feat: enhance Theme scoring logic with 2D gaussian function
- Improved swatch selection accuracy by considering max chroma for each hue.
- Adopted 2D Gaussian function for scoring to better match tones.
perf: replace HashMap and HashSet with FxHashMap and FxHashSet
- Improved performance by using rustc-hash's FxHashMap and FxHashSet.
- Limited usage to internal library components since the algorithm is non-cryptographic.