Extract VM diagnostic handler

docs/PROJECT-STRUCTURE.md

@@ -214,6 +214,8 @@ execution, script-provider access, and shared lifecycle state remain in `Virtual
 `engine/Age.Engine/Vm/VirtualMachine.Values.cs` owns integer arithmetic, bitwise and comparison operations,
 string comparison/concatenation/conversion/move, native byte-length and CP932 operations, and the host-backed
 fullwidth string editor; shared operand storage, addressing, and native-string encoding remain in the coordinator.
+`engine/Age.Engine/Vm/VirtualMachine.Diagnostics.cs` owns diagnostic value/newline accumulation and synchronous
+show-and-clear opcode dispatch; shared diagnostic state and native-context formatting remain in the coordinator.
 
 The disposable `build/page-map-<SCENE>.jsonl` files are produced by editor/development Godot runs and map
 runtime ADV page ordinals to their authoritative script offsets for `tools/locate_page.py`. Packaged exports

docs/remake-architecture-and-roadmap.md

@@ -728,6 +728,12 @@ do not mix mechanical moves with semantic changes.
    through guarded `StepValue`; shared operand storage, addressing, and native-string encoding remain in the
    coordinator. Runtime validation remains green.
 
+   The seventeenth bounded `VirtualMachine.Step` extraction moved diagnostic operand/newline accumulation and
+   synchronous show-and-clear prompt dispatch into `engine/Age.Engine/Vm/VirtualMachine.Diagnostics.cs`. The
+   top-level dispatcher retains all labels and aliases at their existing positions and passes the complete
+   instruction through guarded `StepDiagnostic`; shared accumulator state and native-context formatting remain
+   in the coordinator. Runtime validation remains green.
+
    **Gate:** no externally visible behavior or command changes; generated artifacts are byte-identical where
    deterministic, and the corresponding engine, Python, Godot, and corpus validations remain green after
    each domain move.
@@ -1101,8 +1107,8 @@ Continue step 2 of the **codebase consolidation** maintenance slice: behavior-ne
 by the tracked launcher and layered validation driver. With the planned `Main`, `GodotAdvHost`, and `GfxState`
 domains isolated and the audio, movie, surface/texture, retained-object, animation, presentation, ADV-text,
 text-history, ADV-service, input, timing, persistence, and memory/collection `VirtualMachine.Step` families routed
-through domain handlers, with control-flow/coroutine, process/root-exit/cross-script lifecycle, and scalar and
-string-value dispatch now isolated as well, extract diagnostic accumulation and prompt dispatch next without
-replacing the proven dispatcher or changing public types, commands, and generated output.
+through domain handlers, with control-flow/coroutine, process/root-exit/cross-script lifecycle, value, and
+diagnostic dispatch now isolated as well, extract message-window and system-menu runtime settings
+next without replacing the proven dispatcher or changing public types, commands, and generated output.
 Concrete playthrough blockers may still preempt this bounded maintenance work; the consolidation effort does
 not replace Phase B gameplay validation or the open cross-platform gates.

engine/Age.Engine/Vm/VirtualMachine.Diagnostics.cs

@@ -0,0 +1,29 @@
+using Age.Engine.Model;
+
+namespace Age.Engine.Vm;
+
+public sealed partial class VirtualMachine
+{
+    private int StepDiagnostic(string label, Instruction instruction, int pc)
+    {
+        IReadOnlyList<Operand> a = instruction.Args;
+        switch (label)
+        {
+            case "u00425790": // upstream ABI label
+            case "append-diagnostic-value": // 0x1b2: generic operand text -> EngineCtx accumulator
+                _diagnosticOutput.Append(FormatDiagnosticOperand(a[0]));
+                return pc + 1;
+            case "u004257D0": // upstream ABI label
+            case "append-diagnostic-newline": // 0x1b3: exact native CRLF bytes
+                _diagnosticOutput.Append("\r\n");
+                return pc + 1;
+            case "u004237C0": // upstream ABI label
+            case "show-and-clear-diagnostic": // 0x1b4: synchronous host prompt, then erase
+                _host.ShowDiagnosticMessage(BuildDiagnosticMessage(instruction));
+                _diagnosticOutput.Clear();
+                return pc + 1;
+            default:
+                throw new InvalidOperationException($"Non-diagnostic opcode routed to diagnostic handler: {label}");
+        }
+    }
+}

engine/Age.Engine/Vm/VirtualMachine.cs

@@ -1244,17 +1244,11 @@ public sealed partial class VirtualMachine
                 return StepValue(label, a, pc);
             case "u00425790": // upstream ABI label
             case "append-diagnostic-value": // 0x1b2: generic operand text -> EngineCtx accumulator
-                _diagnosticOutput.Append(FormatDiagnosticOperand(a[0]));
-                return pc + 1;
             case "u004257D0": // upstream ABI label
             case "append-diagnostic-newline": // 0x1b3: exact native CRLF bytes
-                _diagnosticOutput.Append("\r\n");
-                return pc + 1;
             case "u004237C0": // upstream ABI label
             case "show-and-clear-diagnostic": // 0x1b4: synchronous host prompt, then erase
-                _host.ShowDiagnosticMessage(BuildDiagnosticMessage(ins));
-                _diagnosticOutput.Clear();
-                return pc + 1;
+                return StepDiagnostic(label, ins, pc);
             case "is-catalog-resource-unlocked":
             case "save-numbered-slot": // 0x19e
             case "load-numbered-slot-data-only": // 0x19f