Add Luxembourg (LU) tax regime

[nietoarranz]

Summary

Adds a Luxembourg (LU) tax regime so GOBL can build and validate Luxembourg invoices with local VAT rates and n° TVA identity checks.

Included

• Regime definition (EUR, Europe/Luxembourg, credit-note corrections)
• VAT categories/rates: general 17%, intermediate 14%, reduced 8%, super-reduced 3%, plus the temporary one-point reductions that applied in 2023
• Tax identity validation: 8 digits with mod-89 check digits (after standard identity normalization)
• Regime JSON data, package registration, and a sample LU→LU invoice

Design notes

• Followed the same shape as other lightweight EU regimes (definition + categories + identity rules/normalization), using the current rules / norm packages rather than older validate hooks.
• Checksum algorithm taken from the well-known stdnum LU TVA reference; rates and background from AED / Guichet.lu sources linked in the regime definition.
• Identity tests cover valid numbers, bad checksums, length, and non-digit input. Validation runs on the code as stored (post-normalization), so values with separators fail as expected unless normalized first.

Intentionally out of scope

• No Luxembourg-specific org identities or invoice-level regime rules yet (neighbors like BE/NL go further here; happy to add if useful).
• No PEPPOL / B2G addon wiring — the description notes that e-invoicing exists nationally, but this PR only covers the tax regime baseline.
• Corrections limited to credit notes (no debit notes / other correction types) until we have a clear local requirement.
• Historical VAT rates start from 2015; earlier periods were not researched for this first pass.

Feedback welcome on whether identity coverage, rate history, or invoice rules should be expanded before merge.

Test plan

• go test ./regimes/lu/...
• Confirm data/regimes/lu.json matches the Go regime definition
• Build/validate examples/lu/invoice-lu-lu.yaml
• Spot-check rate resolution around the 2023 temporary rates vs current (2024+) rates

[codecov]

Codecov Report

❌ Patch coverage is 90.36145% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.88%. Comparing base (510a545) to head (2ead7a7).
⚠️ Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
regimes/lu/tax_identity.go	75.00%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #918      +/-   ##
==========================================
- Coverage   94.90%   94.88%   -0.03%     
==========================================
  Files         323      325       +2     
  Lines       17639    17722      +83     
==========================================
+ Hits        16741    16816      +75     
- Misses        556      560       +4     
- Partials      342      346       +4     

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