docs: engine diagnostics implementation plan

6 TDD tasks: seam types -> VM emission -> IHost slimming -> Godot rewiring -> CLI --trace -> verify+memory. Parity guarded by default NullTraceSink. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
2026-07-07 15:14:23 -04:00
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+# Engine Diagnostics / Trace Facility Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Give `Age.Engine` a typed, injectable diagnostics seam (`ITraceSink`) that the VM emits engine facts to, and relocate the two diagnostic-only methods (`CallScript`, `OnStub`) off `IHost` onto it.
+
+**Architecture:** The VM emits allocation-free `TraceEvent` structs (Step, FrameEnter, FrameExit, CallScript, Stub, Halt) to an `ITraceSink` supplied at construction; the default `NullTraceSink` makes every existing path byte-identical. One built-in `TextTraceSink` formats events to console/file. Consumers (CLI, Godot, tests) subscribe by supplying a sink instead of reimplementing `IHost`.
+
+**Tech Stack:** C# / .NET 8, xUnit. Solution `engine/AgeEngine.sln`; core `engine/Age.Engine`; CLI `engine/Age.Cli`; tests `engine/Age.Engine.Tests`; Godot project `godot/Himegari.csproj` (.NET, referenced separately).
+
+## Global Constraints
+
+- **Working directory for all commands:** `S:/Game Hacking/Eushully/Himegari/age-reimpl` (the repo root). Branch: `feat/engine-diagnostics` (already created).
+- **Parity is the invariant:** with the default `NullTraceSink`, VM behavior is byte-identical. Existing engine tests (25) and the Godot `--selftest` must stay green. A sink is **observe-only** — it never reads/writes VM state or influences control flow.
+- **Seam rule (unchanged):** `Vm` references only `Model` + `Hosting` + the new `Diagnostics` namespace — never `Sys4`. `Diagnostics` is version-neutral and references no `Sys4`.
+- **No new third-party dependencies.** `Age.Engine` stays dependency-free (Serilog / `EventSource` are deferred to optional future edge sinks, not built here).
+- **Test data is synthesized, never crippled:** build synthetic scenes with `ScriptAssembler`; never disable a feature to keep a golden matching (project principle — see status memory).
+- **Build:** `dotnet build engine/AgeEngine.sln`. **Test:** `dotnet test engine/Age.Engine.Tests/Age.Engine.Tests.csproj`. Filter a class with `--filter FullyQualifiedName~ClassName`.
+- Spec: `docs/superpowers/specs/2026-07-07-engine-diagnostics-design.md`.
+
+---
+
+## File Structure
+
+**New (all in `engine/Age.Engine/Diagnostics/`):**
+- `ITraceSink.cs` — the seam interface (`TracingSteps` gate + `Emit(in TraceEvent)`).
+- `TraceEvent.cs` — `readonly struct` event + `TraceEventKind`/`FrameCause` enums + static factories.
+- `NullTraceSink.cs` — inert default singleton.
+- `TextTraceSink.cs` — the one built-in formatter (console/file).
+
+**New tests:**
+- `engine/Age.Engine.Tests/TraceSinkTests.cs` — type + formatting + VM-emission + gate tests.
+- `RecordingTraceSink` test double — added to `engine/Age.Engine.Tests/TestSupport.cs`.
+
+**New Godot:**
+- `godot/GodotTraceSink.cs` — frontend consumer that queues `CallScript` events for the main thread.
+
+**Modified:**
+- `engine/Age.Engine/Vm/VirtualMachine.cs` — sink field, event emission, `CallScriptDispatches` property.
+- `engine/Age.Engine/Vm/GameSession.cs` — thread the sink through `RunScene`.
+- `engine/Age.Engine/Hosting/IHost.cs`, `Hosting/CaptureHost.cs` — drop `CallScript`/`OnStub`.
+- `engine/Age.Cli/Program.cs` — `--trace` wiring; `run` count from `CallScriptDispatches`; drop 2 methods from `AudioTraceHost`/`GfxTraceHost`.
+- `godot/GodotAdvHost.cs`, `godot/Main.cs` — drop 2 methods + `Dispatched`; wire `GodotTraceSink`.
+- 6 test hosts (`CaptureHost`? no — test hosts: `RecordingHost`, `VoiceCountHost`, `RecHost`, `FakeSizeHost`, and the two `NullHost`s in call-script tests) — drop 2 methods; migrate the 2 call-script assertions.
+- Docs: `docs/tools-reference.md`; status memory + `MEMORY.md`.
+
+---
+
+## Task 1: Diagnostics seam types
+
+Pure data types — no VM changes. Self-contained and fully testable in isolation.
+
+**Files:**
+- Create: `engine/Age.Engine/Diagnostics/ITraceSink.cs`
+- Create: `engine/Age.Engine/Diagnostics/TraceEvent.cs`
+- Create: `engine/Age.Engine/Diagnostics/NullTraceSink.cs`
+- Create: `engine/Age.Engine/Diagnostics/TextTraceSink.cs`
+- Test: `engine/Age.Engine.Tests/TraceSinkTests.cs`
+
+**Interfaces:**
+- Produces:
+  - `interface ITraceSink { bool TracingSteps { get; } void Emit(in TraceEvent e); }`
+  - `enum TraceEventKind { Step, FrameEnter, FrameExit, CallScript, Stub, Halt }`
+  - `enum FrameCause { TopScene, CallScript }`
+  - `readonly struct TraceEvent` with fields `Kind, Pc, Opcode, Depth, Id, Steps, Cause, Name?, Text?, Ins?` and factories `Step(int pc, Instruction ins, int depth)`, `FrameEnter(string name, int depth, FrameCause cause, long id = 0)`, `FrameExit(string name, int depth, string outcome)`, `CallScript(long id, string? name)`, `Stub(int opcode, int pc)`, `Halt(string reason, long steps)`. **`resolved` is implied by `Name != null`.**
+  - `sealed class NullTraceSink { static readonly NullTraceSink Instance; }`
+  - `sealed class TextTraceSink(TextWriter writer, OpcodeTable? table = null, bool includeSteps = false)`
+
+- [ ] **Step 1: Write the seam interface**
+
+Create `engine/Age.Engine/Diagnostics/ITraceSink.cs`:
+
+```csharp
+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);
+}
+```
+
+- [ ] **Step 2: Write the event struct**
+
+Create `engine/Age.Engine/Diagnostics/TraceEvent.cs`:
+
+```csharp
+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 };
+}
+```
+
+- [ ] **Step 3: Write the null and text sinks**
+
+Create `engine/Age.Engine/Diagnostics/NullTraceSink.cs`:
+
+```csharp
+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) { }
+}
+```
+
+Create `engine/Age.Engine/Diagnostics/TextTraceSink.cs`:
+
+```csharp
+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}"));
+}
+```
+
+- [ ] **Step 4: Write the failing tests**
+
+Create `engine/Age.Engine.Tests/TraceSinkTests.cs`:
+
+```csharp
+using System.IO;
+using Age.Engine.Diagnostics;
+using Age.Engine.Model;
+using Xunit;
+
+public class TraceSinkTests
+{
+    [Fact]
+    public void FactoriesSetKindAndFields()
+    {
+        var ins = new Instruction(0x40, 0x55, new[] { new Operand(3, 0x10), new Operand(0, 7) });
+        var step = TraceEvent.Step(0x40, ins, 2);
+        Assert.Equal(TraceEventKind.Step, step.Kind);
+        Assert.Equal(0x55, step.Opcode);
+        Assert.Same(ins, step.Ins);
+        Assert.Equal(2, step.Depth);
+
+        var cs = TraceEvent.CallScript(0x1ab, "ADDITEM");
+        Assert.Equal(TraceEventKind.CallScript, cs.Kind);
+        Assert.Equal(0x1abL, cs.Id);
+        Assert.Equal("ADDITEM", cs.Name);
+    }
+
+    [Fact]
+    public void NullSinkIsInertAndNotTracingSteps()
+    {
+        Assert.False(NullTraceSink.Instance.TracingSteps);
+        NullTraceSink.Instance.Emit(TraceEvent.Halt("x", 1));   // must not throw
+    }
+
+    [Fact]
+    public void TextSinkFormatsEachKind()
+    {
+        var sw = new StringWriter();
+        var sink = new TextTraceSink(sw, table: null, includeSteps: true);
+        sink.Emit(TraceEvent.FrameEnter("SC0000", 1, FrameCause.TopScene));
+        sink.Emit(TraceEvent.CallScript(0x1ab, "ADDITEM"));
+        sink.Emit(TraceEvent.Halt("exit", 27994));
+        var outp = sw.ToString();
+        Assert.Contains("» SC0000 (enter, TopScene)", outp);
+        Assert.Contains("call-script 0x1ab =ADDITEM (resolved)", outp);
+        Assert.Contains("halt: exit @ 27994 steps", outp);
+    }
+}
+```
+
+- [ ] **Step 5: Run the tests — verify they pass**
+
+Run: `dotnet test engine/Age.Engine.Tests/Age.Engine.Tests.csproj --filter FullyQualifiedName~TraceSinkTests`
+Expected: 3 passed. (Types compile; no VM touched yet.)
+
+- [ ] **Step 6: Commit**
+
+```bash
+git add engine/Age.Engine/Diagnostics engine/Age.Engine.Tests/TraceSinkTests.cs
+git commit -m "feat(diagnostics): ITraceSink seam + TraceEvent + Null/Text sinks
+
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>"
+```
+
+---
+
+## Task 2: Emit trace events from the VM
+
+Wire the sink into the VM, emit the six event kinds, add the `CallScriptDispatches` stat, thread the sink through `GameSession.RunScene`. Migrate the two call-script tests that asserted on the (now sink-fed) `CallScript` notification. Parity holds because the default is `NullTraceSink` and the sink is observe-only.
+
+**Files:**
+- Modify: `engine/Age.Engine/Vm/VirtualMachine.cs`
+- Modify: `engine/Age.Engine/Vm/GameSession.cs:27-41`
+- Modify: `engine/Age.Engine.Tests/TestSupport.cs` (add `RecordingTraceSink`)
+- Modify: `engine/Age.Engine.Tests/CallScriptTests.cs:60-79`
+- Modify: `engine/Age.Engine.Tests/CallScriptIntegrationTests.cs:31-35`
+- Test: `engine/Age.Engine.Tests/TraceSinkTests.cs` (add VM-emission + gate tests)
+
+**Interfaces:**
+- Consumes: `ITraceSink`, `TraceEvent`, `NullTraceSink`, `TraceEventKind`, `FrameCause` (Task 1).
+- Produces:
+  - VM constructor gains a 6th optional param `ITraceSink? sink = null`.
+  - `VirtualMachine.CallScriptDispatches` (`long`, get) — count of executed `call-script` opcodes.
+  - `GameSession.RunScene(..., ITraceSink? sink = null)` — 6th optional param, forwarded to the VM.
+  - `RecordingTraceSink` test double: `{ bool TracingSteps init; List<TraceEvent> Events; List<long> CallScriptIds; }`.
+
+- [ ] **Step 1: Write the failing VM-emission and gate tests**
+
+Add to `engine/Age.Engine.Tests/TraceSinkTests.cs` (inside the class; add `using System;`, `using System.Collections.Generic;`, `using System.Linq;`, `using Age.Engine.Sys4;`, `using Age.Engine.Vm;` at the top):
+
+```csharp
+    private static OpcodeTable Table() => OpcodeTableJson.Load(Paths.OpcodesJson);
+
+    [Fact]
+    public void VmEmitsFrameCallScriptAndHaltEvents()
+    {
+        var t = Table();
+        // callee: exit.  caller: call-script 5 ; exit.
+        var callee = ScriptAssembler.Assemble(t, "CALLEE",
+            new List<(int, Operand[])> { (0x2, Array.Empty<Operand>()) }, Array.Empty<string>());
+        var caller = ScriptAssembler.Assemble(t, "CALLER",
+            new List<(int, Operand[])> { (0x3, new[] { new Operand(0, 5) }), (0x2, Array.Empty<Operand>()) },
+            Array.Empty<string>());
+        var sink = new RecordingTraceSink();
+        var vm = new VirtualMachine(caller, t, new RecordingHost(), null,
+                                    new MapProvider(new() { [5] = callee }), sink);
+        vm.Run();
+
+        var kinds = sink.Events.Select(e => e.Kind).ToList();
+        Assert.Equal(TraceEventKind.FrameEnter, kinds[0]);                 // caller enters first
+        Assert.Equal(TraceEventKind.Halt, kinds[^1]);                     // halt is last
+        Assert.Equal(2, sink.Events.Count(e => e.Kind == TraceEventKind.FrameEnter));  // caller + callee
+        Assert.Equal(2, sink.Events.Count(e => e.Kind == TraceEventKind.FrameExit));
+        Assert.Contains(5L, sink.CallScriptIds);
+        Assert.Equal(1, vm.CallScriptDispatches);
+    }
+
+    [Fact]
+    public void StepEventsGatedByTracingSteps()
+    {
+        var t = Table();
+        // mov g[0x10]=7 ; exit  => 2 executed instructions.
+        var body = new List<(int, Operand[])>
+        {
+            (0x55, new[] { new Operand(3, 0x10), new Operand(0, 7) }),
+            (0x2, Array.Empty<Operand>()),
+        };
+        var s = ScriptAssembler.Assemble(t, "S", body, Array.Empty<string>());
+
+        var off = new RecordingTraceSink { TracingSteps = false };
+        new VirtualMachine(s, t, new RecordingHost(), null, null, off).Run();
+        Assert.Empty(off.Events.Where(e => e.Kind == TraceEventKind.Step));
+
+        var on = new RecordingTraceSink { TracingSteps = true };
+        new VirtualMachine(s, t, new RecordingHost(), null, null, on).Run();
+        Assert.Equal(2, on.Events.Count(e => e.Kind == TraceEventKind.Step));
+    }
+```
+
+Add the `RecordingTraceSink` double to `engine/Age.Engine.Tests/TestSupport.cs` (append; add `using System.Linq;` and `using Age.Engine.Diagnostics;` at the top):
+
+```csharp
+/// <summary>Captures every trace event for assertions; TracingSteps is settable so a test can
+/// exercise the Step gate both ways.</summary>
+internal sealed class RecordingTraceSink : ITraceSink
+{
+    public bool TracingSteps { get; init; }
+    public readonly List<TraceEvent> Events = new();
+    public void Emit(in TraceEvent e) => Events.Add(e);
+    public List<long> CallScriptIds =>
+        Events.Where(e => e.Kind == TraceEventKind.CallScript).Select(e => e.Id).ToList();
+}
+```
+
+- [ ] **Step 2: Run the new tests — verify they fail to compile**
+
+Run: `dotnet test engine/Age.Engine.Tests/Age.Engine.Tests.csproj --filter FullyQualifiedName~TraceSinkTests`
+Expected: BUILD FAIL — `VirtualMachine` has no 6-arg constructor; `CallScriptDispatches` undefined.
+
+- [ ] **Step 3: Add the sink + stat to the VM and emit events**
+
+In `engine/Age.Engine/Vm/VirtualMachine.cs`:
+
+Add `using Age.Engine.Diagnostics;` at the top (with the other usings).
+
+Add the field + property near the other fields (after `private int _depth;`, line 19):
+
+```csharp
+    private readonly ITraceSink _sink;
+    public long CallScriptDispatches { get; private set; }
+```
+
+Change the constructor (line 27-28) to accept the sink:
+
+```csharp
+    public VirtualMachine(Script s, OpcodeTable t, IHost host, VmOptions? o = null,
+                          IScriptProvider? provider = null, ITraceSink? sink = null)
+    { _s = s; _t = t; _host = host; _o = o ?? new VmOptions(); _provider = provider;
+      _sink = sink ?? NullTraceSink.Instance; }
+```
+
+Change `Run` (line 97-104) to pass the frame cause and emit the terminal Halt:
+
+```csharp
+    public void Run(int entryOffset = 0)
+    {
+        var top = new ExecFrame(_s, _s.IndexByOffset.TryGetValue(entryOffset, out var idx) ? idx : 0);
+        var outcome = RunFrame(top, FrameCause.TopScene);
+        if (outcome == FrameOutcome.RanOff) HaltReason ??= "pc-out-of-range";
+        else if (outcome == FrameOutcome.Returned) HaltReason ??= "exit";
+        // Halted: HaltReason already set by the halting op.
+        _sink.Emit(TraceEvent.Halt(HaltReason ?? "unknown", Steps));
+    }
+```
+
+Change `RunFrame` (line 106-122) to take the cause, emit FrameEnter/FrameExit, and emit gated Step events:
+
+```csharp
+    private FrameOutcome RunFrame(ExecFrame frame, FrameCause cause, long callId = 0)
+    {
+        var prev = _cur; _cur = frame; _depth++;
+        _sink.Emit(TraceEvent.FrameEnter(frame.Script.Name, _depth, cause, callId));
+        var outcome = FrameOutcome.RanOff;
+        int pc = frame.Pc;
+        while (pc >= 0 && pc < frame.Script.Instructions.Count)
+        {
+            if (Steps >= _o.MaxSteps) { HaltReason ??= "STEP-LIMIT"; outcome = FrameOutcome.Halted; break; }
+            Steps++;
+            if (_sink.TracingSteps) _sink.Emit(TraceEvent.Step(pc, frame.Script.Instructions[pc], _depth));
+            int next = Step(frame.Script.Instructions[pc], pc);
+            if (next == FRAME_RETURN) { outcome = FrameOutcome.Returned; break; }
+            if (next == HALT) { outcome = FrameOutcome.Halted; break; }
+            pc = next;
+        }
+        _sink.Emit(TraceEvent.FrameExit(frame.Script.Name, _depth, outcome.ToString()));
+        _cur = prev; _depth--;
+        return outcome;
+    }
+```
+
+Change the `call-script` case (line 170-182) to count, emit the CallScript event, and pass the cause to the child frame (replaces `_host.CallScript(id)`):
+
+```csharp
+            case "call-script":
+            {
+                long id = a.Count > 0 ? Read(a[0]) : 0;
+                CallScriptDispatches++;
+                if (_provider == null)
+                {
+                    _sink.Emit(TraceEvent.CallScript(id, null));   // stub mode: notify only, no child pushed
+                    return pc + 1;
+                }
+                if (_depth >= _o.CallDepthCap) { HaltReason ??= "call-depth-exceeded"; return HALT; }
+                var child = _provider.GetById(id);
+                _sink.Emit(TraceEvent.CallScript(id, child?.Name));
+                if (child == null) { HaltReason ??= $"callscript-unresolved:0x{id:x}"; return HALT; }
+                var entry = child.IndexByOffset.TryGetValue(0, out var ci) ? ci : 0;
+                var outcome = RunFrame(new ExecFrame(child, entry), FrameCause.CallScript, id);
+                if (outcome == FrameOutcome.Halted) return HALT;   // propagate whole-VM halt up
+                return pc + 1;                                      // Returned / RanOff: resume caller
+            }
+```
+
+Change the `default` stub arm (line 214-215) to emit a Stub event (replaces `_host.OnStub(op)`):
+
+```csharp
+            default:
+                _sink.Emit(TraceEvent.Stub(op, pc)); return pc + 1;
+```
+
+- [ ] **Step 4: Thread the sink through GameSession**
+
+In `engine/Age.Engine/Vm/GameSession.cs`, add `using Age.Engine.Diagnostics;` at the top, then change `RunScene` (line 27-41) to accept and forward the sink:
+
+```csharp
+    public SceneResult RunScene(Script script, OpcodeTable table, IHost host,
+                                VmOptions? options = null, IScriptProvider? provider = null,
+                                ITraceSink? sink = null)
+    {
+        var vm = new VirtualMachine(script, table, host, options, provider, sink);
+        foreach (var kv in Globals) vm.Globals[kv.Key] = kv.Value;
+        foreach (var kv in GlobalStrings) vm.GlobalStrings[kv.Key] = kv.Value;
+
+        vm.Run();
+
+        // Globals are one flat space; last write wins — the engine's single global bank.
+        foreach (var kv in vm.Globals) Globals[kv.Key] = kv.Value;
+        foreach (var kv in vm.GlobalStrings) GlobalStrings[kv.Key] = kv.Value;
+
+        return new SceneResult(vm.Emitted.ToList(), vm.HaltReason, vm.Steps);
+    }
+```
+
+- [ ] **Step 5: Migrate the two call-script assertions off the host notification**
+
+The VM no longer calls `IHost.CallScript`, so tests that asserted on it must read the sink / property instead.
+
+In `engine/Age.Engine.Tests/CallScriptTests.cs`, `CalleeRunsAndControlResumesAfterTheCall` (line 60-67) — add a recording sink and assert on it:
+
+```csharp
+        var host = new NullHost();
+        var sink = new RecordingTraceSink();
+        var vm = new VirtualMachine(caller, t, host, null, new MapProvider(new() { [5] = callee }), sink);
+        vm.Run();
+        Assert.Equal(7, vm.Globals[0x10]);           // callee wrote a shared global
+        Assert.Equal(7, vm.Globals[0x11]);           // caller read it AFTER the call returned
+        Assert.Equal("exit", vm.HaltReason);         // top-level exit
+        Assert.Contains(5L, sink.CallScriptIds);     // dispatch observed via the trace sink
+```
+
+And `MissingProviderFallsBackToStub` (line 69-79):
+
+```csharp
+        var host = new NullHost();
+        var sink = new RecordingTraceSink();
+        var vm = new VirtualMachine(caller, t, host, null, null, sink);   // no provider
+        vm.Run();
+        Assert.Equal("exit", vm.HaltReason);        // did not halt on the call; stub + continue
+        Assert.Contains(5L, sink.CallScriptIds);
+```
+
+In `engine/Age.Engine.Tests/CallScriptIntegrationTests.cs`, `RealScriptExecutesRealSubroutinesAndReturns` (line 31-35) — read the VM stat:
+
+```csharp
+        var host = new NullHost();
+        var vm = new VirtualMachine(script, t, host, null, provider);
+        vm.Run();
+        Assert.Equal(2, vm.CallScriptDispatches); // ADDILLSUB + CALCREVISE both dispatched
+        Assert.Equal("exit", vm.HaltReason);      // subroutines returned; ADDILL reached its own exit
+```
+
+(The `NullHost.CallScript`/`OnStub` methods in these files stay for now — they're removed in Task 3.)
+
+- [ ] **Step 6: Run the full suite — verify green and parity held**
+
+Run: `dotnet test engine/Age.Engine.Tests/Age.Engine.Tests.csproj`
+Expected: all pass (existing 25 + the new TraceSink tests). The parity-critical tests (`RecoverTests`, `SyntheticSceneTests`, `WaitForInputTests`) use the default null sink and are unaffected.
+
+- [ ] **Step 7: Commit**
+
+```bash
+git add engine/Age.Engine/Vm/VirtualMachine.cs engine/Age.Engine/Vm/GameSession.cs \
+        engine/Age.Engine.Tests/TestSupport.cs engine/Age.Engine.Tests/TraceSinkTests.cs \
+        engine/Age.Engine.Tests/CallScriptTests.cs engine/Age.Engine.Tests/CallScriptIntegrationTests.cs
+git commit -m "feat(diagnostics): VM emits trace events + CallScriptDispatches stat
+
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>"
+```
+
+---
+
+## Task 3: Slim IHost — drop CallScript/OnStub
+
+Remove the two diagnostic-only methods from `IHost` and all engine/CLI/test implementers. The VM no longer calls them (Task 2), so this is a pure deletion — except `CaptureHost.CallScriptCount`, whose one consumer (CLI `run`) moves to `vm.CallScriptDispatches`. Godot is handled separately in Task 4.
+
+**Files:**
+- Modify: `engine/Age.Engine/Hosting/IHost.cs:5-6`
+- Modify: `engine/Age.Engine/Hosting/CaptureHost.cs`
+- Modify: `engine/Age.Cli/Program.cs:20` (count via property), `:216-217`, `:266-267` (drop 2 methods each)
+- Modify: `engine/Age.Engine.Tests/TestSupport.cs` (RecordingHost), `GameSessionTests.cs`, `TextureOpsTests.cs`, `TextureGeometryTests.cs`, `CallScriptTests.cs`, `CallScriptIntegrationTests.cs` (drop 2 methods each)
+
+**Interfaces:**
+- Consumes: `VirtualMachine.CallScriptDispatches` (Task 2).
+- Produces: `IHost` with 9 methods (no `CallScript`, no `OnStub`).
+
+- [ ] **Step 1: Remove the two methods from IHost**
+
+Replace `engine/Age.Engine/Hosting/IHost.cs` with:
+
+```csharp
+namespace Age.Engine.Hosting;
+public interface IHost
+{
+    void ShowText(int offset, string text);
+    void WaitForInput();
+    void CreateTexture(int slot, int width, int height);
+    void SetTexture(long resourceId, int slot);
+    void DrawTexture(int slot, int srcX, int srcY, int width, int height, int dstX, int dstY);
+    (int Width, int Height) GetTextureSize(int slot);
+    void PlayBgm(long id);
+    void PlayVoice(long id);
+}
+```
+
+- [ ] **Step 2: Run the build — confirm current state (it still compiles)**
+
+Run: `dotnet build engine/AgeEngine.sln`
+Expected: BUILD SUCCEEDS. Removing the two methods from `IHost` does **not** break compilation — the implementers keep them as ordinary (no-longer-interface) public methods, and `CaptureHost.CallScriptCount`/`.Stubs` still exist, so `Program.cs:20` still resolves. This step just confirms the baseline; Steps 3–5 delete the now-dead members (they are dead because the VM stopped calling them in Task 2). This task is a no-behavior-change refactor — the suite is already green from Task 2 and must stay green.
+
+- [ ] **Step 3: Slim CaptureHost**
+
+Replace `engine/Age.Engine/Hosting/CaptureHost.cs` with:
+
+```csharp
+namespace Age.Engine.Hosting;
+public sealed class CaptureHost : IHost
+{
+    public List<(int Offset, string Text)> Emitted { get; } = new();
+    public void ShowText(int offset, string text) => Emitted.Add((offset, text));
+    public void WaitForInput() { }
+    public void CreateTexture(int slot, int width, int height) { }
+    public void SetTexture(long resourceId, int slot) { }
+    public void DrawTexture(int slot, int srcX, int srcY, int width, int height, int dstX, int dstY) { }
+    public (int Width, int Height) GetTextureSize(int slot) => (0, 0);
+    public void PlayBgm(long id) { }
+    public void PlayVoice(long id) { }
+}
+```
+
+- [ ] **Step 4: Fix the CLI `run` count and drop the trace hosts' dead methods**
+
+In `engine/Age.Cli/Program.cs` line 20, change `runHost.CallScriptCount` to `vm.CallScriptDispatches`:
+
+```csharp
+    Console.WriteLine($"{Path.GetFileName(args[1])}: {vm.Steps} steps, {vm.Emitted.Count} show-text, {vm.CallScriptDispatches} call-scripts (halt: {vm.HaltReason})");
+```
+
+Delete these two lines from `AudioTraceHost` (was 216-217):
+
+```csharp
+    public void CallScript(long id) { }
+    public void OnStub(int opcode) { }
+```
+
+Delete the identical two lines from `GfxTraceHost` (was 266-267).
+
+- [ ] **Step 5: Drop the two methods from every test host**
+
+Delete the `CallScript(...)` and `OnStub(...)` lines from each of these hosts:
+- `engine/Age.Engine.Tests/TestSupport.cs` `RecordingHost` (lines 12-13) — also delete the now-unused `CallScripts` field from line 9 (`public int Waits, CallScripts;` → `public int Waits;`). `Waits` is still asserted; `CallScripts` is not.
+- `engine/Age.Engine.Tests/GameSessionTests.cs` `VoiceCountHost` (lines 31-32).
+- `engine/Age.Engine.Tests/TextureOpsTests.cs` `RecHost` (lines 15-16).
+- `engine/Age.Engine.Tests/TextureGeometryTests.cs` `FakeSizeHost` (lines 13-14).
+- `engine/Age.Engine.Tests/CallScriptTests.cs` `NullHost` — delete `CallScript`/`OnStub` (lines 18-19) and the `Calls` field (line 17: `public List<long> Calls = new();`), now that assertions read the sink.
+- `engine/Age.Engine.Tests/CallScriptIntegrationTests.cs` `NullHost` — delete `CallScript`/`OnStub` (lines 12-13) and the `CallScripts` field (line 10: `public int CallScripts;`), now that the assertion reads `vm.CallScriptDispatches`.
+
+- [ ] **Step 6: Build and run the full suite**
+
+Run: `dotnet test engine/Age.Engine.Tests/Age.Engine.Tests.csproj`
+Expected: build clean, all tests pass. `IHost` now carries only render/audio/input methods.
+
+- [ ] **Step 7: Commit**
+
+```bash
+git add engine/Age.Engine/Hosting/IHost.cs engine/Age.Engine/Hosting/CaptureHost.cs \
+        engine/Age.Cli/Program.cs engine/Age.Engine.Tests
+git commit -m "refactor(hosting): drop CallScript/OnStub from IHost (now trace events)
+
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>"
+```
+
+---
+
+## Task 4: Retire the Godot dispatch hack onto a sink
+
+Replace `GodotAdvHost`'s `Dispatched` queue (the thread-hack the memory calls out) with a `GodotTraceSink`. `Main` reports subroutines from the sink; `GodotAdvHost` sheds the two dead `IHost` methods. Verified by compiling the Godot project (it isn't in the xUnit suite).
+
+**Files:**
+- Create: `godot/GodotTraceSink.cs`
+- Modify: `godot/GodotAdvHost.cs:45-49` (drop `Dispatched` + 2 methods)
+- Modify: `godot/Main.cs:104-105` (wire sink), `:206` (read sink)
+
+**Interfaces:**
+- Consumes: `ITraceSink`, `TraceEvent`, `TraceEventKind` (Task 1); the VM's 6-arg constructor (Task 2).
+- Produces: `GodotTraceSink { ConcurrentQueue<long> CallScripts }`.
+
+- [ ] **Step 1: Create the Godot trace sink**
+
+Create `godot/GodotTraceSink.cs`:
+
+```csharp
+using System.Collections.Concurrent;
+using Age.Engine.Diagnostics;
+
+// Frontend-side trace consumer. Runs on the VM background thread, so it just queues the dispatched
+// call-script ids; the main thread drains them (Godot drops GD.Print from background threads). This
+// replaces the old IHost.CallScript -> GodotAdvHost.Dispatched hack: subroutine visibility is now an
+// engine fact delivered over the trace seam.
+public sealed class GodotTraceSink : ITraceSink
+{
+    public bool TracingSteps => false;
+    public readonly ConcurrentQueue<long> CallScripts = new();
+    public void Emit(in TraceEvent e)
+    {
+        if (e.Kind == TraceEventKind.CallScript) CallScripts.Enqueue(e.Id);
+    }
+}
+```
+
+- [ ] **Step 2: Drop the hack from GodotAdvHost**
+
+In `godot/GodotAdvHost.cs`, delete lines 45-49 (the comment block, `Dispatched` field, `CallScript`, `OnStub`):
+
+```csharp
+    // Records each call-script the VM dispatches (runs on the VM thread, so collect thread-safely and
+    // let the main thread report it — Godot drops GD.Print from background threads).
+    public readonly System.Collections.Concurrent.ConcurrentQueue<long> Dispatched = new();
+    public void CallScript(long id) => Dispatched.Enqueue(id);
+    public void OnStub(int opcode) { }
+```
+
+- [ ] **Step 3: Wire the sink in Main**
+
+In `godot/Main.cs`, add a field near the other VM fields (e.g. beside `_host`):
+
+```csharp
+    private GodotTraceSink _trace = null!;
+```
+
+Change the VM construction (lines 104-105) to create and pass the sink:
+
+```csharp
+        _host = new GodotAdvHost(this, ResourceMap.Load(), scene);
+        _trace = new GodotTraceSink();
+        _vm = new VirtualMachine(script, table, _host, null, provider, _trace);
+```
+
+Change `ReportSubroutines` (line 206) to drain the sink instead of the host:
+
+```csharp
+        while (_trace.CallScripts.TryDequeue(out var id)) ids.Add(id);
+```
+
+- [ ] **Step 4: Build the Godot project — verify it compiles**
+
+Run: `dotnet build godot/Himegari.csproj`
+Expected: build succeeds. (`RunSelfTest`'s headless VM at Main.cs:217 uses a `CaptureHost` with no sink — unaffected; parity of the selftest is preserved.)
+
+- [ ] **Step 5: Commit**
+
+```bash
+git add godot/GodotTraceSink.cs godot/GodotAdvHost.cs godot/Main.cs
+git commit -m "refactor(godot): report subroutines via GodotTraceSink, not the IHost queue
+
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>"
+```
+
+---
+
+## Task 5: CLI `--trace` wiring
+
+Let `run`/`play`/`sweep` attach a `TextTraceSink` to console or file. Absent flag ⇒ `NullTraceSink`.
+
+**Files:**
+- Modify: `engine/Age.Cli/Program.cs` (add `BuildSink` local function; pass sink in `run`/`play`/`sweep`)
+- Modify: `docs/tools-reference.md` (document the flag)
+
+**Interfaces:**
+- Consumes: `TextTraceSink`, `NullTraceSink`, `ITraceSink` (Task 1); `GameSession.RunScene(..., sink)` (Task 2).
+
+- [ ] **Step 1: Add the sink-builder helper**
+
+In `engine/Age.Cli/Program.cs`, add `using Age.Engine.Diagnostics;` at the top, then add this local function just after the `provider` line (line 10):
+
+```csharp
+// --trace [--trace-file <path>] [--trace-steps] → a TextTraceSink to console or file; else inert.
+static ITraceSink BuildSink(string[] a, OpcodeTable tbl)
+{
+    if (!a.Contains("--trace")) return NullTraceSink.Instance;
+    bool steps = a.Contains("--trace-steps");
+    int fi = Array.IndexOf(a, "--trace-file");
+    TextWriter w = (fi >= 0 && fi + 1 < a.Length)
+        ? new StreamWriter(a[fi + 1]) { AutoFlush = true }
+        : Console.Out;
+    return new TextTraceSink(w, tbl, steps);
+}
+```
+
+- [ ] **Step 2: Pass the sink in `run`**
+
+In the `run` block, change the VM construction (line 18-19) to:
+
+```csharp
+    var runHost = new CaptureHost();
+    var vm = new VirtualMachine(script, table, runHost, null, provider, BuildSink(args, table));
+```
+
+- [ ] **Step 3: Pass the sink in `play` and `sweep`**
+
+In `play`, change the scene run (line 103) to pass the sink:
+
+```csharp
+        var r = session.RunScene(script, table, new CaptureHost(), null, provider, BuildSink(args, table));
+```
+
+In `sweep`, the non-seeded path (line 168) similarly:
+
+```csharp
+        var r = session.RunScene(Sys4Loader.Load(scripts[name], table), table, new CaptureHost(), null, provider, BuildSink(args, table));
+```
+
+(Leave the seeded story-explorer path and the `--boot` baseline runs on the default null sink — they run each scene twice for diffing and aren't a tracing target.)
+
+- [ ] **Step 4: Build and smoke-test the flag**
+
+Run: `dotnet build engine/AgeEngine.sln`
+Expected: build succeeds.
+
+Run: `dotnet run --project engine/Age.Cli -- run SC0000.BIN --trace`
+Expected: the normal summary line, preceded by trace lines — a `» SC0000 (enter, TopScene)`, `call-script 0x… =… (resolved)` lines for dispatched subroutines, and a final `halt: … @ … steps`. (Without `--trace-steps`, no per-instruction lines.)
+
+Run: `dotnet run --project engine/Age.Cli -- run SC0000.BIN` (no flag)
+Expected: identical summary line, no trace lines (null sink).
+
+- [ ] **Step 5: Document the flag**
+
+In `docs/tools-reference.md`, under the `Age.Cli` entry, add a line documenting the shared flag:
+
+```
+- `--trace [--trace-file <path>] [--trace-steps]` (on `run`/`play`/`sweep`): stream the engine's
+  diagnostic events (scene enter/exit, call-script dispatch, halts; per-instruction opcodes with
+  `--trace-steps`) to console or a file via the Age.Engine ITraceSink facility. Absent ⇒ no tracing.
+```
+
+- [ ] **Step 6: Commit**
+
+```bash
+git add engine/Age.Cli/Program.cs docs/tools-reference.md
+git commit -m "feat(cli): --trace flag streams engine diagnostics (run/play/sweep)
+
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>"
+```
+
+---
+
+## Task 6: Final verification + status memory
+
+**Files:**
+- Modify: status memory `himegari-port-status.md` + `MEMORY.md` (in `~/.claude/…/memory/`).
+
+- [ ] **Step 1: Full clean build + test**
+
+Run: `dotnet build engine/AgeEngine.sln && dotnet test engine/Age.Engine.Tests/Age.Engine.Tests.csproj`
+Expected: build clean; all tests pass (25 existing + new TraceSink tests). Confirm the count grew only by the added tests and none regressed.
+
+- [ ] **Step 2: Confirm Godot selftest parity (if a Godot runtime is available)**
+
+Run: `dotnet build godot/Himegari.csproj`
+Expected: compiles. If a Godot binary is on hand, `godot --headless --path godot -- --selftest` should still print `SELFTEST OK` (the selftest VM uses the default null sink — parity preserved). If no Godot runtime is available, note that and rely on the compile check.
+
+- [ ] **Step 3: Update the status memory**
+
+Append a completion note to `himegari-port-status.md` (the "ENGINE-LEVEL DIAGNOSTICS" section) and its `MEMORY.md` one-liner: the diagnostics facility is landed — typed `ITraceSink`/`TraceEvent` in `Age.Engine.Diagnostics`, six v1 events, `CallScript`/`OnStub` relocated off `IHost`, `Null`+`Text` sinks, CLI `--trace`, Godot hack retired onto `GodotTraceSink`, parity held. Note the deferred edge-sink adapters (`SerilogTraceSink`, `EventSourceTraceSink`) and future consumers (JSON artifact, divergence differ). Use absolute dates.
+
+- [ ] **Step 4: Commit**
+
+```bash
+git add -A
+git commit -m "docs: record engine diagnostics facility landed
+
+Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>"
+```
+
+---
+
+## Notes for the implementer
+
+- **Line numbers** cite the files as they stand at plan time; if a prior task shifted them, match on the surrounding code shown in each step rather than the number.
+- **`ScriptAssembler.Assemble(table, name, List<(int opcode, Operand[] args)>, string[] strings)`** builds a `Script` through the real loader — use it for synthetic scenes (opcodes: exit=0x2, call-script=0x3 argc1, mov=0x55 argc2; operand types: imm=0, global-int=3, local-int=9).
+- **Why `CallScript`'s `resolved` isn't a field:** it is exactly `Name != null` (a resolved child always has a non-null `Script.Name`; stub-mode and unresolved both leave it null). The `Halt` event still distinguishes the unresolved case via its `callscript-unresolved:` reason.
+- **Parity guard:** if any existing test's offsets/steps/halt change, a sink is doing more than observing — revert to observe-only. The default-null-sink paths must never differ from pre-change behavior.