Giving G1 E10, the extruder motor starts chopping angrily. Setting a search speed (despite the lack of real "search" capability) it moved nicely for a bit, then went back to angry chopping. Trying to adjust the potentiometer does nothing. Odd. Adjusting the potentiometer while it's running a simple back-and-forth doesn't change anything, have to turn it off and on. After adjusting, it now runs stably from Teacup, though slowly and noisily.
D'oh. Simple but consistent miswiring. Green is step (go), yellow is dir (change). Not sure why the Teacup movement is so slow. Using the G0 commands it goes zippy-fast, actually too fast for Y and Z. Setting the max feedrate for all down to 226 from 500 helped the Z, but Y is still unhappy. E is now doing fine.
M104 S100 sets the target temperature to 100ºC. M105 shows that the temperature is going up! It went 5º over before it stopped cooling. Excellent!
Crashed the Z head into the bearing holding the rod. Annoyingly, the extruder house also hits the bearing and supports, after I carefully filed parts of the underlying plate. Need to slice 4cm x 3mm off that, too. Not tonight.
The crashed head: