Display UNTDID unit codes alongside GOBL units

[samlown]

Summary

• display a UNTDID unit code after the localized GOBL unit representation, for example kg (KGM)
• display the UNTDID code by itself when an item has no GOBL unit
• include the unit column for extension-only units
• add focused formatter and line-support tests
• update the unit-focused French invoice example to cover GOBL-only, combined GOBL/UNTDID, and UNTDID-only rendering

This follows invopop/gobl#929. The extension key is read directly so this change remains compatible with the currently released GOBL dependency while that breaking refactor and its downstream dependency migrations are pending.

Validation

• go test ./components/t ./components/bill
• targeted fr-invoice-units.fr snapshot test
• go test ./... -run "^$"
• golangci-lint run ./...
• changed unit formatter functions have 100% coverage

The complete repository test command currently reports existing snapshot drift in party.hide.html, smoobu.hide.html, and smoobu-out.hide.html: their stored German tax-ID labels differ from current renderer output. This PR deliberately does not update those unrelated snapshots.