Add Luxembourg (LU) tax regime

[Lluc24]

Adds Luxembourg (LU) as a new tax regime.

What's included

• regimes/lu — regime definition following the structure of the recently merged Norway regime (Feat/regime no #849):
  • Four VAT rates with full Since-dated history: standard 17%, intermediate ("parking") 14%, reduced 8%, super-reduced 3%. The history includes the temporary one-point reduction in force throughout 2023 (Law of 26 October 2022, Mémorial A No. 534) — the standard, intermediate, and reduced rates dropped to 16/13/7% and were restored on 2024-01-01; the super-reduced rate was excluded from the measure. An invoice dated within 2023 automatically calculates with the temporary rates.
  • TVA number validation: 8 digits where the last two must equal the first six mod 89, plus normalization of the LU prefix, spaces, and dashes.
  • RCS identity (RCS): Luxembourg Business Registers company number, format <section letter> + 1–6 digits (e.g. B263475). Any single uppercase section letter is accepted — the published list of sections (A sole traders, B commercial companies, C GIEs, E civil companies, …) has grown over time, so a closed list risks rejecting valid registrations.
  • Invoice rule: the supplier must carry either a TVA code or an RCS identity. Note this is a structural baseline so businesses below the VAT registration threshold can still issue valid invoices — not a claim that an RCS number legally substitutes for the TVA number, which VAT-registered suppliers must always state.
  • Corrections: credit notes and debit notes, both recognised in Luxembourg practice.
• examples/lu — four documents with golden outputs: standard B2B invoice (17%), multi-rate invoice (17/8/3%), a 2023-dated invoice demonstrating the historical 16%/7% rates, and a credit note.
• Generated data (data/regimes/lu.json, data/rules/lu.json, regime-code schema) and a CHANGELOG entry.

Scope decisions

• No addon. Luxembourg has no domestic B2B e-invoicing mandate; B2G uses Peppol/EN 16931, already covered by the existing eu-en16931-v2017 addon. A lu-* addon would be speculative at this point (pending ViDA).
• No withholding taxes, OSS/IOSS, or VAT-group handling — respectively income-tax mechanisms, EU-level machinery, and registration arrangements invisible at the single-invoice level.
• Pre-2015 rate history (15/12/6/3%) is anchored at a conservative 1992-01-01 baseline; the values are well documented but I could not verify the exact original effective dates under the amended 1979 law against primary sources. Happy to adjust if you have a preferred convention for historical anchors.

Sources

• AED – VAT rates: https://www.aed.public.lu/en/tva/taux-tva.html
• Law of 26 October 2022 (temporary reduction): https://legilux.public.lu/eli/etat/leg/loi/2022/10/26/a534/jo
• Law of 19 December 2014 (2015 increase): https://www.legilux.public.lu/eli/etat/leg/loi/2014/12/19/n1/jo
• European Commission – VAT rates: https://taxation-customs.ec.europa.eu/taxation/vat/eu-vat-rules/vat-rates_en
• Luxembourg Business Registers (RCS): https://www.lbr.lu

Structured sources are also embedded in the regime and category definitions.

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. (97.3% of statements)
• 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.

[codecov]

Codecov Report

❌ Patch coverage is 98.57143% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.79%. Comparing base (166bec4) to head (088601f).

Files with missing lines	Patch %	Lines
regimes/lu/tax_identity.go	93.10%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #858      +/-   ##
==========================================
+ Coverage   94.76%   94.79%   +0.02%     
==========================================
  Files         343      347       +4     
  Lines       18754    18894     +140     
==========================================
+ Hits        17773    17911     +138     
- Misses        597      598       +1     
- Partials      384      385       +1     

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[samlown]

Great PR! A few style changes I think would help.

[Copilot]

Pull request overview

Adds Luxembourg (LU) as a first-class tax regime in GOBL, including VAT rate definitions with historical changes, LU-specific identity validation/normalization, and regime-scoped invoice validation rules, plus examples and regenerated data outputs.

Changes:

• Register new LU regime and expose it through the global regimes registry.
• Implement LU VAT categories (4 rates) with since-dated history (incl. 2023 temporary reduction) and identity validations (TVA mod-89, RCS format).
• Add LU examples (YAML + golden JSON) and regenerate schema/rules/regime JSON, plus a changelog entry.

Reviewed changes

Copilot reviewed 18 out of 22 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
regimes/regimes.go	Registers the LU regime package via blank import.
regimes/lu/lu.go	Defines the LU regime (currency, timezone, corrections, categories, identities) and registers rules/normalizers.
regimes/lu/tax_categories.go	Adds LU VAT category with 4 rates and since-dated history (+ sources).
regimes/lu/tax_identity.go	Adds LU TVA normalization and mod-89 validation rules.
regimes/lu/org_identities.go	Adds LU RCS identity type, normalization, and format validation rules.
regimes/lu/bill_invoices.go	Adds LU invoice rule requiring supplier to have either TVA tax ID code or an RCS identity.
regimes/lu/*_test.go	Unit tests for LU regime construction, identity normalization/validation, and invoice rule.
examples/lu/*.yaml	Adds LU example documents (standard, multi-rate, 2023 historical rate example, credit note).
examples/lu/out/*.json	Golden outputs for the LU examples.
data/schemas/tax/regime-code.json	Adds LU to the regime-code schema enum.
data/rules/lu.json	Generated LU ruleset output.
data/regimes/lu.json	Generated LU regime definition output.
CHANGELOG.md	Notes the addition of the LU regime.

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[samlown]

Still a few issues with the approaches for handling test functions: we should be re-using as much as possible.

[Copilot]

Pull request overview

Copilot reviewed 18 out of 22 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 18 out of 22 changed files in this pull request and generated no new comments.

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