UPDATE: OK, I have found the culprit.

I realize now that when you said to check the values on the map, I needed to do it not just for the alpha, but *especially* for the original image. The black around the moon in the original "incandescent" image had a value of 1 in Photoshop and this is the slight opacity that was sneaking into the render even when the alpha value there was zero.

I guess this means incandescence (whether as a common material attribute or just as a function of a surface shader ) acts upon (makes incandescent) the values it has mapped to it, regardless of the transparency/alpha mask. It's as if it were being calculated *after* the transparency, bringing back to "1" what the transparency had already brought down to "0". This is different from how mapping to color works.

Thanks for the help jsprogg!