Peppol PINT base addon

[migueltorresvalls]

Based on: #885

Add the base Peppol International (PINT) billing addon. Builds on EN 16931 and
re-maps the UNTDID 5305 tax category from the GOBL tax key so non-VAT schemes
(GST) are reported correctly — standard-rated GST as "S" instead of "O".

Changes:

• peppol-pint addon: tax-category normalizer (reuses en16931.TaxCategoryMap) + PINT rules not covered by core/en16931 (IBR-007/080/081/CO-26)
• Export en16931.TaxCategoryMap (was vatKeyMap) for reuse
• AU + NZ (standard and zero-rated) example invoices

Notes:

• Stacked on regimes: add Australia (AU) and New Zealand (NZ) GST tax regimes #885 (AU/NZ regimes); examples depend on them. Rebase after regimes: add Australia (AU) and New Zealand (NZ) GST tax regimes #885 merges.

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.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.18072% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.81%. Comparing base (602103f) to head (1358939).

Files with missing lines	Patch %	Lines
addons/peppol/pint/bill.go	91.66%	1 Missing and 1 partial ⚠️
addons/peppol/pint/tax_combo.go	85.71%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           au-nz-regime     #906      +/-   ##
================================================
+ Coverage         94.80%   94.81%   +0.01%     
================================================
  Files               349      352       +3     
  Lines             19021    19099      +78     
================================================
+ Hits              18033    18109      +76     
- Misses              601      602       +1     
- Partials            387      388       +1     

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