Item base quantity & line unit of measure (EN 16931 BT-149/150/130)

[alvarolivie]

Adds the item price base quantity and a line-level unit of measure for EN 16931.

Part of splitting the original combined item-details PR. Sibling PRs: item attributes (#859), and item identities & buyer item reference.

Changes

• org: base quantity (BT-149) — Item.BaseQuantity, the number of units the item's price refers to (e.g. a price per 100 units). Line sums are calculated as price × quantity ÷ base_quantity. Validated as positive.
• bill: invoiced quantity unit (BT-130) — Line.Unit / SubLine.Unit, distinct from the item's price base quantity unit (Item.Unit, BT-150). Core EN 16931 permits the two to differ (e.g. priced per bushel, invoiced in tonnes); only the PEPPOL layer requires them equal.
• Normalization is lazy by design — a line-only unit seeds Item.Unit (BT-150), but an item-only unit does not populate the line. Existing documents are byte-for-byte unchanged (no digest churn); divergent units are preserved when set explicitly.
• Adds a GB example (examples/gb/invoice-base-quantity.yaml): 250 kg invoiced, priced per 100 kg → sum 300.00.

Converter follow-up (gobl.ubl / gobl.cii)

The @unitCode on the invoiced quantity (BT-130) is mandatory in both UBL and CII, so the converter mapping is:

• BT-130 → InvoicedQuantity/@unitCode (UBL) / BilledQuantity/@unitCode (CII) = Line.Unit if set, else fall back to Item.Unit.
• BT-150 → Price/BaseQuantity/@unitCode (UBL) / BasisQuantity/@unitCode (CII) = Item.Unit, emitted only when a base quantity is present.

A future addons/peppol should enforce PEPPOL-EN16931-R130 (BT-150 == BT-130); core EN 16931 has no such rule.

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 94.76%. Comparing base (595b77a) to head (8b377c2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #881   +/-   ##
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  Coverage   94.75%   94.76%           
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  Files         343      343           
  Lines       18815    18835   +20     
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+ Hits        17829    17849   +20     
  Misses        600      600           
  Partials      386      386           

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