Make atom-group label disambiguation parent-aware

[JFRudzinski]

Analogous to nomad-coe/atomistic-parsers#178

Fixes GROMACS topology groups with identical labels being merged across distinct parent chains during results normalization.

Atom-group labels are now disambiguated parent-first using their finalized ancestry. Equivalent groups under the same parent still aggregate, while same-sized groups under different protein chains receive distinct labels.

Also adds regression coverage confirming that:

• repeated monomers within one parent retain a shared label;
• equivalent groups and monomers under different chains receive distinct labels;
• hierarchy ordering and atom indices remain unchanged;
• generated labels do not collide with existing labels.

Validation: GROMACS tests pass (15 passed, 1 skipped), Ruff passes, and Frame49’s (external test data) normalized SA groups each have (1, 30) chain-local indices.