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/Hosting/CaptureHost.cs

@@ -5,6 +5,7 @@ public sealed class CaptureHost : IHost
     public void ShowText(int offset, string text) => Emitted.Add((offset, text));
     public void WaitForInput() { }
     public void Sleep(long duration) { }
+    public void FrameYield() { }
     public void CreateTexture(int slot, int width, int height) { }
     public void SetTexture(long resourceId, int slot) { }
     public void DrawTexture(int slot, int srcX, int srcY, int width, int height, int dstX, int dstY) { }