refactor(neighbors): improve KD-tree implementation and API
- Replace Box-based node storage with index-based flat array for better memory efficiency
- Rename struct and field names for clarity (Node → KdNode, axis → split_axis, etc.)
- Add comprehensive documentation with examples and performance notes
- Update DBSCAN, FastDBSCAN, and K-means to use new KdTreeSearch API
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.