Add non-linear undo/redo design documentation and improve UndoSystem with backspace batching and GUI integration fixes.

GUIInputHandler.h

@@ -32,6 +32,14 @@ private:
 	// When a printable keydown generated a non-text command, suppress the very next SDL_TEXTINPUT
 	// event produced by SDL for the same keystroke to avoid inserting stray characters.
 	bool suppress_text_input_once_ = false;
+
+	// Universal argument (C-u) state for GUI
+	bool uarg_active_     = false; // an argument is pending for the next command
+	bool uarg_collecting_ = false; // collecting digits / '-' right now
+	bool uarg_negative_   = false; // whether a leading '-' was supplied
+	bool uarg_had_digits_ = false; // whether any digits were supplied
+	int uarg_value_       = 0; // current absolute value (>=0)
+	std::string uarg_text_; // raw digits/minus typed for status display
 };
 
 #endif // KTE_GUI_INPUT_HANDLER_H