RFC-008: RadarChart in djed-viz

Problem

Case summary uses a five-axis confidence spider chart. @alexbayerl/djed-viz provides RankingChart, HeatmapGrid, ScatterPlot, etc., but no radar / spider primitive.

Current Atlas approach

ConfidenceRadar lives under atlas/ui/src/primitives/charts/ConfidenceRadar.tsx — the only sanctioned bespoke data SVG in the workbench until this RFC is resolved.

Proposal

1. Add RadarChart (or SpiderChart) to @alexbayerl/djed-viz with:
   • Typed axis definitions { key, label }[] and { [key]: number } or parallel arrays.
   • Normalized radius 0–1, accessible labels, prefers-reduced-motion respect (static polygon).
   • Theming hooks consistent with other viz components (VizTheme / CSS variables).
2. Migrate Atlas ConfidenceRadar to the shared primitive and delete the local implementation.

Status

Draft — tracking issue in djed-viz repo TBD.