Custom normals for stylized cycles hair strands

Cycles does not properly render custom normals on curve primitives, including geometry nodes curve systems and particle curve systems. Cycles will always render strands with a faux rounding effect, even when using Ribbon-based strands. This makes for blotchy, shaded strands.

This is a feature parity gap between Eevee and Cycles. This locks out stylized normal-based effects that are possible in Eevee, like flat-shaded grass and character fur. In the image below, you can see the stylized normals on the right and the noisy rounded normals on the left.