Add Estonia (EE) tax regime

[sandragc8]

Summary

This PR adds a new tax regime for Estonia (EE). The motivation is the scenario in the assignment: a company already invoicing in one country opens a subsidiary in Estonia, which GOBL did not yet cover.

I used the existing EU regimes (Austria and Ireland) as structural references, since Estonia is an EU member state with a VAT system and the euro.

What's included

• VAT category with four rates:
  • Standard general rate — 24% (current), with historical values kept so past-dated invoices still resolve: 22% since 2024-01-01 and 20% since 2009-07-01.
  • reduced — 13% (accommodation services).
  • super-reduced — 9% (books, press publications, certain medicines).
  • zero — 0% (exports, intra-community supply).
• KMKR VAT identity validation (tax_identity.go): EE + 9 digits, validated after the country prefix is stripped during normalization. Includes the mod-10 weighted checksum (weights [3,7,1,3,7,1,3,7]).
• Credit notes enabled as invoice corrections.
• Regime registered in regimes/regimes.go, generated data/regimes/ee.json, and a CHANGELOG.md entry.

Tests

• regimes/ee/tax_identity_test.go — table tests with real valid KMKR numbers and invalid cases (format, length, un-normalized prefix, bad checksum).
• regimes/ee/ee_test.go — sanity check of the regime definition.
• The global regimes_test.go validates the new RegimeDef automatically.

Sources

VAT rates and effective dates come from the Estonian Tax and Customs Board (EMTA): https://www.emta.ee/en/business-client/taxes-and-payment/value-added-tax/vat-rates-and-supply-exempt-tax/standard-vat-rate

Scope — what I deliberately left out

To keep the change focused and reviewable, I limited it to what the law requires for correct invoicing, in line with the "minimum complexity for legal correctness" principle:

• No e-invoicing addon (PEPPOL / national CTC). Estonia supports e-invoicing, but that belongs in a separate addon rather than the base regime.
• No transitional 20% provisions (cash-accounting / long-term property contracts valid until mid-2025). These depend on time-of-supply rules, not on a standing rate, so modelling them as an active rate would be misleading.
• No full exemptions catalogue (financial, insurance, real estate). The global VAT keys (exempt, reverse-charge, etc.) already cover the common cases.
• No live VIES/EMTA lookup for registration status — that's an application-layer concern; the regime validates format and checksum offline, consistent with the other regimes (AT, IE).

Happy to extend any of the above if you'd prefer it in scope.

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.76%. Comparing base (166bec4) to head (dfac148).
⚠️ Report is 67 commits behind head on main.

Files with missing lines	Patch %	Lines
regimes/ee/tax_identity.go	92.85%	1 Missing and 1 partial ⚠️
regimes/ee/ee.go	97.72%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #871      +/-   ##
==========================================
- Coverage   94.76%   94.76%   -0.01%     
==========================================
  Files         343      345       +2     
  Lines       18754    18887     +133     
==========================================
+ Hits        17773    17898     +125     
- Misses        597      601       +4     
- Partials      384      388       +4     

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[samlown]

Thanks for this! I'd just check the copy in the description, and the rate assignment.

[samlown]

We usually try to avoid adding rates in the description as they're already defined in the tax categories and may become desynchronised.

[samlown]

This I think feels more like an "intermediate" rate as opposed to reduced.

[samlown]

This is probably "reduced", unless you see evidence for something different.

[samlown]

We also lost the PR checklist here.