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);
+}

engine/Age.Engine/Diagnostics/NullTraceSink.cs

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+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) { }
+}

engine/Age.Engine/Diagnostics/TextTraceSink.cs

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+using Age.Engine.Model;
+namespace Age.Engine.Diagnostics;
+
+/// <summary>The one built-in formatter: writes each event as a deterministic text line to a
+/// TextWriter (Console.Out or a file). Indents by frame depth. If an OpcodeTable is supplied, Step
+/// lines show the mnemonic; otherwise the raw opcode. Step lines only appear when includeSteps is set.</summary>
+public sealed class TextTraceSink : ITraceSink
+{
+    private readonly TextWriter _w;
+    private readonly OpcodeTable? _table;
+    private readonly bool _steps;
+
+    public TextTraceSink(TextWriter writer, OpcodeTable? table = null, bool includeSteps = false)
+    { _w = writer; _table = table; _steps = includeSteps; }
+
+    public bool TracingSteps => _steps;
+
+    public void Emit(in TraceEvent e)
+    {
+        string indent = new string(' ', Math.Max(0, e.Depth - 1) * 2);
+        switch (e.Kind)
+        {
+            case TraceEventKind.FrameEnter:
+                _w.WriteLine($"{indent}» {e.Name} (enter, {e.Cause})"); break;
+            case TraceEventKind.FrameExit:
+                _w.WriteLine($"{indent}« {e.Name} ({e.Text})"); break;
+            case TraceEventKind.Step:
+                _w.WriteLine($"{indent}  {e.Pc:x4} {Mnemonic(e.Opcode)} {Args(e.Ins)}"); break;
+            case TraceEventKind.CallScript:
+                _w.WriteLine($"{indent}  call-script 0x{e.Id:x} ={e.Name ?? "?"} " +
+                             $"({(e.Name != null ? "resolved" : "stub/unresolved")})"); break;
+            case TraceEventKind.Stub:
+                _w.WriteLine($"{indent}  {e.Pc:x4} STUB op=0x{e.Opcode:x}"); break;
+            case TraceEventKind.Halt:
+                _w.WriteLine($"halt: {e.Text} @ {e.Steps} steps"); break;
+        }
+    }
+
+    private string Mnemonic(int op) => _table?.Label(op) is { Length: > 0 } l ? l : $"0x{op:x}";
+    private static string Args(Instruction? ins) =>
+        ins == null ? "" : string.Join(" ", ins.Args.Select(o => $"{o.Type}:{o.Value}"));
+}

engine/Age.Engine/Diagnostics/TraceEvent.cs

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+using Age.Engine.Model;
+namespace Age.Engine.Diagnostics;
+
+public enum TraceEventKind { Step, FrameEnter, FrameExit, CallScript, Stub, Halt }
+public enum FrameCause { TopScene, CallScript }
+
+/// <summary>An engine diagnostic fact. A <c>readonly struct</c> with a Kind discriminator and a shared
+/// field set — no per-event heap allocation. Only the fields relevant to a Kind are populated; the
+/// static factories are the only intended constructors.</summary>
+public readonly struct TraceEvent
+{
+    public TraceEventKind Kind { get; private init; }
+    public int    Pc     { get; private init; }
+    public int    Opcode { get; private init; }
+    public int    Depth  { get; private init; }
+    public long   Id     { get; private init; }   // call-script id
+    public long   Steps  { get; private init; }   // total steps at Halt
+    public FrameCause Cause { get; private init; }
+    public string? Name  { get; private init; }    // script/scene name; resolved call-script name (null => unresolved/stub)
+    public string? Text  { get; private init; }    // halt reason; frame outcome
+    public Instruction? Ins { get; private init; } // Step: the instruction (args) by ref, never copied
+
+    public static TraceEvent Step(int pc, Instruction ins, int depth) => new()
+        { Kind = TraceEventKind.Step, Pc = pc, Opcode = ins.Opcode, Ins = ins, Depth = depth };
+    public static TraceEvent FrameEnter(string name, int depth, FrameCause cause, long id = 0) => new()
+        { Kind = TraceEventKind.FrameEnter, Name = name, Depth = depth, Cause = cause, Id = id };
+    public static TraceEvent FrameExit(string name, int depth, string outcome) => new()
+        { Kind = TraceEventKind.FrameExit, Name = name, Depth = depth, Text = outcome };
+    public static TraceEvent CallScript(long id, string? name) => new()
+        { Kind = TraceEventKind.CallScript, Id = id, Name = name };
+    public static TraceEvent Stub(int opcode, int pc) => new()
+        { Kind = TraceEventKind.Stub, Opcode = opcode, Pc = pc };
+    public static TraceEvent Halt(string reason, long steps) => new()
+        { Kind = TraceEventKind.Halt, Text = reason, Steps = steps };
+}