Selas

### Added * Open-sky starlight luminance floor (`starlight_luminance_floor`). * Auto-disable while an Iris/Oculus shader pack is active (`disable_with_shaders`, enabled by default). ### Changed * Recalibrated default natural darkness toward more physically grounded lighting: * higher new-moon and full-moon sky factors * clearer separation between near-black cave darkness and open-sky starlight * cave floor raised slightly to reduce black crush without adding visible ambient light * slightly softer darkness curve and night color grading * slightly brighter Nether / End ambient defaults * When `respect_gamma` is enabled, the brightness slider now lifts Selas target luminance only. * Nether, End, and other skyless dimensions use ambient base light only. ### Fixed * Double gamma application when `respect_gamma` was enabled. * Redundant skyless floor calculation. * Nether / End dimension routing now uses value equality for dimension keys.