Generalize Wikidata cross-link registry to cover OEIS (P829)

[katjabercic]

The WD_OTHER_SOURCES registry in web/slurper/wd_raw_item.py already handles cross-links for nLab, MathWorld, ProofWiki, and EoM. Wire OEIS in the same way using Wikidata property P829 ("OEIS ID", 443 statements as of 2026-06-06).

How the registry works

WD_OTHER_SOURCES maps Item.Source → {wd_property, json_key}. The SPARQL query, a per-source WikidataSlurper, and save_links are all auto-generated from it. Links get label = Link.Label.WIKIDATA (provenance = Wikidata, not the target source). No new Link.Label is needed.

Tasks

• Add Item.Source.OEIS: {"wd_property": "wdt:P829", "json_key": "oeisID"} to WD_OTHER_SOURCES.
• Add an OEISRawItem(OtherWdRawItem) subclass with a url() builder and a case in BaseWdRawItem.raw_item.
• Verify identifier form: OEIS stores A000045; P829 values also include the A prefix — they should match as-is.
• make fix-files clean; tests pass.

Dependency

The OEIS slurper (issue #16) runs after this, this determines which sequences to import.