feat(diagnostics): ITraceSink seam + TraceEvent + Null/Text sinks

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

engine/Age.Engine.Tests/TraceSinkTests.cs

@@ -0,0 +1,44 @@
+using System.IO;
+using Age.Engine.Diagnostics;
+using Age.Engine.Model;
+using Xunit;
+
+public class TraceSinkTests
+{
+    [Fact]
+    public void FactoriesSetKindAndFields()
+    {
+        var ins = new Instruction(0x40, 0x55, new[] { new Operand(3, 0x10), new Operand(0, 7) });
+        var step = TraceEvent.Step(0x40, ins, 2);
+        Assert.Equal(TraceEventKind.Step, step.Kind);
+        Assert.Equal(0x55, step.Opcode);
+        Assert.Same(ins, step.Ins);
+        Assert.Equal(2, step.Depth);
+
+        var cs = TraceEvent.CallScript(0x1ab, "ADDITEM");
+        Assert.Equal(TraceEventKind.CallScript, cs.Kind);
+        Assert.Equal(0x1abL, cs.Id);
+        Assert.Equal("ADDITEM", cs.Name);
+    }
+
+    [Fact]
+    public void NullSinkIsInertAndNotTracingSteps()
+    {
+        Assert.False(NullTraceSink.Instance.TracingSteps);
+        NullTraceSink.Instance.Emit(TraceEvent.Halt("x", 1));   // must not throw
+    }
+
+    [Fact]
+    public void TextSinkFormatsEachKind()
+    {
+        var sw = new StringWriter();
+        var sink = new TextTraceSink(sw, table: null, includeSteps: true);
+        sink.Emit(TraceEvent.FrameEnter("SC0000", 1, FrameCause.TopScene));
+        sink.Emit(TraceEvent.CallScript(0x1ab, "ADDITEM"));
+        sink.Emit(TraceEvent.Halt("exit", 27994));
+        var outp = sw.ToString();
+        Assert.Contains("» SC0000 (enter, TopScene)", outp);
+        Assert.Contains("call-script 0x1ab =ADDITEM (resolved)", outp);
+        Assert.Contains("halt: exit @ 27994 steps", outp);
+    }
+}