Kill stringly-typed command tokens: Add Source menu passes SourceType enum via x:Static CommandParameter (typos break the build, not silently add an Image); webcam gets its own AddWebcamCommand; AddSource keeps Enum.TryParse fallback; docs updated (65 tests passing)

MainWindow.xaml

@@ -4,6 +4,7 @@
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:vm="clr-namespace:ytLive.ViewModels"
+        xmlns:models="clr-namespace:ytLive.Models"
         xmlns:Helpers="clr-namespace:ytLive.Helpers"
         mc:Ignorable="d"
         Title="{Binding WindowTitle}" Height="720" Width="1280"
@@ -249,13 +250,13 @@
                                 Click="AddSourceButton_Click">
                             <Button.ContextMenu>
                                 <ContextMenu DataContext="{Binding PlacementTarget.DataContext, RelativeSource={RelativeSource Self}}">
-                                    <MenuItem Header="Webcam" Command="{Binding AddSourceCommand}" CommandParameter="webcam"
+                                    <MenuItem Header="Webcam" Command="{Binding AddWebcamCommand}"
                                               IsEnabled="{Binding CanAddWebcamToActiveScene}"
                                               ToolTip="One webcam at a time — it's already in your stream"/>
-                                    <MenuItem Header="Screen" Command="{Binding AddSourceCommand}" CommandParameter="screen"/>
-                                    <MenuItem Header="Background" Command="{Binding AddSourceCommand}" CommandParameter="background"/>
+                                    <MenuItem Header="Screen" Command="{Binding AddSourceCommand}" CommandParameter="{x:Static models:SourceType.DisplayCapture}"/>
+                                    <MenuItem Header="Background" Command="{Binding AddSourceCommand}" CommandParameter="{x:Static models:SourceType.Background}"/>
                                     <MenuItem Header="Image" Command="{Binding AddImageCommand}"/>
-                                    <MenuItem Header="Text" Command="{Binding AddSourceCommand}" CommandParameter="text"/>
+                                    <MenuItem Header="Text" Command="{Binding AddSourceCommand}" CommandParameter="{x:Static models:SourceType.TextOverlay}"/>
                                 </ContextMenu>
                             </Button.ContextMenu>
                             <Path Data="M12,4 L12,20 M4,12 L20,12" Stroke="#e94560" StrokeThickness="2"