[BUG] Stable 2.1.x 回归: CardReader::diveToFile() 使用了未终止的 dosSubdirname

预期行为 Directory components extracted by CardReader::diveToFile() should always be valid null-terminated C strings. Functions such as CardReader::fileExists(), file selection, directory traversal, and Power Loss Recovery validation should reliably locate files stored inside subdirectories.