Merges pixel-art-arcade and pixel-art-snes into one pixel-art skill with
named presets (arcade, snes) + parametric overrides. The underlying
pipeline was already identical across both variants — only palette size,
block size, and enhancement strength differed. A single preset-based
function is easier to discover, maintain, and extend (adding a new era
like gameboy or nes is just another preset dict).
Contributor authorship preserved on original additive commit.
