EVJJI theorem twoState_entropy_eq

[oe-parks]

Formalizes the informal lemma tagged EVJJI: a closed form for the
thermodynamic entropy of the two-state canonical ensemble.

Added, both in Physlib/StatisticalMechanics/CanonicalEnsemble/TwoState.lean:

• twoState_entropy_eq with x = β T * (E₁ - E₀) / 2, gives
  S = kB * (log (2 * cosh x) - x * tanh x). Proved through
  thermodynamicEntropy_eq_shannonEntropy, which carries no side conditions,
  rather than through the Helmholtz free energy. That route needs
  0 < T.val, IsFiniteMeasure, and integrability.

• twoState_entropy_eq_T_neq_zero is the same statement carrying T ≠ 0, so
  downstream results on positive temperatures get a matching signature without
  having to know the unconditional version is safe at zero.

Removed the Physlib.Meta.Informal.Basic import, now unused in the file.

I left the primary result unconditional, matching the choice made for
twoState_helmholtzFreeEnergy_eq in #1474. Since β 0 = 0, the RHS is
kB * log 2 at T = 0; the docstring notes this.

Notes to reviewer: look first at the statement, since that's the part that has to be right.
Then hp0 and hp1, the two occupation probabilities in closed form. The rest
is algebra. The corollary and the import removal are a line each.

Second contribution to Physlib. Happy to adjust anything. I'm not particularly attached to the T ≠ 0 restatement if you'd rather it weren't there.

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[jstoobysmith]

Approved - looks good to me. Thanks for this PR