feat(vm): IHost.Sleep seam + dispatch 0xc8 (headless hosts no-op; parity held)

- IHost.Sleep(long duration); VM case "sleep" forwards the raw operand.
- 8 non-Godot hosts no-op Sleep; RecordingHost records it.
- SleepDispatchTests (51 green); sweep unchanged 284 exit / 13 STEP-LIMIT.

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

engine/Age.Engine.Tests/TextureOpsTests.cs

@@ -13,6 +13,7 @@ public class TextureOpsTests
         public int Creates;
         public void ShowText(int o, string t) { }
         public void WaitForInput() { }
+        public void Sleep(long duration) { }
         public void CreateTexture(int slot, int w, int h) => Creates++;
         public void SetTexture(long resId, int slot) => Sets.Add((resId, slot));
         public void DrawTexture(int slot, int srcX, int srcY, int w, int h, int dstX, int dstY) => Draws.Add((slot, w, h));