style(chart): open chart pages and surface/track tokens

- @hm-design/chart: add --hm-chart-track (radial tracks, horizontal bar
  hover band; warm neutral light) and --hm-chart-surface (slice / segment
  gaps, Dot rings, label halos, radial meter center; white by default)
- --hm-chart-bar-name-halo stays an optional override and falls back to
  --hm-chart-surface; official chart.tsx unchanged
- Storybook Chart pages (Bar / Line / Area / Donut / Pie / Radar / Radial):
  open layout without example cards, 2-4 responsive columns, one sample
  data notice per page, surface token set to the content background
- Same sample data, props, tooltip and interaction as the example cards;
  examples entry and consumer example unchanged
- README: surface / track token usage

packages/chart/src/components/line-points.tsx

@@ -50,7 +50,7 @@ export function PointDot({
       cy={y}
       r={radius}
       fill={pointColor(payload, colorKey, fallback)}
-      stroke="var(--hm-sem-bg-surface)"
+      stroke="var(--hm-chart-surface)"
       strokeWidth={2}
     />
   )