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/TestSupport.cs

@@ -14,6 +14,7 @@ internal sealed class RecordingHost : IHost
     public void ShowText(int offset, string text) => Lines.Add((offset, text));
     public void WaitForInput() => Waits++;
     public void Sleep(long duration) => SleptDurations.Add(duration);
+    public void FrameYield() { }
     public void CreateTexture(int slot, int w, int h) { }
     public void SetTexture(long resId, int slot) { }
     public void DrawTexture(int slot, int sx, int sy, int w, int h, int dx, int dy) { }