readCsrgClasses

BuildData's class csrg: <obf> <spigot-internal-name> per line, '#' comments. Read straight into a lookup rather than through mapping-io, which would build a second tree for assembleSpigot to walk.

Exactly one space is what makes it a class row; member rows carry three or four columns. Not a guard against nothing: a field row admitted here would key on its CLASS and overwrite that class's mapping.