Add the Zenodo concept DOI to the citation metadata.

[roytsmart]

This is the follow-up to #28. Zenodo archived v0.3.1 and created the DOIs:

• concept DOI (always the latest version): 10.5281/zenodo.23086313
• v0.3.1: 10.5281/zenodo.23086314

The Zenodo record picked up the CITATION.cff metadata: all three authors with their ORCIDs, and the abstract as its description. The concept DOI never changes, so this is the last citation change the repo needs.

Changes

• CITATION.cff: the concept DOI goes under identifiers, as in Add the Zenodo concept DOI to the citation metadata. optika#248. I didn't use the top-level doi field, so Zenodo can't mistake it for the DOI of a new release.
• README: a Zenodo DOI badge in the header.
• README and docs: the Citation section explains the difference between the concept DOI and the version DOIs, and the BibTeX example now includes the doi field.

Testing

• cffconvert --validate reports: "Citation metadata are valid according to schema version 1.2.0."
• Both DOIs resolve on doi.org.

🤖 Generated with Claude Code

https://claude.ai/code/session_01N6zUsPoxW26srMeWormpe8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fc49ae6) to head (fe55688).

Additional details and impacted files

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##              main       #31   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           24        24           
  Lines          883       883           
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  Hits           883       883           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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