Add citation metadata and link the published JSIAM Letters paper

[t-uda]

Goal

Add repository-level citation metadata for EllPHi and link the now-published JSIAM Letters paper.

Background

The relevant paper appears to be:

• Tomoki Uda, Fast and accurate computation of ellipse tangency time for anisotropic persistent homology, JSIAM Letters 18 (2026), 13--16.
• DOI: https://doi.org/10.14495/jsiaml.18.13
• J-STAGE PDF: https://www.jstage.jst.go.jp/article/jsiaml/18/0/18_13/_pdf/-char/en

Tasks

• Add a root-level CITATION.cff / citation.cff file, following the current Citation File Format schema.
• Include software-level metadata for EllPHi:
  • title/name of the software,
  • repository URL,
  • authors/maintainers,
  • licence if available,
  • release/version metadata if already defined in the repo.
• Add the JSIAM Letters paper as the preferred-citation or otherwise clearly reference it from the CFF file.
• Add or update README/documentation citation instructions so users know how to cite both:
  • the software repository/package, and
  • the published JSIAM Letters paper.
• Ensure the DOI and J-STAGE link are correct before merging.

Acceptance criteria

• GitHub displays citation metadata from the CFF file.
• README or equivalent documentation includes a concise citation section.
• The published paper link and DOI are included and not merely mentioned as forthcoming/preprint.
• Existing package metadata is not contradicted by the new citation metadata.

[t-uda]

Resolved by PR #131 (merged into ellphi-0.1.3): root-level CITATION.cff (CFF 1.2.0, validated) with software metadata and the JSIAM Letters paper as preferred-citation (DOI 10.14495/jsiaml.18.13, resolution verified), plus a README Citation section with separate paper/software BibTeX entries. Docs-site updates deferred to the #118 Pages/MkDocs rework. Note: the GitHub citation widget reflects CITATION.cff once the release branch reaches main.