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

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

engine/Age.Engine/Diagnostics/ITraceSink.cs

@@ -0,0 +1,12 @@
+namespace Age.Engine.Diagnostics;
+
+/// <summary>The engine's diagnostics seam. The VM emits typed <see cref="TraceEvent"/>s here; any
+/// consumer (CLI, Godot, tests) supplies a sink instead of reimplementing IHost. Observe-only:
+/// a sink never reads/writes VM state or influences control flow (that guarantees trace parity).</summary>
+public interface ITraceSink
+{
+    /// <summary>Cheap gate: when false the VM skips constructing per-instruction Step events, keeping
+    /// the hot path (a corpus sweep is ~1.46M instructions) free. Rare events emit regardless.</summary>
+    bool TracingSteps { get; }
+    void Emit(in TraceEvent e);
+}