feat(a2a): worker-thread ADV host + blocking wait-for-input + headless self-test

SELFTEST OK: 186 lines match vm0 trace; real SemaphoreSlim gate + CallDeferred exercised headless.

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

godot/GodotAdvHost.cs

@@ -0,0 +1,34 @@
+using System.Collections.Generic;
+using System.Threading;
+using Age.Engine.Hosting;
+
+public sealed class GodotAdvHost : IHost
+{
+    private readonly Main _main;
+    private readonly SemaphoreSlim _gate = new(0, 1);
+    public volatile bool IsWaiting;
+    public readonly List<(int Offset, string Text)> Captured = new();
+
+    public GodotAdvHost(Main main) => _main = main;
+
+    public void ShowText(int offset, string text)
+    {
+        Captured.Add((offset, text));
+        _main.CallDeferred("AppendLine", text);
+    }
+
+    public void WaitForInput()
+    {
+        _main.CallDeferred("PageBreak");
+        IsWaiting = true;
+        _gate.Wait();
+        IsWaiting = false;
+        _main.CallDeferred("ClearPage");
+    }
+
+    // called from the main thread (click) or the selftest auto-clicker
+    public void SignalInput() { if (_gate.CurrentCount == 0) _gate.Release(); }
+
+    public void CallScript(long id) { }
+    public void OnStub(int opcode) { }
+}