accentHue method
The steered operator (index 0) or = (index 1) hue (tenths),
drawn from r; null when the intent names no accent colour, or names
a harmony (which then places the accents).
Implementation
int? accentHue(Rng r, int index) {
final v = r.nextU32();
if (intent.harmony != null || intent.colors.length < 2) return null;
final c = index == 0 || intent.colors.length < 3
? intent.colors[1]
: intent.colors[2];
if (c.neutral) return null;
return _pick(_HueRange(c.loTenths, c.hiTenths), v);
}