Introduce a new v2 theme system in packages/ui/src/v2/ using
@radix-ui/colors for sand, gold, red, green, amber, and sky scales.
Colors are imported from the library (with P3 wide-gamut support),
mapped to Tailwind via @theme inline, and scoped under
[data-theme="v2"] for isolation from the existing v1 theme. Dark mode
activates via the .dark class on <html>.
Includes contributor docs (contrib/claude/v2-colors.md) and a Cursor
rule (.cursor/rules/v2-color-scale.mdc) for the color system.
Signed-off-by: Émile Ré <emile@probo.com>
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