refactor(app): compose live presentation startup

src/AcDream.App/World/LiveEntityRuntimeSlot.cs

@@ -21,4 +21,31 @@ internal sealed class LiveEntityRuntimeSlot : ILiveEntityRuntimeSource
             throw new InvalidOperationException("The live entity runtime is already bound.");
         Current = runtime;
     }
+
+    public IDisposable BindOwned(LiveEntityRuntime runtime)
+    {
+        Bind(runtime);
+        return new Binding(this, runtime);
+    }
+
+    private void Unbind(LiveEntityRuntime expected)
+    {
+        if (ReferenceEquals(Current, expected))
+            Current = null;
+    }
+
+    private sealed class Binding : IDisposable
+    {
+        private LiveEntityRuntimeSlot? _owner;
+        private readonly LiveEntityRuntime _expected;
+
+        public Binding(LiveEntityRuntimeSlot owner, LiveEntityRuntime expected)
+        {
+            _owner = owner;
+            _expected = expected;
+        }
+
+        public void Dispose() =>
+            Interlocked.Exchange(ref _owner, null)?.Unbind(_expected);
+    }
 }