Slipstream update: the crankarm broke right across the taper for the BB. Don't believe I've ever seen that on a bike that wasn't completely beat to crap.
This one is an Acera 170mm and the closest thing I have handy is an Alivio 175 so I'll keep looking. Turns out several of the fasteners are corroded, the front rim strip had decayed and general lube and massage is indicated. More as it happens.