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

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

engine/Age.Engine/Diagnostics/NullTraceSink.cs

@@ -0,0 +1,11 @@
+namespace Age.Engine.Diagnostics;
+
+/// <summary>The inert default: no step tracing, empty Emit. Supplying this (or null) to the VM
+/// guarantees byte-identical behavior.</summary>
+public sealed class NullTraceSink : ITraceSink
+{
+    public static readonly NullTraceSink Instance = new();
+    private NullTraceSink() { }
+    public bool TracingSteps => false;
+    public void Emit(in TraceEvent e) { }
+}