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/solid-chart.tsx

@@ -89,7 +89,7 @@ function SolidSlice({ sector, hole, sideColor, topColor, center }: SolidSlicePro
   const frontTo = Math.min(to, 360)
   const backFrom = Math.max(from, 0)
   const backTo = Math.min(to, 180)
-  const seam = "var(--hm-sem-bg-surface)"
+  const seam = "var(--hm-chart-surface)"
   // The seam between slices is a surface-colored stroke; on a very thin slice it thins out so the slice stays visible.
   const arcPx = (to - from) * RAD * p.rx
   const seamTop = arcPx < 4 ? 0.5 : arcPx < 10 ? 1 : 2