Add South Korea (KR) tax regime

[aryanv175]

Summary

Adds a new tax regime for South Korea (KR). It follows the structure of the
recently merged Norway regime (regimes/no) and uses the current rules + norm
validation framework.

• VAT (부가가치세) at the standard 10% rate (in place since VAT was introduced on
  1 July 1977), with zero-rating for exports handled via the standard VAT keys.
• Business Registration Number (사업자등록번호) normalization and check-digit
  validation. The algorithm is verified against real, public BRNs in the tests.
• Corrections: credit and debit notes, matching Korea's revised tax invoice
  (수정세금계산서), which can adjust a prior invoice up or down.
• Example invoices (standard, zero-rated export, credit note) and regenerated
  data/regimes/kr.json, data/rules/kr.json, and the regime-code schema enum.

Sources

• Value-Added Tax Act, Art. 30 (tax rate): https://elaw.klri.re.kr/eng_service/lawView.do?hseq=53110&lang=ENG
• National Tax Service (NTS): https://www.nts.go.kr/english/

Scope decisions (and what I deliberately left out)

• VAT is the correct category for Korea: 부가가치세 allows input-tax credits, which
  is the defining property of a VAT (unlike a consumption/sales tax).
• No org-identity / Corporate Registration Number (법인등록번호): the CRN is not used
  to identify parties on Korean tax invoices — the BRN is — so I didn't add an identity
  type that wouldn't appear on invoices, and I couldn't verify its check digit against a
  known-valid number.
• No invoice-level "supplier tax ID required" rule: Korea has simplified taxpayers
  (간이과세자) below the VAT-registration threshold, so a blanket requirement would be
  wrong. Kept the regime minimal; format/agency-specific rules belong in a future addon.
• No e-invoicing addon: the NTS e-Tax Invoice system (전자세금계산서) is a large CTC
  system and out of scope for a base regime.

Checklist

• Opened as a draft
• Read the CONTRIBUTING.md guide
• Self-reviewed the code
• Tests with ≥90% coverage (KR package is at ~95%)
• Added example GOBL documents
• Added source links for tax data
• Ran go generate . so schema and regime data are up to date
• Fixed all linter warnings (go vet, gofmt)
• Nil-checks on tax-identity handling
• Updated CHANGELOG.md
• Requested a review from Copilot and addressed feedback
• Marked ready and requested review from @samlown

[aryanv175]

Ready for review @samlown — thanks!

[codecov]

Codecov Report

❌ Patch coverage is 96.82540% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.76%. Comparing base (225e39d) to head (790cca4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
regimes/kr/tax_identity.go	92.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #878   +/-   ##
=======================================
  Coverage   94.75%   94.76%           
=======================================
  Files         343      345    +2     
  Lines       18815    18878   +63     
=======================================
+ Hits        17829    17890   +61     
- Misses        600      601    +1     
- Partials      386      387    +1     

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[Copilot]

Pull request overview

Adds a new South Korea (KR) tax regime to the GOBL regimes registry, including VAT definitions, BRN (사업자등록번호) normalization/validation, and example documents, and regenerates the derived JSON rule/regime artifacts and schema enum.

Changes:

• Register the new kr regime and define KR VAT (10%) category/rate metadata.
• Add KR tax identity normalization + BRN check-digit validation with unit/integration tests.
• Add KR example invoices/credit note and regenerate data/regimes/kr.json, data/rules/kr.json, and the regime-code schema enum.

Reviewed changes

Copilot reviewed 14 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
regimes/regimes.go	Registers the new KR regime via blank import.
regimes/kr/kr.go	Defines and registers the KR regime (VAT scheme, timezone, corrections, norm/rules wiring).
regimes/kr/tax_categories.go	Adds KR VAT category and 10% general rate with sources.
regimes/kr/tax_identity.go	Implements KR BRN validation rules + check-digit algorithm hook.
regimes/kr/kr_test.go	Regime + invoice calculation/validation tests for KR.
regimes/kr/tax_identity_test.go	Normalization + validation tests for KR BRN handling.
regimes/kr/tax_identity_internal_test.go	Direct guard-rail test for isValidBRN type assertion behavior.
examples/kr/invoice-kr-standard.yaml	Source example for a standard KR invoice.
examples/kr/invoice-kr-zero-rated.yaml	Source example for a zero-rated export KR invoice.
examples/kr/credit-note-kr.yaml	Source example for a KR credit note (correction).
examples/kr/out/invoice-kr-standard.json	Generated output for standard KR invoice example.
examples/kr/out/invoice-kr-zero-rated.json	Generated output for zero-rated KR invoice example.
examples/kr/out/credit-note-kr.json	Generated output for KR credit note example.
data/schemas/tax/regime-code.json	Adds KR to the regime code enum schema.
data/rules/kr.json	Generated rules artifact for KR tax identity validation.
data/regimes/kr.json	Generated regime definition artifact for KR.
CHANGELOG.md	Documents addition of the KR regime.

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