Well, no good. Time to buy a new microwave.
I took apart the Sharp. However, I simply can't find anything wrong with it; All of the parts look perfect, and all of the circuits I tested -- including the main fuse, the light fuse and the temp fuse -- are all fine. The magnetron looks factory-perfect, and the photos I can find of dead magnetrons online seem to indicate that there should be something visibly wrong with it. Plus, the DMM thinks the magnetron is fine.
So, something isn't working in the microwave, but it's completely unclear what. The only things I haven't tested are the high-voltage capacator and the transformer, neither of which I have he equipment to test safely.
Given that I can replace the whole thing for $150, I think it's time to do so. Next time, I need to do a better job filing the warranty paperwork.
BTW, this web site is invaluable if you ever want to try to repair your own microwave:
http://www.repairfaq.org/sam/micfaq.htm Thank you, Mr. Goldwasser.
BTW, does anyone know where I can dispose of a defunct microwave? There's a lot of steel and parts in this thing; it seems like a shame to just toss it in the trash.