feat(ui): centralize retail selection state

src/AcDream.Plugin.Abstractions/IPluginHost.cs

@@ -10,5 +10,6 @@ public interface IPluginHost
     IPluginLogger Log { get; }
     IGameState State { get; }
     IEvents Events { get; }
+    ISelectionService Selection { get; }
     IUiRegistry Ui { get; }
 }

src/AcDream.Plugin.Abstractions/ISelectionService.cs

@@ -0,0 +1,19 @@
+namespace AcDream.Plugin.Abstractions;
+
+/// <summary>A plugin-safe selection transition matching retail's selected-target callback.</summary>
+public readonly record struct SelectionChangedEvent(
+    uint? PreviousObjectId,
+    uint? SelectedObjectId);
+
+/// <summary>
+/// Shared selected-object service. Plugins read and change the same selection
+/// used by the world, radar, inventory, paperdoll, and toolbar.
+/// </summary>
+public interface ISelectionService
+{
+    uint? SelectedObjectId { get; }
+    uint? PreviousObjectId { get; }
+    event Action<SelectionChangedEvent> Changed;
+    bool Select(uint objectId);
+    bool Clear();
+}