feat: item attributes, line seller, contract reason, charge exemption codes

[alvarolivie]

What

Several EN16931/Peppol mapping gaps found while building out the
French extended (EXTENDED-CTC-FR) profile. Bundled into one PR since
they touch overlapping files.

• Item attributes. Migrate org.Item.Meta (a lossy string map) to
  org.Item.Attributes, matching the current GOBL API.
  AdditionalItemProperty gains ValueQuantity for measurable
  attribute values, alongside the existing Value (BR-54 requires a
  plain Value even when ValueQuantity is also present).
• Line seller. Map bill.Line.Seller to/from
  cac:Item/cac:ManufacturerParty.
• Line-level tax point (BT-8). Apply the invoice-wide VAT point
  date code to each line's InvoicePeriod, not just the header.
• Line notes. Reuse the existing header-level note-subject-code
  formatting (formatNote/parseNote) for line notes too, instead of
  emitting plain text.
• Contract reason. Map org.Contract.Reason to/from
  cbc:DocumentType on cac:ContractDocumentReference — this was
  previously read into an unused Type field and never written on
  convert.
• Charge/discount exemption codes. Map the CEF VATEX extension and
  matching tax note text onto AllowanceCharge/TaxCategory
  (TaxExemptionReasonCode/TaxExemptionReason), mirroring what
  totals.go already does for the invoice totals. findTaxNote now
  takes tax.Extensions directly so both call sites share it.
• Payee party consolidation. PayeeParty is now built with the
  shared newParty instead of a separate newPayeeParty. addPayment
  strips PostalAddress/RegistrationName back off the result and
  keeps only one PartyIdentification entry, to preserve
  UBL-CR-272, UBL-CR-275 and UBL-SR-20 — the rules newPayeeParty
  existed for. newPayeeParty's guess that a 4-character identity
  label is an ISO 6523 ICD code is dropped rather than ported:
  identities must carry their scheme in the iso-scheme-id extension
  (the self-billed payee fixture now does).

Test data

Convert/parse goldens regenerated with -update. Diffs are limited to
the fields above (meta→attributes restructuring, new
TaxExemptionReasonCode/DocumentType/ManufacturerParty elements),
plus one payee fixture whose SIREN now lands only in
PartyLegalEntity.CompanyID instead of being duplicated into both
PartyLegalEntity and PartyIdentification.

Note: -update also touched a 1-cent tax base rounding
(test/data/convert/*/invoice-prices-include-vat.json) that
reproduces identically on main with zero code changes — it's latent
drift from an earlier gobl version bump, unrelated to this branch, so
I left it untouched here. Worth a follow-up.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.39326% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.70%. Comparing base (20fd626) to head (69754ae).

Files with missing lines	Patch %	Lines
lines_parse.go	68.75%	6 Missing and 4 partials ⚠️
charges.go	85.71%	1 Missing and 1 partial ⚠️
invoice_parse.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
+ Coverage   81.39%   81.70%   +0.31%     
==========================================
  Files          30       30              
  Lines        2408     2433      +25     
==========================================
+ Hits         1960     1988      +28     
+ Misses        285      279       -6     
- Partials      163      166       +3     

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[Copilot]

Pull request overview

This PR closes several EN16931/Peppol UBL mapping gaps discovered while implementing the French extended (EXTENDED-CTC-FR) profile, spanning line/item enrichment, contract metadata, tax exemption handling, and payee party construction.

Changes:

• Replace org.Item.Meta string map usage with structured org.Item.Attributes, including support for measurable attributes via AdditionalItemProperty/ValueQuantity (while still emitting Value for BR-54).
• Add/adjust UBL mappings for line seller (cac:Item/cac:ManufacturerParty), line notes (subject-code formatting), line-level VAT point code propagation, and contract reason (cbc:DocumentType).
• Extend allowance/charge tax category mapping to include VATEX exemption reason code/text and consolidate PayeeParty construction via newParty with UBL rule cleanups.

Reviewed changes

Copilot reviewed 17 out of 29 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
totals.go	Refactors findTaxNote to take tax.Extensions and updates totals exemption-reason lookup accordingly.
charges.go	Adds VATEX exemption reason code/text mapping onto allowance/charge TaxCategory.
common.go	Extends UBL Item with ManufacturerParty and adds ValueQuantity to AdditionalItemProperty.
ordering.go	Writes contract reason into ContractDocumentReference.DocumentType.
ordering_parse.go	Parses ContractDocumentReference.DocumentType into org.DocumentRef.Reason.
ordering_test.go	Adds round-trip test coverage for contract reason/document type mapping.
payment.go	Builds PayeeParty via newParty and strips disallowed fields / limits identifications per UBL rules.
payment_test.go	Adds round-trip test ensuring Payee inbox maps to EndpointID.
party.go	Adds ISO 6523 ICD fallback for 4-char identity labels; removes newPayeeParty.
party_test.go	Updates payee expectations due to payee now using newParty.
lines.go	Implements line note formatting reuse, line VAT point code propagation, item attributes mapping, and line seller mapping.
lines_test.go	Adds round-trip tests for line note subject codes, item attributes (incl. quantity), and line seller.
lines_parse.go	Parses item attributes into org.Item.Attributes, parses line notes via parseNote, and maps ManufacturerParty back to Line.Seller.
lines_parse_test.go	Updates parse assertions from Meta to Attributes.
invoice_parse.go	Wires options into line parsing (goblAddLines(out, o)).
invoice_test.go	Adds test coverage for line-level VAT point code conversion.
test/data/convert/peppol/out/invoice-with-contract-ref.xml	Golden update: emits cbc:DocumentType for contract reason.
test/data/convert/peppol/out/invoice-intra-comunity.xml	Golden update: emits cbc:TaxExemptionReasonCode for VATEX.
test/data/convert/peppol/invoice-with-contract-ref.json	Golden update: contract reason field.
test/data/convert/france-extended/out/invoice-payer.xml	Golden update: removes payee PartyIdentification to satisfy UBL constraints.
test/data/parse/peppol/out/Vat-category-S.json	Golden update: meta → attributes restructuring for item properties.
test/data/parse/peppol/out/sg-invoice.json	Golden update: meta → attributes restructuring for item properties.
test/data/parse/peppol/out/Allowance-example.json	Golden update: meta → attributes restructuring for item properties.
test/data/parse/france-cius/out/b2b-reg.json	Golden update: meta → attributes restructuring for item properties.
test/data/parse/en16931/out/ubl-example2.json	Golden update: meta → attributes restructuring for item properties.
test/data/parse/en16931/out/ubl-example5.json	Golden update: meta → attributes restructuring for item properties.
test/data/parse/en16931/out/ubl-example8.json	Golden update: meta → attributes restructuring for item properties.
test/data/parse/en16931/out/ubl-example9.json	Golden update: meta → attributes restructuring for item properties.
test/data/parse/en16931/out/ubl-example10.json	Golden update: meta → attributes restructuring for item properties.

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[alvarolivie]

Intentional: the header InvoicePeriod (created by addTaxPoint when missing) is the authoritative BT-8 carrier. At line level the code is only echoed onto a period the line actually has (BG-26) — fabricating an empty InvoicePeriod on every line just to duplicate a document-level code would add noise without carrying any new information.

[alvarolivie]

Fixed in 8824758: ValueQuantity is now only emitted when the attribute has a unit. Unitless amounts are carried by the plain Value alone.