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

Controls/index.md

@@ -17,6 +17,10 @@ See [`schema.md`](../schema.md) for the memory-map conventions.
 | `SceneThumbnailStrip.xaml.cs` | Code-behind for the strip above |
 | `LeftPanel.xaml.cs` | Code-behind for the panel above |
 
+The Add Layer menu exposes one standalone recent-alert Ticker per scene. The
+properties panel edits its independent `Ticker` settings; Stream Alerts retains
+its separate current-event ticker controls.
+
 ## Window ↔ control contract
 
 The shell (`MainWindow.xaml(.cs)`) wires up each control and routes the few