Theme asset validation

dotnet run --project tools/validation/AtomUI.ThemeAssetsValidation/AtomUI.ThemeAssetsValidation.csproj -c Release

The tool commits a real Headless Avalonia application with the official control packages and Gallery. It forces each selected theme resource factory, including deferred exported themes and known resource keys, through its normal application owner bridge. It fails on a factory or contract exception, reports per-package materialization counts, and disposes the scope before printing the final summary. CI runs this independently of test selection.

This checks factory materialization and ownership. Native window rendering, control-template layout and user interaction continue to require their focused UI tests and platform validation.