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/CallScriptIntegrationTests.cs.bak

@@ -0,0 +1,49 @@
+using Age.Engine.Hosting;
+using Age.Engine.Sys4;
+using Age.Engine.Vm;
+using Xunit;
+
+public class CallScriptIntegrationTests
+{
+    private sealed class NullHost : IHost
+    {
+        public void ShowText(int o, string t) { }
+        public void WaitForInput() { }
+        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) { }
+        public (int Width, int Height) GetTextureSize(int s) => (0, 0);
+        public void PlayBgm(long id) { }
+        public void PlayVoice(long id) { }
+    }
+
+    [Fact]
+    public void RealScriptExecutesRealSubroutinesAndReturns()
+    {
+        // ADDILL.BIN unconditionally call-scripts ADDILLSUB then CALCREVISE at entry, then exits.
+        // With execution on, both subroutines load, run, and return, so ADDILL reaches its own exit.
+        var t = OpcodeTableJson.Load(Paths.OpcodesJson);
+        var provider = Sys4ScriptProvider.Load(t);
+        var script = Sys4Loader.Load(Paths.Scripts()["ADDILL.BIN"], t);
+        var host = new NullHost();
+        var vm = new VirtualMachine(script, t, host, null, provider);
+        vm.Run();
+        Assert.Equal(2, vm.CallScriptDispatches); // ADDILLSUB + CALCREVISE both dispatched
+        Assert.Equal("exit", vm.HaltReason);      // subroutines returned; ADDILL reached its own exit
+    }
+
+    [Fact]
+    public void BunkiTopLevelRetReturnsCleanlyAsSubroutine()
+    {
+        // BUNKI.BIN ends with a top-level `ret` (empty intra-call stack). Called as a subroutine it
+        // must return to the caller, not underflow-halt. Drive it directly.
+        var t = OpcodeTableJson.Load(Paths.OpcodesJson);
+        var provider = Sys4ScriptProvider.Load(t);
+        var bunki = provider.GetById(0x143);   // BUNKI.BIN
+        Assert.NotNull(bunki);
+        var vm = new VirtualMachine(bunki!, t, new NullHost(), null, provider);
+        vm.Run();
+        // Reaching a frame-return at the top = clean "exit"; never "ret-underflow".
+        Assert.NotEqual("ret-underflow", vm.HaltReason);
+    }
+}