Add release validation gates

[galjos]

Summary

• require a real DFTB+ screen in Linux CI
• add a reproducible anthraquinone redox regression
• publish benchmark pages in the documentation

Validation

• 522 passed, 7 skipped; 96% coverage
• required DFTB+ workflow passed
• strict docs, lint, and package checks passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #134   +/-   ##
=======================================
  Coverage        ?   95.81%           
=======================================
  Files           ?       34           
  Lines           ?     2864           
  Branches        ?        0           
=======================================
  Hits            ?     2744           
  Misses          ?      120           
  Partials        ?        0           

Flag	Coverage Δ
unittests	95.81% <ø> (?)

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