Add OKLCH struct and conversion methods (sRGB ↔ OKLab ↔ OKLCH) using
Björn Ottosson's matrices. Includes:
- SimpleColor.ToOKLCH() and FromOKLCH() for color conversion
- SimpleColor.ClampOKLCH() for bounding lightness/chroma
- SimplePalette.ClampOKLCH(), NormalizeLightness(), NormalizeChroma()
for palette-level OKLCH operations
- Comprehensive tests with round-trip, known values, and clamping
