refactor(baoyu-article-illustrator): extract palette as independent third dimension
- Create palettes/ directory with macaron, warm, neon palette files
- Move macaron from styles/ to palettes/ (color scheme, not rendering style)
- Extract warm palette variant from vector-illustration into shared palette
- Add neon palette extracted from retro style colors
- Update architecture from Type × Style to Type × Style × Palette
- Add Palette Gallery and override rules to styles.md
- Add Palette Override section to prompt-construction.md
- Update presets table with Palette column
- Add palette selection step to workflow
- Add preferred_palette to EXTEND.md schema
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