feat(theme-studio): add the ramp UI in the palette

A ramp button on the palette controls opens a panel that generates a tonal ramp from the current color and previews the steps. Each step is a swatch labeled with its derived name (blue, blue+1, blue-1) and a clamp badge when the color left the sRGB gamut, so an out-of-gamut step is visible before it's added. The n, stepL, and chroma-ease controls default to 2 / 0.08 / 0.5 and re-preview live.

Clicking a step adds it to the palette; "add all" adds the lot. Steps insert adjacent to the source swatch in -n..+n order. A name collision is flagged and skipped rather than overwriting an existing color, and a generated hex that already matches another entry is added but flagged as a duplicate.

This is Phase 2, the DOM around the pure ramp() from Phase 1. A new #ramptest browser gate pins the step count, the ordered insertion after the source, the collision skip, and the clamp badge on an out-of-gamut step.

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