Armed with a diode, more croc-wires, and a Ritter Sport marzipan, I'm going to get that motor sorted out once and for all.
First test that the LED does what I want it to, by running it across the Y motor. Ok, so that wasn't a multi-color LED as I hoped, but I can at least see it change intensity when the motor starts and stops.
Black-Red: Lights up a bit, but doesn't change when "extruding".
Green-Red: Lights up a bit, but doesn't change when "extruding".
After carefully marking the wires the wrong way (RGBK instead of RBGK), I put the extruder wires into the Z axis jack. The light lights up more now when I run the Z motor. Wiring the motor back up, I get - no movement whatsoever.
Finally took some better wires and wired directly from the Z jack to the motor, carefully matching the colors. Motor moves not the least bit, even though the heater is at full. I can only conclude that this motor is no more. It is an ex-motor. It has ceased to be.
Curious: I set the LED back on the extruder wires, but didn't actually wire the extruder. When I do the first extrude after the power cycle, I still hear a "chunk" sound. That can't be the extruder, then, like I thought. Some kind of cross-wiring? It appears the X axis will sometimes move a bit when starting to extrude, to get away from homing. So does Y and Z. But that doesn't give that "chunk" sound.
Powercycled again and looked carefully. The "chunk" is indeed the X-axis moving a tiny bit the first time. So the actual extruder was dead the whole time.
FabLab has no spare compatible stepper motors, nor do I. So I can't get that much further.
Tried wiring up the extruder wires to an incompatible stepper motor. Didn't get any reaction out of it with any combination of wires, but then I don't know if it works or if I'm giving it enough power to work or anything, so that doesn't prove much.
In conclusion, I have an extruder stepper motor that is almost certainly dead, wiring for it that is possibly faulty, and still no prints. *sigh*