Side note: verified that the Speedy Gonzales thing is actually DMA and HDMA updating the MDR, not NMI as first suspected.

It brings up entirely new problems by exposing that HDMA is asserting MDR after an opcode's read cycle from which the HDMA triggers; which is going to require a ground-up rethink of the MDR and HDMA sync subsystems. But anyway for right now, even without that, just let H/DMA update the MDR and Speedy will work.