uy: Add tax regime for Uruguay with IVA rates and RUT validation

[CarlosMari]

Summary

Adds a new tax regime for Uruguay (UY), implementing:

• IVA (VAT) tax categories: 22% standard rate (tasa básica) and 10% reduced rate (tasa mínima), both effective since Law 18.083 (2007-07-01).
• RUT validation: Full 12-digit structure validation including modulo-11 check digit algorithm
• RUT normalization: Strips hyphens, spaces, and "UY" prefix

Design decisions

• Scope: This is a base tax regime only. Invoice-level validation (e.g., requiring supplier tax ID) is intentionally left for a future CFE addon, consistent with how co and gb handle this separation.
• Zero/exempt rates: Inherited from GlobalVATKeys(), no local rate value definitions needed since they work with NoPercent: true.
• RUT "001" field: Positions 9-11 are always "001" per the OECD TIN specification. Branch identification is handled separately via CdgDGISucur in the CFE XML, not within the RUT itself.

Sources

• IVA rates: Título 10, Texto Ordenado 2023, Art. 18
• RUT structure: OECD TIN document, DGI RUT numbering
• Checksum algorithm: Cross-verified against python-stdnum and alfius/uy-rut

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.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[codecov]

Codecov Report

❌ Patch coverage is 96.93878% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.13%. Comparing base (e36c17e) to head (2a5b98a).
⚠️ Report is 89 commits behind head on main.

Files with missing lines	Patch %	Lines
regimes/uy/tax_identity.go	94.59%	1 Missing and 1 partial ⚠️
regimes/uy/uy.go	98.36%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #799      +/-   ##
==========================================
+ Coverage   93.11%   93.13%   +0.02%     
==========================================
  Files         334      336       +2     
  Lines       18086    18184      +98     
==========================================
+ Hits        16840    16935      +95     
- Misses        876      878       +2     
- Partials      370      371       +1     

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[samlown]

Looks good! Minot observation regarding the source links, but if this was up to date with the latest rules based validation changes, it'd be good to go 👍