Extract standalone animations package and sample

[wieslawsoltes]

Summary

• extract reusable animation, composition, and transition helpers into the independently shippable Xaml.Behaviors.Animations package
• preserve existing custom behavior APIs through adapters and type forwards
• add a standalone tabbed animations sample with one page per feature
• add headless/unit coverage and update solution, package, README, and DocFX documentation

Validation

• dotnet build AvaloniaBehaviors.slnx --no-restore --nologo
• dotnet test AvaloniaBehaviors.slnx --no-build --no-restore --nologo (505 passed, 4 skipped)
• dotnet pack src/Xaml.Behaviors.Animations/Xaml.Behaviors.Animations.csproj --no-build -c Debug --nologo
• DocFX build completed with the existing documentation warnings
• standalone sample launch visually verified on Linux