Whoops!
I’m not sure (I can’t see any joints) but I believe that the DT<->HT braze has failed, because those two tubes should not be able to flex at the lug. Time to drop the fork and fire up the torch, I guess? Nope, I think the headset + crown race seat are mismatched? I replaced the crown race, put the fork back together, and bunny-hopped the machine down the block without any creaks or joints breaking apart, so I’m less convinced that I messed up the brazes. Time to scrounge through my pile of 1" headsets and see if any of them are either (a) unthreaded or (b) can have the threads ground out to become unthreaded.