feat: add IHost.FrameYield per-opcode hook (no-op headless, parity held)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine.Tests/GameSessionTests.cs

@@ -30,6 +30,7 @@ public class GameSessionTests
         public void ShowText(int o, string t) => Emitted.Add(o);
         public void WaitForInput() { }
         public void Sleep(long duration) { }
+        public void FrameYield() { }
         public void CreateTexture(int s, int w, int h) { }
         public void SetTexture(long r, int s) { }
         public void DrawTexture(int s, int sx, int sy, int w, int h, int dx, int dy) { }