Correct v2.4.0 citation metadata

[berrygrant]

What changed

• replace the citation frozen at v2.0.0 with version-aware R package citation metadata
• cite the version-specific 2.4.0 Zenodo DOI (10.5281/zenodo.21864533)
• add CITATION.cff for GitHub and future Zenodo deposits
• declare Grant M. Berry as the software creator, consistent with DESCRIPTION; retain AI assistance in the README acknowledgment
• add regression coverage and a release-tag guard for citation/package version drift
• document how users can obtain the installed release's citation and BibTeX

Root cause

inst/CITATION was last updated for the 2.0.0 rename and remained hard-coded while later releases changed. Zenodo inferred creators from GitHub contributor statistics because the repository had no explicit CFF or Zenodo metadata file.

Follow-through after merge

• move v2.4.0 and 2.4.0 to the corrected merge commit
• update the GitHub release disclosure
• correct the existing Zenodo 2.4.0 record's creators, license, description, and archived source ZIP within Zenodo's post-publication correction window

Validation

• R CMD build . produced phontrast_2.4.0.tar.gz
• R CMD check --no-manual phontrast_2.4.0.tar.gz — Status: OK
• installed citation("phontrast") reports version 2.4.0 and DOI 10.5281/zenodo.21864533
• generated BibTeX reports the same version and DOI
• CITATION.cff parses as YAML and agrees with DESCRIPTION
• git diff --check