FA(3): missing rendering of GTU for invoice lines

[Emilz4466]

Describe the bug
For FA(3) invoices, ksef-fop does not render the GTU field for invoice lines (FaWiersz) in the generated PDF.
In KSeF, the GTU designation is displayed for invoice lines when present. In ksef-fop, this information is missing from the positions table.

To Reproduce
Steps to reproduce the behavior:

1. Prepare an FA(3) XML invoice containing GTU in Fa/FaWiersz
2. Generate a PDF from the XML using ksef-fop
3. Open the generated PDF
4. Check the positions table and note that GTU is not displayed for invoice lines

Sample XML :

9_simple.xml

Expected behavior
The PDF should display GTU for each invoice line (FaWiersz) when the field is present in XML.
A dedicated GTU column should be shown in the positions table.

Desktop (please complete the following information):

• OS: Linux Ubuntu
• Version 25.04

Additional context
GTU exists in the FA(3) schema as part of FaWiersz:
• src/test/resources/Schemat_FA_VAT(3)_v1-0E.xsd

However, the FA(3) invoice positions table does not render crd:GTU:
• src/main/resources/templates/fa3/invoice-rows.xsl

This means invoice lines containing GTU markings are currently visualized incompletely compared to KSeF.