feat(a2a): add IHost.WaitForInput hook (wait-for-input 0x72); trace parity preserved

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

engine/Age.Engine/Hosting/CaptureHost.cs

@@ -7,4 +7,5 @@ public sealed class CaptureHost : IHost
     public void ShowText(int offset, string text) => Emitted.Add((offset, text));
     public void CallScript(long id) => CallScriptCount++;
     public void OnStub(int opcode) { Stubs.TryGetValue(opcode, out var c); Stubs[opcode] = c + 1; }
+    public void WaitForInput() { }
 }