Add Iceland (IS) tax regime

[jaimerabazo]

Summary

Adds Iceland (IS) as a new tax regime. Completes Nordic coverage alongside dk, se and no.

Changes

• regimes/is/ — regime, VSK category (24% / 11%, since 2015), kennitala validation, IdentityTypeKennitala.
• Tests at 95.5% coverage; example invoice in examples/is/.
• changelog entry, regenerated schemas.

Design decisions

• Single kennitala model. Iceland uses one national identifier for persons and companies; the distinction is the first digit (0–3 person, 4–7 company, 8–9 temporary). Modeled as one type with IsCompany / IsPerson helpers rather than mirroring Sweden's split.
• VAT-ID restricted to company-form. Person and temporary kennitalas rejected — only companies hold VSK numbers.
• Current rates only. Modeled post-2015 rates; pre-2015 (25.5% / 7%) omitted to keep the MVP focused, can be backfilled symmetrically.

Out of scope

• TS-236 addon. Iceland's B2G Peppol BIS 3.0 CIUS. Natural follow-up under addons/is/ts236/. Left out here because it requires reading the full spec and ideally a domain-expert review.
• Custom scenarios beyond bill.InvoiceScenarios().

Sources

• Registers Iceland — kennitala
• Skatturinn — VSK
• OECD — Iceland TIN
• EC — eInvoicing in Iceland

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.

[jaimerabazo]

@codecov full review

[jaimerabazo]

@copilot full review

[codecov]

Codecov Report

❌ Patch coverage is 96.49123% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.77%. Comparing base (595b77a) to head (004e4ce).

Files with missing lines	Patch %	Lines
regimes/is/tax_identity.go	92.50%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #887      +/-   ##
==========================================
+ Coverage   94.75%   94.77%   +0.01%     
==========================================
  Files         343      346       +3     
  Lines       18815    18986     +171     
==========================================
+ Hits        17829    17994     +165     
- Misses        600      603       +3     
- Partials      386      389       +3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.