Implement diagnostic output opcodes

engine/Age.Engine/Vm/DiagnosticOutputState.cs

@@ -0,0 +1,17 @@
+using System.Text;
+
+namespace Age.Engine.Vm;
+
+/// <summary>
+/// EngineCtx-lifetime accumulator used by opcodes 0x1b2 through 0x1b4. GameSession shares this
+/// state across fresh scene VMs just as native AGE retains the embedded string across script frames.
+/// </summary>
+public sealed class DiagnosticOutputState
+{
+    private readonly StringBuilder _text = new();
+
+    public string PendingText => _text.ToString();
+
+    internal void Append(string value) => _text.Append(value);
+    internal void Clear() => _text.Clear();
+}