feat(pdftract-206o6): implement scientific_paper profile with fixtures and tests
Author profiles/builtin/scientific_paper.yaml per Phase 7.10 YAML schema:
- Match predicates: text_contains (Abstract, References, doi:, arXiv:, Bibliography)
- Structural predicates: has_math, heading_depth, page_count
- Extraction tuning: xy_cut reading order for 2-column layout
- Fields: title, authors, abstract, doi, journal, publication_date, references
Add 5 fixtures covering diverse scientific paper types:
- arXiv preprint (CC-BY license)
- PLOS ONE journal article
- IEEE-style 2-column paper
- Nature-style single-column with sidebar
- ACM/IEEE conference proceedings
Add comprehensive regression tests in test_scientific_paper.rs:
- Profile schema validation
- Fixture structure verification
- Expected output consistency checks
- Match predicate validation
- Fixture diversity verification
- xy_cut reading order verification
- DOI regex format validation
Co-Authored-By: Claude Code (claude-opus-4-7) <noreply@anthropic.com>
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