fix(theme-studio): clamp generated palette spans to the bg/fg bounds

Spanning a color generated steps toward pure black and white, so a column could produce colors darker than bg or lighter than fg. I changed regenColumn to ramp the dark side toward the darker ground endpoint and the light side toward the lighter one, bounded by bg and fg. Pure black/white duplicates are still skipped, and callers that pass no ground fall back to the old black/white ramp.

Node tests cover the bounded span and the no-ground fallback. The #counttest gate asserts the regenerated column stays within the bg/fg bounds.

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