feat: add Latvia tax regime

[patricia-drt]

Summary

Introduced new tax regime for Latvia (LV) with VAT category support, including the standard, intermediate and reduced rates with comprehensive tax identity validation for both legal entities and individuals.

What's included

regimes/lv — regime definition.

• VAT categories and rates:

  • Standard rate (21%) – applied to most goods and services. History anchored at 2013-01-01.
  • Intermediate rate (12%) – first reduced rate applied to essential goods and services (medicines, food supplies, etc.). History anchored at 2013-01-01.
  • Reduced rate (5%) – second reduced rate applied to books, press, and media publications. History anchored at 2018-01-01.

  Note: Latvia does not currently have a super-reduced VAT rate (< 5% as per EU directive).

• VAT identity validation: Complete Latvian VAT/tax identity code validation:

  • 11-digit format (LV prefix optional, normalized away)
  • Legal entities (codes starting with 4-9): MOD-11 checksum validation using official weights
  • Legacy personal codes (DDMMYYCXXXX format): Date validation including leap year handling, century digit support (0=18xx, 1=19xx, 2=20xx)
  • Modern personal codes (starting with 3X, post-July 2017): Second digit validation (2-9 range)
  • Normalization strips LV prefix, removes hyphens, and yields clean numeric codes

• Corrections: Credit notes and debit notes both recognized in Latvian VAT practice, following GOBL's default correction handling.

• Scenarios: Reverse charge scenario for B2B intra-community supplies with automatic tax extension.

examples/lv — three invoice examples with golden outputs:

1. invoice-standard.json — standard B2B invoice with 21% VAT
2. invoice-reduced.json — multi-rate invoice with 5% and 12% VAT rates
3. invoice-intermediate.json — B2C invoice with 12% VAT (food/medicines)

Generated data: data/regimes/lv.json, data/rules/lv.json, and regime-code schema via go generate.

Scope decisions

Decision	Reasoning
No addon	Latvia has no domestic B2B e-invoicing mandate; B2G uses Peppol/EN 16931, already covered by the existing eu-en16931-v2017 addon. A lv-* addon would be speculative at this point (pending ViDA implementation).
No withholding taxes	No Latvian VAT-specific withholding mechanisms at the invoice level.
No OSS/IOSS handling	EU-level machinery for one-stop-shop, handled at a higher level than single-invoice logic.
No VAT-group handling	Registration arrangements are invisible at the single-invoice level.
No supplier tax ID requirement rule	Unlike some EU countries, Latvia does not mandate VAT registration below the threshold. The regime provides validation when a tax ID is present, but does not enforce it.
Rate history anchored at 2013-01-01	This represents when Latvia joined the Eurozone. Earlier rates (pre-2013 at 22%) are documented but not included; happy to extend if primary sources confirm exact effective dates.

Sources

• State Revenue Service (VID) – VAT rates: https://www.vid.gov.lv/en/value-added-tax
• Ministry of Finance – Tax rates: https://www.fm.gov.lv/lv/tax-rates
• Value Added Tax Law: https://likumi.lv/ta/en/en/id/253451-value-added-tax-law
• EU Council Directive 2006/112/EC: https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32006L0112
• Personal code (Personas kods) – Official documentation embedded in tax_identity.go comments

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.

[codecov]

Codecov Report

❌ Patch coverage is 86.72566% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.71%. Comparing base (225e39d) to head (3b5f200).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
regimes/lv/tax_identity.go	80.76%	9 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #879      +/-   ##
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- Coverage   94.75%   94.71%   -0.05%     
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  Files         343      345       +2     
  Lines       18815    18928     +113     
==========================================
+ Hits        17829    17927      +98     
- Misses        600      609       +9     
- Partials      386      392       +6     

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