It's probably not a very satisfying workaround, but this all seems to work (including in an AppImage built with Qt 6.8.2) if you simply embed the svg filter define (from Metal_Filter.svg) in the xhtml of Section0001.xhtml rather than defining it in a separate file. I included your latest changes to rebuild the AppImage.
Then it's simply a matter of changing the filter url from "url(../Images/Metal_Filter.svg#metal)" to simply url(#metal):
Code:
<p><img style="filter: url(#metal)" alt="Image1" src="../Images/Image1.svg"/></p>