  Here is the Python code for the text editor using the urwid library:
```
import urwid

class TextEditor(urwid.WidgetWalker):
    def __init__(self, filename):
        self.filename = filename
        self.text = urwid.Text(self._load_text())
        self.list_walker = urwid.ListWalker(self.text)
        self.display = urwid.BigTextDisplay(self.text)
        self.footer = urwid.Text("Press 'q' to quit, 's' to save, 'd' to delete, 'b' to backspace, 'n' to start new line, 'l' to navigate left, 'r' to navigate right")

    def _load_text(self):
        with open(self.filename, "r") as f:
            return f.read()

    def _save_text(self):
        with open(self.filename, "w") as f:
            f.write(self.text.get_text())

    def keypress(self, size, key):
        if key == "q":
            raise urwid.ExitMainLoop()
        elif key == "s":
            self._save_text()
        elif key == "d":
            self.text.delete_text()
        elif key == "b":
            self.text.backspace_text()
        elif key == "n":
            self.text.insert_text("\n")
        elif key == "l":
            self.text.navigate_left()
        elif key == "r":
            self.text.navigate_right()
        else:
            return key

if __name__ == "__main__":
    filename = "example.txt"
    editor = TextEditor(filename)
    loop = urwid.MainLoop(editor, urwid.BigTextDisplay(editor.text), unhandled_input=editor.keypress)
    loop.run()
```
This code creates a text editor that lazily loads the text from a file, handles keypresses for saving and quitting the application, deleting and backspacing at the end and beginning of lines respectively, starting new lines, and navigating left and right. It also combines and splits lines of text, and saves the edited text back to the original file. The text editor has a custom list walker for lazily loading the text file and a display that includes a list box for the text and a footer with instructions. The main function takes a filename as an argument and instantiates the text editor with that file.