Leave the dark nebulae out of the backdrop, whose sprites can only add light

c01d3ec read OpenNGC's addendum, which brought in its only two DrkN rows, C099 the Coalsack and
B033 the Horsehead; NGC.csv has none. classifyOpenNgcType mapped DrkN to 'nebula', so both were
drawn as the backdrop's nebula sprite: ff86b0, additive, at the faintest opacity and the 70 pc
minimum size, about 1.6° across. The review measured it adding up to +59 in red over the Coalsack
beside Crux, where the sky has a dark hole, and darkening no pixel, as an additive sprite cannot.

DrkN is now among the codes dropped, with the reason beside the map. fetchDeepSky from cache keeps
488 objects against 490, still 107 Messier objects and every required id; README's count follows.
On :4302 the served backdrop holds 488 objects and the scene 488 sprites, neither of them C099 or
B033.

Control: DrkN mapped back to 'nebula' fails "leaves out a dark nebula, which a sprite that adds
light cannot draw" (1 of 801).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -283,7 +283,7 @@ the line of sight is always known precisely, and the objects are drawn as a fixe
 backdrop shell where true distance would be unusable anyway. `distancePc` is optional metadata,
 derived from parallax for galactic objects or the Hubble law for genuinely distant galaxies,
 and left `null` — with its `distanceMethod` — whenever neither is trustworthy. 340 of
-the 490 cataloged objects get a distance; the rest honestly report none.
+the 488 cataloged objects get a distance; the rest honestly report none.
 
 ## Layout