feat(theme-studio): show worst-case contrast for overlay faces

The five covered overlay faces (region, hl-line, highlight, lazy-highlight, isearch) now show the worst-case floor over their foreground set instead of one fg/bg pair. The cell starts with "worst:", names the limiting foreground by role and hex, then gives the ratio and a PASS/FAIL verdict, so a tint that clears the default text but fails the darkest token can't hide. The foreground set is the live syntax-token colors plus the default fg; the verdict is WCAG AA by default. Package and other UI rows keep their single-pair readout.

A syntax-color edit now repaints the covered faces, since their floors depend on the whole token palette, not just their own fg/bg. An out-of-scope face falls through to the single-pair cell, and a face whose set resolves empty reads "no fg set" rather than a bogus ratio.

Phase 4 of the palette-ramps spec. A #contrasttest browser gate pins the readout, the keyword-blue limiting case, the single-pair fallback, and the no-set message.

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