Add button field value extraction distinguishing pushbutton, checkbox,
and radio button types via /Ff flags. Extracts selected state and
appearance state name (/Yes, /Off, custom).
- New module: forms/value_button.rs with ButtonKind enum and ButtonValue
- Updated FormFieldValue::Button variant with kind and state_name fields
- 15 unit tests covering all button types and edge cases
- Fixed CCITTFaxDecoder test syntax blocking test execution
Closes: pdftract-66pgk
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
