Wire up file operations in ted

[Copilot]

The ted demo exposed File menu entries for New/Open/Save/Save As, but the actions were placeholders. This wires those operations to the editor document and adds focused coverage for file I/O behavior.

• File operations

  • Added NewFile, OpenFile, SaveFile, and SaveFileAs.
  • Tracks CurrentFilePath.
  • Uses Terminal.Gui open/save dialogs by default.

• Testability

  • Added injectable dialog and file I/O delegates so tests can exercise behavior without interactive UI.

• Status display

  • Updates the status bar filename from <untitled> to the current file name.

app.ShowOpenDialog = () => filePath;
Assert.True (app.OpenFile ());
Assert.Equal ("from disk", app.Editor.Document!.Text);

app.Editor.Document!.Text = "after";
Assert.True (app.SaveFile ());
Assert.Equal ("after", File.ReadAllText (filePath));