extract_formulas/extract_figures derive paper_id from Path(pdf_path).stem, which
need not equal papers.id (and won't, given canonical ids). Inserting that stem
violated the formulas/figures -> papers(id) FK on rich (.pdf) ingest. The DELETE
already used paper.id; the INSERTs now do too.
Strengthened the rich-ingest test: the fake formula/figure carry a filename-stem
paper_id distinct from papers.id, and the test asserts the inserted rows are
keyed on papers.id.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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