Add Türkiye (TR) tax regime

[mrdanwa]

• New Türkiye tax regime (regimes/tr/) with:
  • VKN (10-digit) tax identity validation with checksum verification
  • TCKN (11-digit) org identity validation with checksum verification
  • KDV (VAT) rates: standard 20%, reduced 10%, super-reduced 1%, with historical rates back to 1999
  • Corrective invoice type
  • Supplier TaxID or TCKN identity required on all invoices
  • 5 example invoices: standard, reduced, corrective, exempt, tckn
  • Registered in regimes/regimes.go, generated data/regimes/tr.json and updated regime-code.json
  • CHANGELOG updated

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, if appropriate.
• Added links to the source of the changes in tax regimes or addons, either structured or in the comments.
• 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.
• Marked this PR as ready for review.

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.

[Copilot]

Pull request overview

This pull request adds a comprehensive tax regime implementation for Türkiye (TR) to the GOBL tax library. The implementation includes tax identity validation for both company (VKN) and individual (TCKN) tax numbers with checksum verification, VAT (KDV) rate definitions with historical rates dating back to 1999, and support for corrective invoices in compliance with Turkish tax law.

Changes:

• Added complete Türkiye tax regime with VKN (10-digit) and TCKN (11-digit) tax identity validation using checksum algorithms
• Defined KDV (VAT) rates: standard 20%, reduced 10%, super-reduced 1%, with historical rate changes since 1999
• Implemented corrective invoice type as the only valid correction method per Tax Procedure Law Article 227

Reviewed changes

Copilot reviewed 14 out of 18 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
regimes/tr/tr.go	Main regime definition with corrections configuration and validation/normalization hooks
regimes/tr/tax_identity.go	VKN and TCKN validation logic with checksum verification algorithms
regimes/tr/tax_identity_test.go	Comprehensive tests for tax identity normalization and validation
regimes/tr/tax_categories.go	KDV rate definitions with historical rates and source documentation
regimes/tr/invoices.go	Invoice validation requiring supplier tax ID
regimes/tr/invoices_test.go	Invoice validation tests
regimes/regimes.go	Registration of TR regime
examples/tr/*.yaml	Four example invoices demonstrating standard, reduced, exempt, and corrective scenarios
examples/tr/out/*.json	Generated JSON outputs for example invoices
data/regimes/tr.json	Generated regime data file
data/schemas/tax/regime-code.json	Added TR regime code to schema
CHANGELOG.md	Documented new TR regime addition

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[samlown]

Thanks for providing this! A few changes are required around the tax identity handling and the use of non-public or official sources. Some good stuff here though!

[codecov]

Codecov Report

❌ Patch coverage is 95.69892% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.07%. Comparing base (926130f) to head (26f1e03).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
regimes/tr/tax_identity.go	90.47%	2 Missing and 2 partials ⚠️
regimes/tr/identities.go	95.00%	1 Missing and 1 partial ⚠️
regimes/tr/invoices.go	94.11%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #743      +/-   ##
==========================================
+ Coverage   93.05%   93.07%   +0.02%     
==========================================
  Files         333      337       +4     
  Lines       17785    17971     +186     
==========================================
+ Hits        16549    16727     +178     
- Misses        870      874       +4     
- Partials      366      370       +4     

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[mrdanwa]

Thanks for the review Sam. You were right, TCKN is a national identity number used domestically by individuals and sole traders and not a valid tax ID for international trade. Only VKN serves that purpose. I've moved TCKN to org.Identity and updated supplier validation to require either a VKN (via TaxID) or a TCKN (via Identities). Also replaced sources with official publications, added a regime description, fixed lint issues, and added a TCKN example invoice.