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

UndoSystem.h

@@ -40,6 +40,8 @@ private:
 private:
 	Buffer &buf_;
 	UndoTree &tree_;
+	// Internal hint for Delete batching: whether next Append() should prepend
+	bool pending_prepend_ = false;
 };
 
 #endif // KTE_UNDOSYSTEM_H