feat(gtm): Add Guatemala DPI validator

[AngusHsu]

Description

Add support for Guatemala DPI (Documento Personal de Identificación).

Parent Epic

#14 - Add support for new countries

Guatemala DPI Format

• 13 digits
• Format: NNNN NNNNN NNNN
• Contains municipality and department codes

Tasks

• Research and verify format
• Create src/countries/gtm/ directory
• Implement DPI validator class
• Implement validate() with checksum (if any)
• Implement parse() function
• Add to main dispatcher
• Add to getCountryIdFormat()
• Write comprehensive tests (80%+ coverage)

Acceptance Criteria

• Validation works correctly
• Handles format with/without spaces
• Test coverage 80%+

Labels

enhancement, new-country, guatemala