Class AtmosphereLutsFeature

Bakes the atmosphere transmittance + multi-scattering LUTs at startup and publishes the resulting texture handles into Frame.extras each frame. Other features consume the LUTs by reading those keys:

• AtmosphereFeature reads the multi-scatter LUT to add 2nd-order+ in-scatter on top of its existing single-scatter integral (sky brightening, indoor ambient lift).
• The sample-local HQ cloud pass reads the transmittance LUT to attenuate the sun color by the air column at each cloud sample (the visual coupling that gives clouds their deep amber-red look at sunset).

Register this feature BEFORE its consumers in the engine — features run in registration order, so the LUTs need to be in frame.extras before the consumers' addPasses runs.