Turns out- not the clutch. So much for assumptions. No, the reason I heard a terrific bang and didn't go anywhere? My axle broke. (The drive axle that connects to the transmission.) That's...unusual. Well, it does explain why I didn't have any of the usual signs of a dying clutch. Also the lack of forward motion. And it's a cheaper repair than a clutch.
But. It means that the clutch repair still waits for me sometime in the future. Hmm. Better start swatching.