toRGBaClipped

Converts this color to ColorRGBa, gamut-mapping it first if it falls outside the RGB gamut. Unlike toRGBa -- which converts directly and can produce components outside [0, 1] for an out-of-gamut color -- this holds lightness (l) and hue (the a/b direction) fixed and reduces chroma to find_gamut_intersection's boundary, the "reduce saturation until it fits" gamut-mapping strategy. A final ColorRGBa.clip guards against the tiny floating-point overshoot the boundary search can leave right at the edge of the gamut.