Accessible colors, clearer tokens

Design tokens should be obvious. This release flattens the preset so tokens are easier to find, retunes the color palette for accessibility, and renames the concept that governs how light or dark a color is.

Improved

• A flatter token preset. The design-tokens preset was flattened and its variable composables renamed, so tokens sit closer to the surface and read more directly.
• An accessible palette. The color palette was updated for accessibility, and swatches now show their hex values so you can see exactly what you're picking.

Breaking

• colorLightness is now colorLevel. "Level" describes the step in a color scale more plainly than "lightness" did. The rename runs across the theme, docs, and Storybook — update any references.