Item identity scopes for EN 16931 (BT-155/156/157/158)

[alvarolivie]

Adds item identity scopes covering the EN 16931 item identification family.

Part of splitting the original combined item-details PR. Sibling PRs: item attributes (#859), and item base quantity & units (#881).

Changes

• org: identity scopes — org.Identity.Scope gains four scopes for item identification, each documented inline with when to use it:
  • seller (BT-155) — the seller's own article number / SKU for the item.
  • buyer (BT-156) — the buyer's own catalogue code for the item.
  • legal (BT-157) — a registered-scheme identifier such as the GS1 GTIN (also used by parties for company registration numbers).
  • class (BT-158) — a classification-scheme category such as UNSPSC, CPV, or HS, rather than a unique identifier.
• eu-en16931: identity rules — class identities require the untdid-item-type extension (BT-158); items allow at most one legal identity (BT-157); legal item identities require iso-scheme-id (BR-64), enforced at the item level since party legal identities need no scheme.
• data/catalogues/untdid — new untdid-item-type-version extension (BT-158-2).
• Adds a GB example (examples/gb/invoice-item-identities.yaml) with all four item-identity scopes.

The buyer's item reference (BT-156) is now modelled as a buyer-scoped item identity rather than a Line.BuyerItemRef field — consistent with the UBL Item identification family (Sellers/Buyers/Standard/CommodityClassification).

Note

This branch regenerates data/rules/eu-en16931.json, which also drops the stale GOBL-EU-EN16931-TAX-COMBO-06 (BR-E-10) assertion. That Go rule was already removed in v0.501.0 but the generated data on main was never regenerated — a pre-existing drift fix, not a behaviour change here.

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 94.77%. Comparing base (595b77a) to head (d729816).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #882      +/-   ##
==========================================
+ Coverage   94.75%   94.77%   +0.01%     
==========================================
  Files         343      343              
  Lines       18815    18869      +54     
==========================================
+ Hits        17829    17883      +54     
  Misses        600      600              
  Partials      386      386              

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[Copilot]

Pull request overview

This PR expands org.Identity.Scope to cover the EN 16931 item identification family (BT-155/156/157/158) and adds EN 16931 addon validation to constrain item identities (including required extensions), alongside regenerated schemas/rules data and a GB example demonstrating usage.

Changes:

• Add new identity scopes class, seller, and buyer (and clarify legal scope usage for items) in org.Identity, including validation + JSON Schema enum updates.
• Add EN 16931 addon rules: (1) at most one legal identity per item, (2) legal item identities require iso-scheme-id, (3) class identities require untdid-item-type.
• Add UNTDID extension key untdid-item-type-version and add example invoice exercising all four item identity scopes.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
org/identity.go	Adds new identity scope constants, documents scope intent, and expands scope validation + JSON Schema enum.
org/identity_test.go	Updates rule error assertion text and JSON Schema scope enum expectations.
addons/eu/en16931/en16931.go	Registers new EN 16931 identity-level ruleset.
addons/eu/en16931/org.go	Adds EN 16931 item identity constraints and class-scope extension requirement.
addons/eu/en16931/org_test.go	Adds tests for new scope/extension requirements and item-level legal identity constraints.
addons/eu/en16931/org_internal_test.go	Adds white-box tests for defensive guards in helper predicates.
catalogues/untdid/untdid.go	Introduces untdid-item-type-version extension key constant.
data/rules/org.json	Regenerates org rules JSON to reflect expanded identity scope set.
data/rules/eu-en16931.json	Regenerates EN 16931 rules JSON to include new identity/item identity assertions and remove stale assertion.
data/schemas/org/identity.json	Regenerates identity JSON Schema to include the new scope enum values.
examples/gb/invoice-item-identities.yaml	Adds GB YAML example showing all four item identity scopes and required extensions.
examples/gb/out/invoice-item-identities.json	Adds generated JSON output for the new GB item identities example.
CHANGELOG.md	Documents the new scopes, EN 16931 identity rules, new UNTDID extension, and the regenerated-data drift fix.

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