docs: add FlashPDE arXiv citation

[factnn]

Summary

• link the published arXiv preprint from both READMEs
• replace the placeholder BibTeX with the official arXiv metadata
• synchronize CITATION.cff with the paper title and identifiers

Verified against arXiv:2607.18020.

Summary by CodeRabbit

• Documentation
  • Updated English and Chinese citation guidance to link directly to the arXiv publication page.
  • Revised BibTeX examples with arXiv-specific metadata and citation format.
  • Updated citation metadata to include DOI and arXiv identifiers.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 48087ab1-cc72-42aa-8640-ada0c5e12452

📥 Commits

Reviewing files that changed from the base of the PR and between b365b51 and 069ed07.

📒 Files selected for processing (3)

• CITATION.cff
• README.md
• README_zh-CN.md

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📝 Walkthrough

Walkthrough

Citation metadata was updated in CITATION.cff, README.md, and README_zh-CN.md to reference the arXiv record, DOI, repository, and arXiv-specific BibTeX fields.

Changes

Citation metadata synchronization

Layer / File(s)	Summary
Canonical citation metadata CITATION.cff	Adds DOI and arXiv identifiers, reorganizes repository and arXiv URLs, updates the title entry, and removes the preprint note.
README citation blocks README.md, README_zh-CN.md	Replaces PDF and GitHub citation references with the arXiv page and updates BibTeX entries from @article to @misc with arXiv-specific fields.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related PRs

• factnn/FlashPDE#2: Updates the same citation metadata and README citation sections for the 2026 arXiv paper.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: adding the FlashPDE arXiv citation and related documentation updates.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/arxiv-citation

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