np.linalg.eig/eigvals return complex128 for real symmetric input from NumPy 2.5.0

Bisected np.linalg.eig(np.diag([2.0,-1.0]))[0].dtype: float64 on 1.26.4, 2.0.2, 2.1.3, 2.2.6, 2.3.5, 2.4.6; complex128 on 2.5.0 and 2.5.4 (eigvals too). Not documented by a versionchanged note in numpy/linalg/_linalg.py. Consequence: np.log of a negative eigenvalue silently gives pi*j instead of nan+RuntimeWarning; float(w[0]) only warns. Use eigh/eigvalsh for symmetric/Hermitian input, or check np.allclose(w.imag, 0) and take .real explicitly; don't rely on the dtype when supporting NumPy < 2.5 and >= 2.5.