ITA - Fattura elettronica - Emissione - Sconto fisso

This is a technical bridge module that connects two existing features: fixed-amount discounts on invoice lines and the Italian electronic invoice export. It installs itself automatically when both features are present, so users do not need to enable it manually.

What changes for users:

• If you enter a fixed discount amount (not a percentage) on an invoice line, that discount is written into the electronic invoice XML as a discount ("Sconto") entry with the amount rounded to two decimals.
• The discount appears in the same place where percentage discounts already appear in the generated file, so the resulting XML remains valid for the Italian tax authority.
• No new menus, buttons or screens are added; the behaviour is applied automatically when you use the existing electronic invoice export wizard.

The module touches the electronic invoice export wizard (wizard.export.fatturapa) and extends its discount handling method. It relies on the fixed discount field provided by the fixed discount module and on the electronic invoice data structures from the Italian localization. Only fixed discounts are added; percentage discounts continue to be handled by the base module.