feat: add overlay manager

- compose visible overlays into rendered lines

- add runtime APIs for showing and hiding overlays

- restore focus when overlays close

- add help overlay example

src/Example/Program.cs

@@ -1,6 +1,7 @@
 using TinyTUI;
 using TinyTUI.Components;
 using TinyTUI.Input;
+using TinyTUI.Overlay;
 using TinyTUI.Rendering;
 using TinyTUI.Runtime;
 using TinyTUI.Stdio;
@@ -22,7 +23,16 @@ var input = new TinyTUI.Components.Input(textMeasurer) { Prompt = "请输入: "
 input.OnChanged = value => UpdateStatus(statusText, value, textMeasurer, renderer);
 input.OnSubmitted = value =>
 {
-    AddEvent(eventsText, $"Submitted: {value}");
+    if (value.Trim() == "/help")
+    {
+        AddEvent(eventsText, "Overlay: help opened");
+        runtime.ShowOverlay(new HelpOverlay(runtime, textMeasurer));
+    }
+    else
+    {
+        AddEvent(eventsText, $"Submitted: {value}");
+    }
+
     input.Clear();
 };
 input.OnCanceled = () => done.Set();
@@ -32,7 +42,7 @@ UpdateStatus(statusText, input.Value, textMeasurer, renderer);
 var page = new Container();
 page.Add(new Text("TinyTUI 基础组件示例"));
 page.Add(new Text("======================"));
-page.Add(new Text("测试方式: 输入文本 Backspace 删除 Enter 提交 Esc 退出"));
+page.Add(new Text("测试方式: 输入文本 Backspace 删除 Enter 提交 /help 打开弹层 Esc 退出"));
 page.Add(new Text(string.Empty));
 page.Add(new Box(statusText, textMeasurer) { Title = "状态" });
 page.Add(new Text(string.Empty));
@@ -78,3 +88,39 @@ static void AddEvent(Text eventsText, string message)
     var lines = eventsText.Value.Split('\n').Skip(1).Append($"- {message}").TakeLast(8);
     eventsText.Value = "最近事件:\n" + string.Join('\n', lines);
 }
+
+/// <summary>
+/// 用于手动验证 overlay 的帮助弹层
+/// </summary>
+file sealed class HelpOverlay(ITuiRuntime runtime, ITextMeasurer textMeasurer) : IInputComponent
+{
+    private readonly Box _box = new(
+        new Text(
+            "Overlay 示例\n" +
+            "\n" +
+            "这个弹层由 Runtime 显示\n" +
+            "OverlayManager 会把它合成到基础页面上\n" +
+            "显示时焦点会切到弹层\n" +
+            "关闭后焦点会恢复到输入框\n" +
+            "\n" +
+            "按 Enter 或 Esc 关闭"),
+        textMeasurer)
+    {
+        Title = "Help",
+    };
+
+    /// <inheritdoc />
+    public IReadOnlyList<string> Render(int width)
+    {
+        return _box.Render(width);
+    }
+
+    /// <inheritdoc />
+    public void HandleInput(TuiInputEvent input)
+    {
+        if (input is { Kind: TuiInputEventKind.Key, Value: KeyNames.Enter or KeyNames.Escape })
+        {
+            runtime.HideOverlay();
+        }
+    }
+}