Since then, my dead computer (now renamed Lazarus) sprang back into total usability mode when I plugged it in one last time yesterday before consigning it to the trash heap. Like nothing had ever gone wrong! Like the final message it had given me that the disk "cannot be repaired" before the screen went black was just an illusion! Like the many fixes I found online for resetting PRAM and SMC and Safe Boot mode and Disk Utility which did nothing were just a dream.
Anyway, it's acting completely normal now, and I've been able to not only find the foreground file but also ID its origin, which is on Rt 2 in Concord, Mass, just as one drives over the Sudbury River.
So here is: