Validate total entropy regression value

[galjos]

Scope

Validate the expected total entropy value used by the thermochemistry regression test.

Notes

• identify the reference calculation or formula
• update the assertion or fixture if the current value is not the intended reference

[galjos]

Resolved: the asserted total entropy (99.467956 cal/mol/K, test_thermo_aq_2) is confirmed correct against ASE IdealGasThermo for the same anthraquinone-dianion geometry/frequencies. An independent ASE reference matches only at the physically correct symmetry number σ=4 (σ=1→102.22, σ=2→100.85, σ=4→99.46801) — and the tool derives σ=4 itself via system.py::rotational_symmetry_number, so the match isn't fitted. It is also internally self-consistent (trans 41.90 + rot 29.30 + vib 28.26 + elec 0 = 99.468).

The entropy engine was additionally cross-validated in PR #69 (test_S_H_G_match_ase over 13 molecules incl. charged species, and entropy vs experiment).