Add pdftract grep subcommand with ripgrep-style flag compatibility.
Implements all flags from the plan options table with proper defaults:
- Literal match mode by default (-F style)
- -E for full regex mode
- -i for case-insensitive search
- -w for word boundaries
- -v for invert match
- -l, -c for output modes
- -j for thread control
- --ocr, --json, --highlight DIR
- --progress/--no-progress/--progress-json
- Feature-gated behind 'grep' feature flag
Unit tests cover all flag combinations and edge cases.
Stub implementation exits with code 2 pending 7.8.2-7.8.10.
Closes: pdftract-4xu46
