@spec parse(String.t()) :: {:ok, Sys.Linux.Fstab.Spec.t()} | {:error, :invalid_format}

Parse a single fstab line into a Spec.

fstab fields are whitespace-separated: device mount_point fs_type options [dump] [pass]. The dump/pass columns are ignored; options is split on commas. Blank lines and # comments yield {:error, :invalid_format}.