feat(ui): centralize retail selection state

tests/AcDream.App.Tests/UI/InteractionStateTests.cs

@@ -0,0 +1,32 @@
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public sealed class InteractionStateTests
+{
+    [Fact]
+    public void Modes_HaveOneOwnerAndOneDeduplicatedTransitionStream()
+    {
+        var state = new InteractionState();
+        var changes = new List<InteractionModeTransition>();
+        state.Changed += changes.Add;
+
+        Assert.True(state.EnterUse());
+        Assert.False(state.EnterUse());
+        Assert.True(state.EnterExamine());
+        Assert.True(state.EnterUseItemOnTarget(0x100u));
+        Assert.True(state.Clear());
+
+        Assert.Equal(4, changes.Count);
+        Assert.Equal(InteractionModeKind.None, state.Current.Kind);
+        Assert.Equal(0x100u, changes[2].Current.SourceObjectId);
+    }
+
+    [Fact]
+    public void UseItemOnTarget_RejectsMissingSource()
+    {
+        var state = new InteractionState();
+        Assert.Throws<ArgumentOutOfRangeException>(() => state.EnterUseItemOnTarget(0));
+        Assert.Equal(InteractionMode.None, state.Current);
+    }
+}