Function fadeRejectHeadroom

x / nearCull at which smoothstep(nearCull, 2*nearCull, x) reaches reject — the top of the band inside which every geometry type is already suppressed.

Solved numerically rather than written down as a literal, so retuning the reject threshold moves the derived constraint instead of quietly invalidating it. smoothstep is monotone on [0, 1], so bisection converges.