feat: support theme-aware (dark mode) sequential color palettes

[mistercrunch]

Opening this PR as a quick POC to test whether sequential color palettes can look good in dark mode using transparency, and it seems like they just work — thanks to d3-interpolate doing the right thing when given rgba(...).

Palette preview

Initial takeaways

• A lot of existing palettes avoid going all the way to pure white or black, probably to stay somewhat neutral across themes — but that middle still ends up clashing in dark mode. Using alpha (e.g. fading to transparent) gives us a cleaner fallback.
• I don’t want to change how any existing palettes render in light mode. Changing the output given the same data would be a soft compatibility break — and making things both backward-compatible and theme-aware might get tricky.
• Easiest path forward: introduce new theme-aware palettes (e.g. "Safe Diverging", "Dark-Optimized Sequential") and mark older ones as “legacy”.

Experimenting

• highjack the color scheme registry and flip the brightness (hsb) of the color if/when in dark mode
• looks ok? unclear

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Bonus: Categorical palettes

Our categorical palettes often cycle through a few base colors (say 5) and vary them with lightness to generate more (say 3 shades per base color → 15 total). We could explore using alpha here too — generating lighter/darker variants using opacity instead of tweaking RGB directly. Same caveats around preserving light-mode compatibility apply.

[korbit-ai]

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[mistercrunch]

Experimenting with flipping the brightness on existing palettes:

[mistercrunch]

gpt had some decent ideas around categorizing palettes in a fancier PalettePicker:

• 🌒 Adaptive Palettes
• 🧱 Fixed Palettes
• 🎨 Legacy Palettes

[mistercrunch]

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