App Settings: transition type and length picker

Cut/Fade/Move and a 100-2000 ms length, bound to the view model; changes save with the layout.
Test: TransitionPickerTests. Not pushed.

ytLive.Tests/TransitionPickerTests.cs

@@ -0,0 +1,54 @@
+using System;
+using System.ComponentModel;
+using Xunit;
+using ytLive.Services;
+using ytLive.ViewModels;
+
+namespace ytLive.Tests;
+
+/// <summary>
+/// The App Settings transition picker drives the view model: type and length changes
+/// notify the bound controls.
+/// </summary>
+[Collection("RealApp")]
+public sealed class TransitionPickerTests
+{
+    private readonly RealAppHost _app;
+
+    public TransitionPickerTests(RealAppHost app) => _app = app;
+
+    [Fact]
+    public void Transition_Type_And_Length_Notify_The_Settings_Picker()
+    {
+        _app.Run(Run);
+    }
+
+    private static void Run()
+    {
+        var tempDb = System.IO.Path.Combine(System.IO.Path.GetTempPath(), $"ytLlive-transition-{Guid.NewGuid():N}.db");
+        MainViewModel.LayoutPathOverride = tempDb;
+        try
+        {
+            using (var schema = new LayoutStore(tempDb)) { }
+            Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
+
+            var vm = new MainViewModel();
+            var raised = new System.Collections.Generic.List<string>();
+            ((INotifyPropertyChanged)vm).PropertyChanged += (_, e) => raised.Add(e.PropertyName ?? "");
+
+            vm.SelectedTransitionType = TransitionType.Move;
+            vm.TransitionDurationMs = 750;
+
+            Assert.Equal(TransitionType.Move, vm.SelectedTransitionType);
+            Assert.Equal(750, vm.TransitionDurationMs);
+            Assert.Contains(nameof(MainViewModel.SelectedTransitionType), raised);
+            Assert.Contains(nameof(MainViewModel.TransitionDurationMs), raised);
+        }
+        finally
+        {
+            MainViewModel.LayoutPathOverride = null;
+            Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
+            try { System.IO.File.Delete(tempDb); } catch { }
+        }
+    }
+}