feat(ui): centralize retail selection state

docs/architecture/acdream-architecture.md

@@ -102,7 +102,11 @@ retained gameplay tree, LayoutDesc importer, window runtime, and panel
 controllers. Neither presentation stack owns independent game-state truth.
 Plugins register retained gameplay markup through the BCL-only
 `AcDream.Plugin.Abstractions.IUiRegistry`; they do not import App or ImGui
-assemblies.
+assemblies. Core `SelectionState` is the sole selected-object owner for world,
+radar, inventory, paperdoll, toolbar, use/examine consumers, and plugins;
+`IPluginHost.Selection` exposes that same state and retail-style old/new callback.
+Temporary pointer modes are separate App orchestration in `InteractionState` and
+must never become a competing selection owner.
 
 Every retained gameplay window has a typed `RetailWindowHandle` and at most one
 `IRetainedPanelController` lifecycle owner. Multi-controller windows use
@@ -164,6 +168,7 @@ src/
     IPluginHost.cs            -> done
     IGameState.cs             -> done
     IEvents.cs                -> done
+    ISelectionService.cs      -> done
   
   AcDream.App/               Layer 1 + Layer 4 wiring
     Rendering/