feat(diagnostics): VM emits trace events + CallScriptDispatches stat

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

engine/Age.Engine.Tests/CallScriptIntegrationTests.cs

@@ -31,8 +31,8 @@ public class CallScriptIntegrationTests
         var host = new NullHost();
         var vm = new VirtualMachine(script, t, host, null, provider);
         vm.Run();
-        Assert.Equal(2, host.CallScripts);       // ADDILLSUB + CALCREVISE both dispatched
-        Assert.Equal("exit", vm.HaltReason);     // subroutines returned; ADDILL reached its own exit
+        Assert.Equal(2, vm.CallScriptDispatches); // ADDILLSUB + CALCREVISE both dispatched
+        Assert.Equal("exit", vm.HaltReason);      // subroutines returned; ADDILL reached its own exit
     }
 
     [Fact]

engine/Age.Engine.Tests/CallScriptTests.cs

@@ -58,12 +58,13 @@ public class CallScriptTests
             OP_MOV, 3, 0x11, 3, 0x10,
             OP_EXIT);
         var host = new NullHost();
-        var vm = new VirtualMachine(caller, t, host, null, new MapProvider(new() { [5] = callee }));
+        var sink = new RecordingTraceSink();
+        var vm = new VirtualMachine(caller, t, host, null, new MapProvider(new() { [5] = callee }), sink);
         vm.Run();
         Assert.Equal(7, vm.Globals[0x10]);           // callee wrote a shared global
         Assert.Equal(7, vm.Globals[0x11]);           // caller read it AFTER the call returned
         Assert.Equal("exit", vm.HaltReason);         // top-level exit
-        Assert.Contains(5L, host.Calls);             // host notified
+        Assert.Contains(5L, sink.CallScriptIds);     // dispatch observed via the trace sink
     }
 
     [Fact]
@@ -72,10 +73,11 @@ public class CallScriptTests
         var t = Table();
         var caller = Asm(t, "CALLER", OP_CALLSCRIPT, 0, 5, OP_EXIT);
         var host = new NullHost();
-        var vm = new VirtualMachine(caller, t, host, null, null);   // no provider
+        var sink = new RecordingTraceSink();
+        var vm = new VirtualMachine(caller, t, host, null, null, sink);   // no provider
         vm.Run();
         Assert.Equal("exit", vm.HaltReason);        // did not halt on the call; stub + continue
-        Assert.Contains(5L, host.Calls);
+        Assert.Contains(5L, sink.CallScriptIds);
     }
 
     [Fact]

engine/Age.Engine.Tests/TestSupport.cs

@@ -1,4 +1,6 @@
 using System.Collections.Generic;
+using System.Linq;
+using Age.Engine.Diagnostics;
 using Age.Engine.Hosting;
 using Age.Engine.Model;
 
@@ -36,3 +38,14 @@ internal sealed class AnyProvider : IScriptProvider
     public AnyProvider(Script s) => _s = s;
     public Script? GetById(long id) => _s;
 }
+
+/// <summary>Captures every trace event for assertions; TracingSteps is settable so a test can
+/// exercise the Step gate both ways.</summary>
+internal sealed class RecordingTraceSink : ITraceSink
+{
+    public bool TracingSteps { get; init; }
+    public readonly List<TraceEvent> Events = new();
+    public void Emit(in TraceEvent e) => Events.Add(e);
+    public List<long> CallScriptIds =>
+        Events.Where(e => e.Kind == TraceEventKind.CallScript).Select(e => e.Id).ToList();
+}

engine/Age.Engine.Tests/TraceSinkTests.cs

@@ -1,10 +1,17 @@
+using System;
+using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using Age.Engine.Diagnostics;
 using Age.Engine.Model;
+using Age.Engine.Sys4;
+using Age.Engine.Vm;
 using Xunit;
 
 public class TraceSinkTests
 {
+    private static OpcodeTable Table() => OpcodeTableJson.Load(Paths.OpcodesJson);
+
     [Fact]
     public void FactoriesSetKindAndFields()
     {
@@ -41,4 +48,49 @@ public class TraceSinkTests
         Assert.Contains("call-script 0x1ab =ADDITEM (resolved)", outp);
         Assert.Contains("halt: exit @ 27994 steps", outp);
     }
+
+    [Fact]
+    public void VmEmitsFrameCallScriptAndHaltEvents()
+    {
+        var t = Table();
+        // callee: exit.  caller: call-script 5 ; exit.
+        var callee = ScriptAssembler.Assemble(t, "CALLEE",
+            new List<(int, Operand[])> { (0x2, Array.Empty<Operand>()) }, Array.Empty<string>());
+        var caller = ScriptAssembler.Assemble(t, "CALLER",
+            new List<(int, Operand[])> { (0x3, new[] { new Operand(0, 5) }), (0x2, Array.Empty<Operand>()) },
+            Array.Empty<string>());
+        var sink = new RecordingTraceSink();
+        var vm = new VirtualMachine(caller, t, new RecordingHost(), null,
+                                    new MapProvider(new() { [5] = callee }), sink);
+        vm.Run();
+
+        var kinds = sink.Events.Select(e => e.Kind).ToList();
+        Assert.Equal(TraceEventKind.FrameEnter, kinds[0]);                 // caller enters first
+        Assert.Equal(TraceEventKind.Halt, kinds[^1]);                      // halt is last
+        Assert.Equal(2, sink.Events.Count(e => e.Kind == TraceEventKind.FrameEnter));  // caller + callee
+        Assert.Equal(2, sink.Events.Count(e => e.Kind == TraceEventKind.FrameExit));
+        Assert.Contains(5L, sink.CallScriptIds);
+        Assert.Equal(1, vm.CallScriptDispatches);
+    }
+
+    [Fact]
+    public void StepEventsGatedByTracingSteps()
+    {
+        var t = Table();
+        // mov g[0x10]=7 ; exit  => 2 executed instructions.
+        var body = new List<(int, Operand[])>
+        {
+            (0x55, new[] { new Operand(3, 0x10), new Operand(0, 7) }),
+            (0x2, Array.Empty<Operand>()),
+        };
+        var s = ScriptAssembler.Assemble(t, "S", body, Array.Empty<string>());
+
+        var off = new RecordingTraceSink { TracingSteps = false };
+        new VirtualMachine(s, t, new RecordingHost(), null, null, off).Run();
+        Assert.Empty(off.Events.Where(e => e.Kind == TraceEventKind.Step));
+
+        var on = new RecordingTraceSink { TracingSteps = true };
+        new VirtualMachine(s, t, new RecordingHost(), null, null, on).Run();
+        Assert.Equal(2, on.Events.Count(e => e.Kind == TraceEventKind.Step));
+    }
 }