Need to formally define the integration of irradiance in XRLightProbe's sphericalHarmonicsCoefficients and its relationship with the discrete samples accessed by XRReflectionProbe

XRReflectionProbe's cube map should be described as representing discrete samples of incoming light, integrated only over the area represented by the pixel at each mip level.

XRLightProbe's sphericalHarmonicsCoefficients represent the samples that would be sampled by a fully diffuse material. The integration for the SH would be defined as covering a semi-sphere centered around the incoming light vector. For such a wide integration, 9 SH coefficients are able to reach 1% accuracy for any particular vector.

It is possible to accurately generate XRLightProbe SH coefficients from an XRReflectionProbe's cube map though integration of semi-sphere's; however, the reverse conversion can only represent a low frequency detail approximation.

No file, test, or entry point is named. Start with the issue's definitions of XRLightProbe sphericalHarmonicsCoefficients and XRReflectionProbe cube-map samples, then determine how the proposal should formally specify their integration and relationship; done means the specification clearly states both representations and the limits of conversion between them.