feat: add IHost.FrameYield per-opcode hook (no-op headless, parity held)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine/Hosting/CaptureHost.cs

@@ -5,6 +5,7 @@ public sealed class CaptureHost : IHost
     public void ShowText(int offset, string text) => Emitted.Add((offset, text));
     public void WaitForInput() { }
     public void Sleep(long duration) { }
+    public void FrameYield() { }
     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) { }

engine/Age.Engine/Hosting/IHost.cs

@@ -4,6 +4,7 @@ public interface IHost
     void ShowText(int offset, string text);
     void WaitForInput();
     void Sleep(long duration);
+    void FrameYield();
     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);

engine/Age.Engine/Vm/VirtualMachine.cs

@@ -122,6 +122,7 @@ public sealed class VirtualMachine
             Steps++;
             if (_sink.TracingSteps) _sink.Emit(TraceEvent.Step(pc, frame.Script.Instructions[pc], _depth));
             int next = Step(frame.Script.Instructions[pc], pc);
+            _host.FrameYield();
             if (next == FRAME_RETURN) { outcome = FrameOutcome.Returned; break; }
             if (next == HALT) { outcome = FrameOutcome.Halted; break; }
             pc = next;