feat(headless): complete portable single-session host

src/AcDream.Headless/Hosting/HeadlessGenerationResetHost.cs

@@ -0,0 +1,25 @@
+using AcDream.Runtime;
+using AcDream.Runtime.Entities;
+
+namespace AcDream.Headless.Hosting;
+
+/// <summary>
+/// A direct host owns no graphical projections, but still acknowledges every
+/// exact canonical retirement edge required by Runtime generation reset.
+/// </summary>
+internal sealed class HeadlessGenerationResetHost
+    : IRuntimeGenerationResetHost
+{
+    public void RetireEntityProjection(RuntimeEntityRecord entity)
+    {
+        ArgumentNullException.ThrowIfNull(entity);
+    }
+
+    public void DrainEntityProjectionBoundary()
+    {
+    }
+
+    public void CompleteEntityProjectionRetirement()
+    {
+    }
+}