Add CorrectionNormalizer callback to tax.CorrectionDefinition

[pmenendz]

• Adds CorrectionNormalizer callback type and Normalize field on tax.CorrectionDefinition, allowing addons to define custom logic for routing extensions between
  the invoice and the preceding document reference during Correct().
• Adds Extensions.Clone() to tax.Extensions and CorrectionOptionsValue() accessor on bill.Invoice.
• Implements CorrectionNormalizer in ar-arca-v4 to copy original tax extensions to preceding and route the new doc-type to the invoice.
• Breaking: Migrates es-verifactu-v1 and es-sii-v1 from the normalizer hack (which moved ExtKeyDocType from preceding.Ext to inv.Tax.Ext on every Calculate()) to CorrectionNormalizer (which routes it during Correct() only). Code that called Calculate() on a manually-constructed invoice with ExtKeyDocType on a preceding document ref and expected it to be moved to inv.Tax.Ext will no longer see that behavior, the extension will remain on preceding.Ext. Really weird case. I don't think anyone is using this.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.40%. Comparing base (5cf58e9) to head (8027c7c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #815      +/-   ##
==========================================
+ Coverage   92.28%   92.40%   +0.11%     
==========================================
  Files         370      370              
  Lines       23242    23295      +53     
==========================================
+ Hits        21450    21525      +75     
+ Misses       1357     1339      -18     
+ Partials      435      431       -4     

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[Copilot]

Pull request overview

This PR adds a new correction-time normalization hook (CorrectionNormalizer) so addons/regimes can route/adjust tax extensions during Invoice.Correct() (instead of relying on Calculate()-time hacks), along with supporting utilities and migrations in affected addons.

Changes:

• tax: introduce CorrectionNormalizer + CorrectionDefinition.Normalize, and chain normalizers when merging correction definitions.
• tax/bill: add Extensions.Clone() and Invoice.CorrectionOptionsValue() to support safe, correction-scoped normalization.
• Addon migrations: implement Normalize for ARCA v4 and migrate Verifactu/SII to use the new correction hook; adjust tests + changelog accordingly.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tax/extensions.go	Adds Extensions.Clone() to allow safe copying of extension maps.
tax/extensions_test.go	Tests clone behavior (nil/empty/independent copy).
tax/corrections.go	Adds CorrectionNormalizer and CorrectionDefinition.Normalize; chains normalizers in Merge().
tax/corrections_test.go	Expands unit tests for correction set lookup, merging, and normalizer chaining.
bill/invoice.go	Adds internal correctionOptions storage + CorrectionOptionsValue() accessor for normalizers.
bill/invoice_correct.go	Invokes correction normalizer during Correct(), sets preceding earlier, clones option extensions.
bill/invoice_correct_test.go	Adds invoice-level tests covering normalization and ensuring options aren’t leaked.
addons/ar/arca/bill_invoice.go	Implements ARCA correction normalizer to copy original tax extensions to preceding and route doc-type to invoice.
addons/ar/arca/bill_invoice_test.go	Updates correction definition assertions and adds correction-normalization behavior tests.
addons/es/verifactu/bill.go	Migrates doc-type routing to CorrectionNormalizer during Correct().
addons/es/verifactu/bill_test.go	Adjusts expectations now that preceding extensions aren’t forcibly emptied by Calculate().
addons/es/sii/bill.go	Migrates doc-type routing to CorrectionNormalizer during Correct().
addons/es/sii/bill_test.go	Adjusts expectations now that preceding extensions aren’t forcibly emptied by Calculate().
data/addons/ar-arca-v4.json	Removes explicit correction types from addon data (types still provided by the AR regime).
CHANGELOG.md	Documents the new hook, migrations, and extension cloning behavior.

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