Thanks to @Henry for confirming, by several similarly observed whacko problems the past two days, that my problem is indeed caused by a bug in AS. Follow his links to become convinced yourself. I'll let Henry file the bug report.
So there's no fixing my problem in situ without great effort--wait for bug fix.... which could take awhile.
So here's what I did to work around it and get back to work (it took less than 10 minutes):
- create new project with no activity
(Do the rest inside the IDE, not using Windows or DOS.)
- copy all
.java files into the java folder in the IDE (it's ...\app\src\main\java\)
- copy all
res folders into the res folder (...\app\src\main\res\)
- copy all
asset files into main folder (...\app\src\main\assets\)
- copy
androidmanifest.xml into main folder (...\app\src\main)
- do a global replace of old project name to new project name
I didn't change any of the files in the new project's ...\app\ folder because compilation was normal.
(I'll tell you this: when I saw what I THOUGHT were mangled FILES and looked at the History and saw nothing resembling what should have been there, I got a sick feeling, but I immediately opened Windows Explorer, selected all of the project's java files, and opened them, en masse, with Notepad++. Whew. No problems seen. (I didn't really think AS could actually mangle the files and I doubt it ever could. But it did wake me up at 5am!))