Add external thermochemistry benchmarks

[galjos]

Summary

• add an explicit rotational symmetry-number override
• add checksum-pinned anthraquinone and QM9 benchmarks
• correct near-linear geometry classification
• document benchmark scope and results

Validation

• 528 passed, 7 skipped; 96% coverage
• anthraquinone: 0.021320 mV MAE, 0.334662 mV maximum
• QM9: 1,000 molecules; 3.211252 microhartree maximum Gibbs error
• documentation builds with warnings treated as errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.81%. Comparing base (f17ec21) to head (478bca7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #135   +/-   ##
=======================================
  Coverage   95.81%   95.81%           
=======================================
  Files          34       34           
  Lines        2864     2866    +2     
=======================================
+ Hits         2744     2746    +2     
  Misses        120      120           

Flag	Coverage Δ
unittests	95.81% <100.00%> (+<0.01%)	⬆️

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