Line-items currency when not unionized

Set currency on Line-items when that step is not unionized with Payment or Payout. You can itemize VAT, fees, and product amounts in one currency even when checkout settles in another, without forcing every line to match the payment step.

• Define currency in line-items features when the action stands alone, so item labels and modifiers display in the currency your catalog uses.
• When line-items share a union with payment or payout, set currency on those linked actions only; line items inherit it so customers do not see conflicting currency codes in the same checkout.