feat(omn): Add Oman national ID validator

[AngusHsu]

Description

Add support for Oman National ID (البطاقة الشخصية).

Parent Epic

#14 - Add support for new countries

Oman National ID Format

• Research needed on exact format
• Likely 8-9 digits with check digit

Tasks

• Research and verify format
• Create src/countries/omn/ directory
• Implement NationalId 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
• Format documented
• Test coverage 80%+

Labels

enhancement, new-country, oman