Namespace Editor Document Utils

Includes the Rope<T> data structure (a balanced B-tree for efficient insert/delete in large sequences), Deque<T> (double-ended queue), CompressingTreeList<T> (run-length compressed list), FileReader (encoding-detecting file reader), IFreezable (immutability pattern), and various string/text helpers.

These types are adapted from AvaloniaEdit's utility layer and have no dependency on Terminal.Gui.