Oh, crikey: Windows 10 "Inaccessible boot device". Good old "bootrec /fixboot" didn't work, and any claims that "automatic repair" does anything useful is apparently bollocks.

So for the first time ever, I had to use a restore point to revert the system to where it was before Xamarin was installed. The prime suspect for this cock-up is Intel HAXM, since I don't belive anything else in Xamarin possesses the power to accomplish a boot loop.