TestPlugin was still WPF (net10.0-windows, UseWpf, System.Windows,
TomsToolbox, the old ICSharpCode.ILSpy namespace), so it could not load
into the cross-platform app. Re-target it to net10.0 and map each
extension point onto the port's contracts: the custom Language, About-page
addition, context-menu entry, main-menu/toolbar command, and an Avalonia
options page whose view the ViewLocator resolves by naming convention. The
command needs [ImportingConstructor] for System.Composition to satisfy its
service dependency. Reference it build-only from the tests (no DLL copied
into the test output) so a composition test can load it without the
headless app auto-loading the plugin into every other test.
Assisted-by: Claude:claude-opus-4-8:Claude Code
Siegfried Pammer
461faeda58