Use pymatgen-core for symmetry

[galjos]

Summary

• Replace the full pymatgen dependency with pymatgen-core >= 2026.5.18.
• Handle monoatomic and overlapping atom symmetry cases.
• Add symmetry edge-case regression coverage.

Validation

• python -m pytest -q
• Clean install with pymatgen-core only

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.78%. Comparing base (22ab27f) to head (2d6e8b1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   93.54%   93.78%   +0.23%     
==========================================
  Files          21       21              
  Lines        1178     1190      +12     
==========================================
+ Hits         1102     1116      +14     
+ Misses         76       74       -2     

Flag	Coverage Δ
unittests	93.78% <100.00%> (+0.23%)	⬆️

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