Well, that answered that question

The source of the under-sink leak is found—a handyman-engineering job that finally came apart.  L theorizes the house shifting so much in the drought was what pulled it apart; I think it may have just got banged into by something being shoved under there.  L’s sister, an architect with construction-industry experience, thinks I’m capable of fixing it myself if I will.  Her observation was “PVC is a lot easier to work with than copper or black iron.”  I’m going to think about it, and see how I feel this weekend, ’cos fixing it right will require pulling out and redoing the P-trap and cross-piece going to the main wastepipe.