feat(publication): add GenGA arXiv publication

[shunk031]

Motivation

Add a standalone publication page for the GenGA arXiv preprint.

Changes

• Add GenGA publication metadata, authors, abstract, and tags.
• Add Preprint and PDF links for arXiv:2608.05478.
• Add the generated featured.png thumbnail.
• Use an empty summary for standalone preprints to avoid unintended rendered description and OGP text.

Testing

• Frontmatter lint: passed for 193 files; 3 pre-existing casing warnings remain.
• pytest: 54 passed.
• Hugo build: passed; 844 pages generated.
• featured.png: 640x480.
• arXiv URL: HTTP 200.
• Full lychee scan: not rerun because it hangs.