bound mean-cut partition scan in crawl_decision_tree

[jmestwa-coder]

mean-cut color quantization in cgif_rgb can split a node on the wrong side of its range:

• both partition scans assumed the float mean sits strictly between the node's min and max color along cutDim
• a frequency-weighted mean rounds to the channel extreme for skewed inputs, so a scan runs past idxMax/idxMin and creates a child with an empty range
• get_mean then divides a zero frequency sum, and the NaN mean is cast to uint8_t in get_palette_from_decision_tree (undefined behavior; the over-scan can also read past pFrequDense)

Bounded both scans to the node range and kept the split non-empty.