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.Tests/WaitForInputTests.cs

@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+using Age.Engine.Hosting;
+using Age.Engine.Sys4;
+using Age.Engine.Vm;
+using Xunit;
+
+public class WaitForInputTests
+{
+    private sealed class CountHost : IHost
+    {
+        public int Waits;
+        public List<int> Emitted = new();
+        public void ShowText(int offset, string text) => Emitted.Add(offset);
+        public void CallScript(long id) { }
+        public void OnStub(int opcode) { }
+        public void WaitForInput() => Waits++;
+    }
+
+    [Fact]
+    public void WaitForInputFiresAndEmittedIsStable()
+    {
+        var table = OpcodeTableJson.Load(Paths.OpcodesJson);
+        var script = Sys4Loader.Load(Paths.Scripts()["SC0000.BIN"], table);
+        var host = new CountHost();
+        var vm = new VirtualMachine(script, table, host);
+        vm.Run();
+        Assert.True(host.Waits > 0, "wait-for-input (0x72) should fire at least once");
+        Assert.Equal(186, host.Emitted.Count);   // unchanged vs the A1 SC0000 trace
+        Assert.Equal("exit", vm.HaltReason);
+    }
+}