feat(pdftract-dtpwa): implement contract profile per Phase 7.10 schema
- Rewrite profiles/builtin/contract/profile.yaml following Phase 7.10 schema
with match predicates, extraction tuning, and field extractors
- Create tests/fixtures/profiles/contract/ directory with 5 expected outputs
- Add comprehensive regression tests in tests/profiles/test_contract.rs
- Profile extracts: parties, effective_date, term, governing_law, signatures
Fixtures cover: NDA, employment agreement, MSA, service agreement, real estate purchase
Closes: pdftract-dtpwa
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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