feat(alerts): add scene-local recent ticker

Model the AlertBox and standalone ticker with shared settings; scene tickers render the in-memory active and queued events without saving history. Keep AlertBox content active-event-only.

Ref: https://obsproject.com/kb/obs-studio-overview

Models/Source.cs

@@ -21,7 +21,9 @@ public enum SourceType
     AlertBox,
     /// <summary>A privacy blur: blurs the frame beneath it inside a rect or ellipse. Live scene
     /// only, always below the brand logo, several allowed (strongest wins on overlap).</summary>
-    Blur
+    Blur,
+    /// <summary>A scene-local feed of recent alert events.</summary>
+    Ticker
 }
 
 /// <summary>Distinguishes how a media source presents its content (TASK 21).</summary>
@@ -66,6 +68,19 @@ public enum AlertDisplayMethod
 /// </summary>
 public class Source : SceneElement
 {
+    private readonly Ticker _ticker = new();
+
+    public Source()
+    {
+        _ticker.PropertyChanged += (_, e) =>
+        {
+            if (e.PropertyName == nameof(Ticker.IsEnabled)) Raise(nameof(AlertShowTicker));
+            if (e.PropertyName == nameof(Ticker.DisplayMethod)) Raise(nameof(AlertDisplayMethod));
+        };
+    }
+
+    public Ticker Ticker => _ticker;
+
     private SourceType _type;
     public SourceType Type
     {
@@ -80,6 +95,7 @@ public class Source : SceneElement
                 Raise(nameof(IsWebSource));
                 Raise(nameof(IsTextOverlay));
                 Raise(nameof(IsScoreboard));
+                Raise(nameof(IsTicker));
             }
         }
     }
@@ -88,18 +104,19 @@ public class Source : SceneElement
     /// and sizes it in the preview, so it needs the same selection box, grab and resize handle
     /// (creator ruling 2026-11-01).</summary>
     public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox
-        or SourceType.AlertBox or SourceType.Blur;
+        or SourceType.AlertBox or SourceType.Blur or SourceType.Ticker;
 
     /// <summary>Live captures, chat, web sources and media are dynamic; static art is fixed.</summary>
     public override ElementKind Kind => Type is SourceType.DisplayCapture or SourceType.WindowCapture
         or SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource or SourceType.AlertBox
-        or SourceType.Blur
+        or SourceType.Blur or SourceType.Ticker
         ? ElementKind.Dynamic : ElementKind.Static;
 
     public override bool IsWebSource => Type == SourceType.WebSource;
     public override bool IsTextOverlay => Type == SourceType.TextOverlay;
 
     public override bool IsScoreboard => Type == SourceType.Scoreboard;
+    public bool IsTicker => Type == SourceType.Ticker;
 
     /// <summary>The permanent brand logo layer (one per scene, directly above the background).
     /// Locked against removal; its image comes from the App Settings brand logo.</summary>
@@ -156,7 +173,7 @@ public class Source : SceneElement
     /// VideoImageSource, and never displayed (the same shape as the invisible-alert defect).</summary>
     private bool IsFrameSource =>
         Type is SourceType.ChatBox or SourceType.WebSource or SourceType.MediaSource
-             or SourceType.AlertBox or SourceType.Scoreboard;
+             or SourceType.AlertBox or SourceType.Scoreboard or SourceType.Ticker;
 
     public override ImageSource? DisplaySource => IsLiveCapture
         ? (VideoImageSource ?? _imageSource)
@@ -259,11 +276,10 @@ public class Source : SceneElement
         set => Set(ref _alertUseDefaultVideo, value);
     }
 
-    private bool _alertShowTicker = true;
     public bool AlertShowTicker
     {
-        get => _alertShowTicker;
-        set => Set(ref _alertShowTicker, value);
+        get => _ticker.IsEnabled;
+        set => _ticker.IsEnabled = value;
     }
 
     private double _alertVideoVolume = 1.0;
@@ -295,11 +311,10 @@ public class Source : SceneElement
     /// <summary>True once a file is chosen — gates the loop switch and the clear button.</summary>
     public bool HasAlertAudio => !string.IsNullOrWhiteSpace(_alertAudioPath);
 
-    private AlertDisplayMethod _alertDisplayMethod = AlertDisplayMethod.TickerScroll;
     public AlertDisplayMethod AlertDisplayMethod
     {
-        get => _alertDisplayMethod;
-        set => Set(ref _alertDisplayMethod, value);
+        get => _ticker.DisplayMethod;
+        set => _ticker.DisplayMethod = value;
     }
 
     public override bool IsAlertBox => Type == SourceType.AlertBox;