refactor(runtime): extract the live object frame

src/AcDream.App/World/LiveEntityRuntimeSlot.cs

@@ -0,0 +1,24 @@
+namespace AcDream.App.World;
+
+internal interface ILiveEntityRuntimeSource
+{
+    LiveEntityRuntime? Current { get; }
+}
+
+/// <summary>
+/// One-time composition bridge for the resource-activation/runtime cycle.
+/// It owns no identity or entity state; all authority remains in the bound
+/// <see cref="LiveEntityRuntime"/>.
+/// </summary>
+internal sealed class LiveEntityRuntimeSlot : ILiveEntityRuntimeSource
+{
+    public LiveEntityRuntime? Current { get; private set; }
+
+    public void Bind(LiveEntityRuntime runtime)
+    {
+        ArgumentNullException.ThrowIfNull(runtime);
+        if (Current is not null)
+            throw new InvalidOperationException("The live entity runtime is already bound.");
+        Current = runtime;
+    }
+}