sejm_interpellations data files (scoped fix for #24)

This is the review fix for #24 (sejm_interpellations), delivered as a separate PR because PR refs are author-scoped and this account cannot push to refs/pr/24 or to plain branches on this repo (pre-receive hook declined). It is one clean commit on main @ 34e7ea3 — exactly 4 files, +172 / −0:

• data/sejm_interpellations/sejm_interpellations.md (datasheet — scrubber named + NERGAL gate, inclusion sentence + caveats)
• data/sejm_interpellations/sejm_interpellations.parquet (25,950 documents / 28,180,895 cl100k_base tokens; LFS object already in the repo store)
• data/sejm_interpellations/sejm_interpellations.stats.json
• artifacts/source_findings.md — one appended entry (33 lines)

Against #24's review items:

1. Release bookkeeping dropped — no CHANGELOG / README / VERSION / RELEASE_DATE / make_docs changes; the old branch's whole-file sources.py rewrite, AGENTS.md, build_dynaword.py and 464-line findings drift are gone. The registry entry, src/fetch_sejm_interpellations.py and src/test_sejm_interpellations_contract.py are already on main in newer form — untouched here, so #28 cannot regress.
2. Shared diffs dropped — source_findings.md gets exactly one appended entry on top of main's version.
3. PII — the datasheet states which scrubber ran on the data (the scrub_pii pattern pass from #20 — phones → [Telefon]; e-mail / PESEL / NIP / REGON / IBAN → [PII]) and that it predates #38: the shard must be re-run through SlayerLab/NERGAL before it is admitted to a release — interpellations quote constituency casework naming private persons. Merge with release: None is fine meanwhile.
4. Before/after of the header-chrome stripping is in the #24 thread (live /body endpoint through extract_body()/normalize_document()).
5. Inclusion + caveats — in the datasheet: the written Q&A register (ministry-voiced replies + argumentative MP questions) is a document type the corpus otherwise lacks; caveats: single term (10th), private citizens named in casework.

Verification: python3 -m pytest src/test_sejm_interpellations_contract.py — 5 passed (entry, fetcher and test are main's). Merging this closes the #24 review items; #24 itself can then be closed as superseded, or its author can fast-forward refs/pr/24 to this commit before merging.

Thanks — superseded by #24, which is now rebased onto main and carries this PR's datasheet and findings entry.