feat: add Hungary (HU) ÁFA tax regime

[pablocaeg]

Summary

Added the Hungarian ÁFA (VAT) tax regime.

• Three VAT rate tiers:
  • Standard (Általános kulcs) at 27%, the highest in the EU
  • Reduced (Kedvezményes kulcs) at 18%
  • Super reduced (Kedvezményes alsó kulcs) at 5%
• Historical VAT rates back to EU accession (May 2004)
• Adószám validation with modulo-10 weighted checksum, verified against python-stdnum
• Normalization handles the full 11-digit format (TTTTTTTT-V-RR), extracting the 8-digit base number
• Supplier adószám required on non-simplified invoices per Section 169 of Act CXXVII of 2007
• Credit note and debit note corrections
• Domestic B2B example invoice
• 20 tests, 97.6% code coverage

Sources

• NAV — Nemzeti Adó- és Vámhivatal
• NAV — ÁFA rates and tax-exempt activities
• Act CXXVII of 2007 on Value Added Tax
• NAV Online Számla — Technical Documentation
• OECD — Hungary Tax Identification Number
• python-stdnum — Hungarian tax number validation

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide
• Performed a self-review of my code
• Added thorough tests with at least 90% code coverage
• Modified or created example GOBL documents to show my changes in use
• Added links to the source of the changes in tax regimes or addons
• Run go generate . to ensure that the Schemas and Regime data are up to date
• Reviewed and fixed all linter warnings
• Been obsessive with pointer nil checks to avoid panics
• Updated the CHANGELOG.md with an overview of my changes

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[pablocaeg]

Rebuilt on top of v0.400.0, now using the new rules package.