Implement compile-time phf::Set of 20,000 common English words for
dictionary coverage scoring in readability analysis (Phase 4.7).
Key changes:
- Added wordlist-en-20k.txt (20k frequency-sorted English words)
- Extended build.rs to generate phf::Set from wordlist
- Added layout/wordlist.rs module with is_english_word() API
- Added wordlist benchmarks (< 100 ns lookup achieved)
Test results:
- All 9 unit tests pass
- Benchmarks: 13-62 ns per lookup (well under 100 ns requirement)
- Binary size: Estimated ~200-220 KB (within 250 KB limit)
Closes: pdftract-9wevc
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
