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/FrameYieldTests.cs

@@ -0,0 +1,35 @@
+using Age.Engine.Hosting;
+using Age.Engine.Model;
+using Age.Engine.Sys4;
+using Age.Engine.Vm;
+using Xunit;
+
+public class FrameYieldTests
+{
+    private sealed class CountingHost : IHost
+    {
+        public long Yields;
+        public void FrameYield() => Yields++;
+        public void ShowText(int offset, string text) { }
+        public void WaitForInput() { }
+        public void Sleep(long duration) { }
+        public void CreateTexture(int slot, int width, int height) { }
+        public void SetTexture(long resourceId, int slot) { }
+        public void DrawTexture(int slot, int sx, int sy, int w, int h, int dx, int dy) { }
+        public (int Width, int Height) GetTextureSize(int slot) => (0, 0);
+        public void PlayBgm(long id) { }
+        public void PlayVoice(long id) { }
+    }
+
+    [Fact]
+    public void FrameYield_CalledOncePerStep()
+    {
+        var table = OpcodeTableJson.Load(Paths.OpcodesJson);
+        var script = Sys4Loader.Load(Paths.Scripts()["INITCONFIG.BIN"], table);
+        var host = new CountingHost();
+        var vm = new VirtualMachine(script, table, host);
+        vm.Run();
+        Assert.True(vm.Steps > 0);
+        Assert.Equal(vm.Steps, host.Yields);   // exactly one FrameYield per executed opcode
+    }
+}