Implement country-specific PEPPOL validation rules

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Summary

Implement country-specific validation rules for PEPPOL BIS Billing 3.0 (DK, IT, NL, NO, SE).

Background

PR #32 added foundational PEPPOL support with 84 auto-generated rules, but only implemented 5 core rules. The remaining 79 rules include important country-specific validations that need to be implemented.

Country-Specific Rules

Denmark (DK-R-*)

• DK-R-002: Legal entity requirement
• DK-R-003: ItemClassification UNSPSC version
• DK-R-004 through DK-R-016: Various Danish supplier requirements

Italy (IT-R-*)

• IT-R-001: Seller tax registration identifier format (11-16 chars)
• IT-R-002: Postal address line 1 required
• IT-R-003: City required
• IT-R-004: Post code required

Netherlands (NL-R-*)

• NL-R-001 through NL-R-009: Dutch supplier and credit note requirements

Norway (NO-R-*)

• NO-R-001: VAT number format validation
• NO-R-002: "Foretaksregisteret" requirement

Sweden (SE-R-*)

• SE-R-001 through SE-R-013: Swedish VAT number and organization number validations

Implementation

The rules are already defined in rules/peppol.go. Need to:

1. Implement validation logic in check_peppol.go
2. Detect seller/buyer country from postal address or VAT registration
3. Apply country-specific rules conditionally
4. Add comprehensive test coverage for each country

References

• PEPPOL schematron: https://github.com/OpenPEPPOL/peppol-bis-invoice-3/blob/master/rules/sch/PEPPOL-EN16931-CII.sch
• PEPPOL BIS Billing 3.0 spec: https://docs.peppol.eu/poacc/billing/3.0/bis/

Related

Part of #24 (PEPPOL BIS Billing 3.0 support)
Depends on #32