I don't understand why this worked, but I found [this comment] in Redhat Bugzilla (it doesn't seem to be a bug report specifically about that...but anyway):
Parameter 'tools' is mandatory [...]
However, if I create the packages directory in ~/.arduino15, compilation now works.
mkdir ~/.arduino15/packages
Seemed like an easy thing to try, and voila -- after an hour of fuss.
I do not understand why this is so (nothing ends up in that directory, either) but I'm leaving this in hopes it may save someone else some time.