Add VQM24 transition-state benchmark

[galjos]

Summary

• resolve ambiguous near-linear mode counts
• add VQM24 transition-state regressions
• add a checksum-verified 1,000-structure benchmark

Closes #138

Validation

• 535 passed, 7 skipped; 96% coverage
• VQM24 benchmark passed
• strict documentation build passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.82%. Comparing base (a5383a2) to head (3354b9f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
+ Coverage   95.81%   95.82%   +0.01%     
==========================================
  Files          34       34              
  Lines        2866     2877      +11     
==========================================
+ Hits         2746     2757      +11     
  Misses        120      120              

Flag	Coverage Δ
unittests	95.82% <100.00%> (+0.01%)	⬆️

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