An ARIA radiogroup with roving tabindex (#827, REQ-010/011/018): three
arrow-key-navigable ≥44px segments with text labels, full-word aria-labels and
≤360px abbreviations, semantic-token colours that hold contrast in dark mode,
defaulting to Datum. When disabled it stays in place, retains its selection, and
announces a screen-reader reason — deliberately distinct from #780's
aria-pressed layer toggles.
Refs #827
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
