Add Finland regime

[rilla]

Summary

• Introduced new tax regime for Finland (FI) with the following VAT categories:
  • standard (25.5%)
  • reduced (13.5%)
  • super-reduced (10%)
• Added historical VAT rates (2013-2026)
• Implemented Y-tunnus (Business ID) validation based on MOD 11
• Added a B2B invoice example GOBL document
• Added tests for all regime-specific logic
• Corrections left undefined: Finnish VAT law doesn't restrict correction to specific document types, so GOBL's default permissive behavior applies
• No supplier tax ID validation: Finland has a €20,000 annual turnover threshold for VAT registration (since 2025). Sellers below it aren't VAT-registered and have no obligation to include a VAT identification number on invoices.
• Updated CONTRIBUTING.md to remove stalled references to deleted template and provide more detailed instructions for new regime additions.

Sources

• Verohallinto - VAT Rates
• Verohallinto - VAT Invoice Requirements
• Verohallinto - Business ID Check Digit
• Finlex - Arvonlisäverolaki 1501/1993
• Finlex - Act on Electronic Invoicing 241/2019

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 97.46835% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.77%. Comparing base (225e39d) to head (f00183b).

Files with missing lines	Patch %	Lines
regimes/fi/tax_identity.go	92.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #748      +/-   ##
==========================================
+ Coverage   94.75%   94.77%   +0.01%     
==========================================
  Files         343      345       +2     
  Lines       18815    18894      +79     
==========================================
+ Hits        17829    17906      +77     
- Misses        600      601       +1     
- Partials      386      387       +1     

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[Copilot]

Pull request overview

Adds a new Finland (FI) tax regime to GOBL, including VAT categories/rates, Y-tunnus (Business ID) validation, invoice requirements, and supporting generated data/examples.

Changes:

• Registered the new fi regime and added FI to the regime-code JSON schema enum.
• Implemented FI VAT categories (with historical rates) plus FI-specific validation/normalization (tax identity + invoice requirements) with tests.
• Added FI example invoice documents and generated regime definition data, plus updated contributor guidance and changelog.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
regimes/regimes.go	Registers the new Finland regime via blank import.
regimes/fi/fi.go	Defines FI regime metadata and dispatches validation/normalization.
regimes/fi/tax_categories.go	Declares FI VAT categories and historical rate values.
regimes/fi/tax_identity.go	Adds Y-tunnus checksum validation logic.
regimes/fi/invoices.go	Enforces FI invoice supplier Tax ID requirements.
regimes/fi/tax_identity_test.go	Tests for FI tax ID validation and normalization.
regimes/fi/invoices_test.go	Tests invoice validation requirement and date-based VAT calculations.
examples/fi/invoice-fi-fi.yaml	Adds an FI invoice example input document.
examples/fi/out/invoice-fi-fi.json	Adds generated JSON output for the FI example.
data/schemas/tax/regime-code.json	Adds "FI" to the schema enum list.
data/regimes/fi.json	Adds generated FI regime definition data.
CONTRIBUTING.md	Updates regime-addition guidance and references.
CHANGELOG.md	Notes the new FI regime addition.

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[samlown]

Very nice! Not much to change here. We just need to double check the situation with Finish Tax Identities to ensure a VAT ID is required for any invoice.

[rilla]

Thanks for the early review, @samlown, caught me in the middle of some final polish. I've addressed your (and Copilot's) comments and this is now ready for a final review.

[samlown]

Sorry! Too keen :-) I should have waited for the non-draft state! Just let me know when ready.

[rilla]

Since #750 has been approved, I've amended the historic rates definitions so they reflect effective dates. I've also removed invoices_test.go as that behavior will be covered in tax/rate_def_test.go once the fix is merged.

[samlown]

Excelent.

[rilla]

Hey @samlown, I've updated this branch with master and ported the regime to use validation rules.

[Copilot]

Pull request overview

Copilot reviewed 11 out of 12 changed files in this pull request and generated 3 comments.

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[samlown]

LGTM!