feat: palette operations

[qTipTip]

First step in making Pylette a library not just for extracting palettes, but also working with, transforming, and combining palettes.

• add oklab_to_srgb inverse round-trip
• add delta_e OKLab perceptual difference
• add InvalidHarmonyError
• add HarmonyKind enum
• add weighted_oklab_mean and frequency normalization
• add dedup for exact-duplicate colors
• add perceptual sort by OKLab lightness
• add merge_similar perceptual clustering
• add OKLab gradient interpolation
• add color harmonies
• add Hypothesis tests
• document palette operations
• copy colors on return for true immutability and make normalize_frequencies public