two-mode Gaussian entanglement of the comb

New module entangle: two-mode Gaussian entanglement computed directly from the covariance matrices the package already produces.

• ppt_symplectic_eigenvalue: partial-transpose symplectic eigenvalue from the local-symplectic invariants (Simon, PRL 84, 2726 (2000)) - a necessary and sufficient entanglement test for 1x1-mode Gaussian states.
• • logarithmic_negativity: E_N = max(0, -ln(2 nu_tilde / hbar)) (Vidal and Werner, PRA 65, 032314 (2002)).
• • duan_epr_sum: the symmetric Duan-Simon EPR sum, with its separability bound.
• • entanglement_report: all of the above for every sideband pair of a comb state.
• • ppt_eigenvalue_direct: an independent cross-check via an explicit partial transpose, asserted in the tests to agree with the invariant route to 1e-9.
    Validation in the test suite: the two-mode squeezed vacuum reproduces E_N = 2r exactly; the photonic-molecule twin beams are PPT-entangled below threshold in a regime where the Duan sum alone misses the entanglement.

Also: README Status section reconciled with the release history.