feat(forcefield): add SixthPower mixing rule and aliases for Class-II CFF

[Eng-LO]

Summary

This pull request completes Class-II 9-6 / PCFF force field support in RASPA3 by adding the Sixth-Power (Waldman-Hagler) mixing rule, common string aliases, and dedicated unit tests, resolving #66.

Following the addition of the core CFF9_6 and CFFEpsilonSigma functional forms, this PR provides:

1. Sixth-Power (Waldman-Hagler) Mixing Rule (MixingRule::SixthPower):
   Enables automatic cross-interaction computation for Class-II force fields (e.g. PCFF, SciPCFF, COMPASS) where:
   $$\sigma_{ij} = \left(\frac{\sigma_i^6 + \sigma_j^6}{2}\right)^{1/6}, \quad \varepsilon_{ij} = 2 \sqrt{\varepsilon_i \varepsilon_j} \frac{\sigma_i^3 \sigma_j^3}{\sigma_i^6 + \sigma_j^6}$$
   Recognizes "SixthPower", "Waldman-Hagler", "sixthpower", and "waldman-hagler" in force_field.json.
2. String Aliases in VDWParameters::stringToEnum:
   Accepts standard user notation including "cff_eps_sigma", "class2", "class-2", "pcff", "cff96", and "cff_9_6", eliminating invalid interaction string errors during input parsing.
3. Dedicated Unit Tests (cff_cross_mixing.cpp):
   Adds tests for alias resolution, analytical sixth-power parameter mixing, energy evaluation across distances, and spatial gradient factors using the PairDerivatives framework.
4. Cross-Engine Benchmark Parity:
   To independently verify numerical correctness and thermodynamic sampling against reference codes (LAMMPS pair_style lj/class2/coul/cut, pair_modify mix sixthpower), a full polyethylene–methane benchmark (6,020-atom host, 300 K) was performed:
   • Deterministic single-point parity: interaction energy matches LAMMPS to $2.8 \times 10^{-11}$ relative difference.
   • Thermodynamic chemical potential consistency: $\mu_{\text{ex}} = 10.40 \pm 0.19\text{ kJ/mol}$ (RASPA3 Widom/CFCMC) vs $13.06 \pm 2.23\text{ kJ/mol}$ (LAMMPS Widom), consistent within $1.2\sigma$.
     Full benchmark datasets, inputs, and raw output logs are archived at:
     https://github.com/Eng-LO/cff96-polyethylene-benchmark

Closes #66