Implement Cyprus tax regime

[xMigma]

Summary

• VAT (Value Added Tax / ΦΠΑ) with the four rates currently in force
  and their historical values:
  • 19% general
  • 9% higher reduced
  • 5% lower reduced
  • 3% super-reduced
• Cyprus TIN normalization and format validation.
• Invoice requirements and prescribed wording for special VAT scenarios.
• Tests, an example invoice, generated regime data, and a changelog entry.

Implementation notes

• Corrections remain unrestricted because the available guidance describes
  credit notes but does not establish that all corrections must use a specific
  document type.
• The EUR 85 simplified retail invoice threshold is not enforced because GOBL's
  generic simplified tag does not represent all its eligibility conditions.
• VAT representative details are not modelled because the invoice schema has no
  dedicated tax representative party.
• TIN validation checks the published format only. The final letter is a control
  character, but no public validation algorithm was found.

Feedback on these scope decisions would be appreciated.

Sources

• VAT invoice requirements
• Current VAT rates
• Historical VAT rates
• TIN format

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 97.26027% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.00%. Comparing base (8652979) to head (2e9df00).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
regimes/cy/tax_identity.go	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #934   +/-   ##
=======================================
  Coverage   94.99%   95.00%           
=======================================
  Files         327      330    +3     
  Lines       17732    17805   +73     
=======================================
+ Hits        16845    16916   +71     
- Misses        547      548    +1     
- Partials      340      341    +1     

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