feat(text): persist independent text layer settings

Add additive defaults and round-trip tests for per-layer text content and formatting. Model the static text source after OBS text-source conventions: https://obsproject.com/kb/text-sources

Models/Source.cs

@@ -41,6 +41,13 @@ public enum MediaPlaybackState
     Paused
 }
 
+public enum TextLayerAlignment
+{
+    Left,
+    Center,
+    Right
+}
+
 public enum ClipShape
 {
     Traditional,
@@ -391,6 +398,58 @@ public class Source : SceneElement
 
     public override bool IsChatBox => Type == SourceType.ChatBox;
 
+    private string _textContent = "Text";
+    public string TextContent
+    {
+        get => _textContent;
+        set => Set(ref _textContent, value ?? string.Empty);
+    }
+
+    private string _textFontFamily = "Segoe UI";
+    public string TextFontFamily
+    {
+        get => _textFontFamily;
+        set => Set(ref _textFontFamily, string.IsNullOrWhiteSpace(value) ? "Segoe UI" : value.Trim());
+    }
+
+    private double _textFontSize = 48;
+    public double TextFontSize
+    {
+        get => _textFontSize;
+        set => Set(ref _textFontSize, double.IsFinite(value) ? Math.Clamp(value, 8, 300) : 48);
+    }
+
+    private string _textColor = "#FFFFFF";
+    public string TextColor
+    {
+        get => _textColor;
+        set => Set(ref _textColor, IsValidTextColor(value) ? value.ToUpperInvariant() : "#FFFFFF");
+    }
+
+    private bool _textBold;
+    public bool TextBold { get => _textBold; set => Set(ref _textBold, value); }
+
+    private bool _textItalic;
+    public bool TextItalic { get => _textItalic; set => Set(ref _textItalic, value); }
+
+    private bool _textUnderline;
+    public bool TextUnderline { get => _textUnderline; set => Set(ref _textUnderline, value); }
+
+    private TextLayerAlignment _textAlignment;
+    public TextLayerAlignment TextAlignment
+    {
+        get => _textAlignment;
+        set => Set(ref _textAlignment, Enum.IsDefined(value) ? value : TextLayerAlignment.Left);
+    }
+
+    private static bool IsValidTextColor(string? value)
+    {
+        if (value is not { Length: 7 or 9 } || value[0] != '#') return false;
+        for (var i = 1; i < value.Length; i++)
+            if (!Uri.IsHexDigit(value[i])) return false;
+        return true;
+    }
+
     public override bool IsCountdown => Type == SourceType.Countdown;
 
     private bool _chatPreviewEnabled = true;