Add support for additional calculation engines

[galjos]

Scope

Add at least one calculation engine beyond the current DFTB+ path.

Notes

• define the engine interface before adding implementations
• include fixtures and tests for parsing, units, and thermochemistry inputs

[galjos]

Resolved: two calculation engines beyond DFTB+ were added and wired end-to-end.

• tblite GFN-xTB (in-process), PR Add a GFN-xTB engine (tblite) alongside DFTB+ #68 — ThermoScreening/calculator/xtb.py → thermo/api.py::xtb_thermo (engine="xtb").
• Native xtb (xtb --ohess, with --uhf/--chrg/--alpb), PR Add a native-xtb engine with implicit solvation for open-shell species #70 — ThermoScreening/calculator/xtb_cli.py::run_xtb → api.py::xtb_cli_thermo (engine="xtb-cli").

Dispatch: thermo/screening.py::screen validates {dftb+, xtb, xtb-cli}; cli/thermo.py exposes --engine/--method. All engines feed run_thermo through a uniform (optimised ASE Atoms, energy in Hartree, sorted real frequencies) contract. Covered by tests/calculator/test_xtb.py and test_xtb_cli.py.

A formal engine ABC/registry and extending the file-driven single-molecule execute() path to xtb are worthwhile follow-ups but out of scope here.