Fixed color palette dithering to preserve PLTE chunk

Refactored palette creation to use ImageMagick's mpr: (memory program
register) instead of temp files, eliminating file I/O and cleanup issues.

Key fixes:
- Skip png:exclude-chunks for color palettes (was removing PLTE chunk)
- Apply saturation boost BEFORE normalize (order matters)
- Skip strip() for color palettes (was corrupting colormap)
- Enable normalize + saturationBoost by default for color palettes

Also extracted streamToBuffer() and configureOutputFormat() utilities
to reduce duplication across processing functions.

Author: ikraamg