refactor(ui): own retained controller lifetimes

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

@@ -0,0 +1,42 @@
+using System.Collections.Generic;
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public sealed class RetainedPanelControllerGroupTests
+{
+    [Fact]
+    public void Lifecycle_ForwardsInOwnershipOrder_AndDisposesInReverseExactlyOnce()
+    {
+        var calls = new List<string>();
+        var first = new RecordingController("first", calls);
+        var second = new RecordingController("second", calls);
+        var group = new RetainedPanelControllerGroup(first, second);
+        var focus = new UiPanel();
+
+        group.OnShown();
+        group.OnDescendantFocusChanged(focus);
+        group.OnHidden();
+        group.Dispose();
+        group.Dispose();
+
+        Assert.Equal(
+        [
+            "first:shown", "second:shown",
+            "first:focus", "second:focus",
+            "second:hidden", "first:hidden",
+            "second:dispose", "first:dispose",
+        ], calls);
+    }
+
+    private sealed class RecordingController(
+        string name,
+        List<string> calls) : IRetainedPanelController
+    {
+        public void OnShown() => calls.Add($"{name}:shown");
+        public void OnHidden() => calls.Add($"{name}:hidden");
+        public void OnDescendantFocusChanged(UiElement? focusedDescendant)
+            => calls.Add($"{name}:focus");
+        public void Dispose() => calls.Add($"{name}:dispose");
+    }
+}