You’re passing a Decoration where a Range<Decoration> is expected. You line decorations apply to a specific line, and need to point at the start of the line (auto_dir.range(0) for the first line).
Nope, that’s not a requirement that has come up before.
Also, the library assumes the editor has a single direction (from the computed style of the cm-content element). I don’t think putting dir=auto on every line should break anything in too hard a way, but you will probably, for lines different from the main editor’s direction, see some non-ideal behavior.