Implements Phase 5.6.3: FeatureSignals extraction computed during Phase 4 assembly.
- Added profiles/signals.rs module with PageSignalAccumulator and extract_feature_signals()
- Predefined text patterns: currency symbols, ISO dates, INVOICE, WHEREAS, Abstract, References, page numbers, bullets, math operators
- Per-page signal extraction: text content, fonts, table count, heading depth, glyph density
- Document-level aggregation: page count, font diversity, presence flags (signature field, form field, math operators, bullet lists, footer page numbers)
- All regex patterns compiled once via OnceLock for performance
- 23 unit tests covering all functionality
Closes: pdftract-49cn
51cb277535