Add CITATION.cff

[LiNk-NY]

This automated PR adds or updates CITATION.cff using the
cffr R package (cffr::cff_write()).
Citation metadata is derived from the package DESCRIPTION, and the
auto-citation is also included as a references entry.

Changes

• Added / updated CITATION.cff – generated by cffr::cff_write() with
  the auto-citation included under references

How citations are handled

• If inst/CITATION exists in the repository it is automatically used by
  cff_write() as the preferred-citation in CITATION.cff.
• Otherwise, citation metadata is derived solely from DESCRIPTION.
• The auto-citation (equivalent to citation(".", auto = TRUE)) is always
  included under the references key so that both the preferred and the
  software citation are present.

Why

A CITATION.cff file makes it easy for users and tools (GitHub, Zenodo,
Zotero, etc.) to cite your package correctly.

Customisation

You can further customise the generated file by editing CITATION.cff
directly after this PR is merged. See the
cffr vignette for
details.

Opened by the repo-audits workflow (run #24527883700).