feat(ui): centralize retail selection state

src/AcDream.App/UI/InteractionState.cs

@@ -0,0 +1,54 @@
+namespace AcDream.App.UI;
+
+public enum InteractionModeKind
+{
+    None,
+    Use,
+    Examine,
+    UseItemOnTarget,
+}
+
+public readonly record struct InteractionMode(
+    InteractionModeKind Kind,
+    uint SourceObjectId = 0)
+{
+    public static InteractionMode None => new(InteractionModeKind.None);
+}
+
+public readonly record struct InteractionModeTransition(
+    InteractionMode Previous,
+    InteractionMode Current);
+
+/// <summary>
+/// Single App-layer owner for retail's pointer interaction mode. Core selection
+/// remains session truth; this state represents temporary UI orchestration.
+/// </summary>
+public sealed class InteractionState
+{
+    public InteractionMode Current { get; private set; } = InteractionMode.None;
+
+    public event Action<InteractionModeTransition>? Changed;
+
+    public bool EnterUse() => Set(new InteractionMode(InteractionModeKind.Use));
+
+    public bool EnterExamine() => Set(new InteractionMode(InteractionModeKind.Examine));
+
+    public bool EnterUseItemOnTarget(uint sourceObjectId)
+    {
+        if (sourceObjectId == 0)
+            throw new ArgumentOutOfRangeException(nameof(sourceObjectId));
+        return Set(new InteractionMode(InteractionModeKind.UseItemOnTarget, sourceObjectId));
+    }
+
+    public bool Clear() => Set(InteractionMode.None);
+
+    private bool Set(InteractionMode mode)
+    {
+        if (Current == mode)
+            return false;
+        InteractionMode previous = Current;
+        Current = mode;
+        Changed?.Invoke(new InteractionModeTransition(previous, mode));
+        return true;
+    }
+}