Implement Phase 7.4.4: AcroForm + XFA field combiner with XFA-wins
precedence. This enables pdftract to handle hybrid PDF forms that
contain both AcroForm and XFA representations.
- Add FormFieldValue enum with Text, Button, Choice, Signature variants
- Add ChoiceValue enum for single/multiple choice selections
- Implement combine() function that merges AcroForm and XFA fields
with XFA values taking precedence on collision
- Implement XFA boolean string conversion ("true"/"false"/"1"/"0")
to Button selected state
- Preserve AcroForm type hints when XFA provides the value
- Emit diagnostics for field name collisions
- Sort output alphabetically by field name
Closes: pdftract-2qum
