EQlib, workflow publish.yml, environment pypi)
• Requires Python ≥ 3.12 (breaking for users on older versions)
• Follow-up idea: release a modernized HyperJet to PyPI, then CI can install it normally