feat(engine): --trace-json emitter for the differential oracle

JsonOffsetTraceSink records every executed instruction offset (bytecode word
index) of one target script, in order, filtered to the scene's own frame
(call-script subroutines excluded) to match the Frida engine tracer's
per-codebase filter. Wired as 'trace <SCENE.BIN> [--boot] --trace-json <out>'.
Observe-only; sweep path and trace parity untouched. +2 xUnit tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

engine/Age.Engine/Diagnostics/JsonOffsetTraceSink.cs

@@ -0,0 +1,30 @@
+using System.Collections.Generic;
+namespace Age.Engine.Diagnostics;
+
+/// <summary>Records the script-relative offset of every instruction executed in one target script, in
+/// execution order — the VM side of the differential offset-path oracle (docs/engine-re.md). Offsets are
+/// the instruction's <see cref="Model.Instruction.Offset"/> (the bytecode WORD index, same unit the engine
+/// tracer emits as <c>(pc-codebase)/4</c>), so the two sequences are directly comparable.
+///
+/// Filters to the target frame: only Step events whose innermost running script is the target are recorded,
+/// so call-script subroutines into OTHER scripts are excluded — matching the engine trace's per-codebase
+/// filter, which isolates the scene from boot/system scripts. Observe-only (emits nothing, touches no VM
+/// state) so trace parity is preserved.</summary>
+public sealed class JsonOffsetTraceSink : TraceSinkBase
+{
+    private readonly string _target;
+    private readonly List<int> _offsets = new();
+
+    public JsonOffsetTraceSink(string targetScript) { _target = targetScript; }
+
+    /// <summary>Executed offsets of the target script, in order.</summary>
+    public IReadOnlyList<int> Offsets => _offsets;
+
+    public override bool TracingSteps => true;
+
+    protected override void OnEvent(in TraceEvent e)
+    {
+        if (e.Kind == TraceEventKind.Step && CurrentScript == _target && e.Ins is { } ins)
+            _offsets.Add(ins.Offset);
+    }
+}