[dxnnystyles]

Summary

retries 404s once with a project-named browser user agent and checks DOI handles when publisher URLs return 403.

Type

• Audit
• Explainer
• Bug fix
• Other

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Linked issue

Closes #1173
Closes #1176

Verification

• Focused external-link tests: 18 passed; full suite: 1,314 passed, 1 skipped, 828 warnings.
• Ruff passed. The four Kaggle URLs named in check_external_links.py reports Kaggle dataset pages as broken (404) #1173 returned HTTP 200 in a bounded live check on 2026-10-26.
• DOI resolver responses are mocked in tests; no live DOI request was made. Resolver code 200 means the handle exists but has no values; this checker treats a DOI with no resolution values as broken because it has no URL or 10320/loc target to resolve.
• make lint stops at pre-existing closed issue references in ROADMAP.md (JSON string cells "NA" and "" are missing in the web engine and ordinary values in Python #1036, XLSX date cells are labelled 2020-01-01 00:00:00 by Python and by their serial number (43831) by the web engine #1039, Numeric column headers keep their type in Python (the integer 2020) and are text in the web engine #1045, An empty header cell is named Unnamed: 1 by pandas and an empty string by the web engine #1046, Two long group labels that share their first 29 characters still print identically after the ellipsis fix #1038); the other lint checks passed.

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[github-actions]

@yakew7 @ahmdkaml @propcgamer20-png - new PR touching a path you own, please review.