- Fix Token::Keyword to use b"..." .to_vec() instead of static strings
- Improve unknown keyword diagnostics to show actual keyword bytes
- Remove unused has_valid_line_ending variable in stream keyword lexer
- Add stream_header_valid_line_endings test for stream keyword validation
All hex string lexer tests pass (16 unit tests + 2 proptests).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Bead-Id: pdftract-2hm4