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/radial-chart.tsx

@@ -193,12 +193,12 @@ export function HMRadialMeterChart({
   return (
     <ChartContainer config={config} role="group" aria-label={label} className={cn(CHART_BOX, className)}>
       <RadialBarChart data={[row]} startAngle={startAngle} endAngle={endAngle} innerRadius={innerRadius} outerRadius={outerRadius}>
-        {/* Official: the first circle is the muted track, the second (smaller) one is the surface color that cuts it into a ring. */}
+        {/* Official: the first circle is the track (official muted, HM --hm-chart-track), the second (smaller) one is the chart surface color that cuts it into a ring. */}
         <PolarGrid
           gridType="circle"
           radialLines={false}
           stroke="none"
-          className="first:fill-muted last:fill-[var(--hm-sem-bg-surface)]"
+          className="first:fill-[var(--hm-chart-track)] last:fill-[var(--hm-chart-surface)]"
           polarRadius={[...trackRadii]}
         />
         <PolarAngleAxis type="number" domain={[0, maxOf([item.value])]} tick={false} axisLine={false} />
@@ -269,7 +269,7 @@ export function HMRadialStackedChart({
               stackId="a"
               fill={`var(--color-${item.key})`}
               cornerRadius={5}
-              stroke="var(--hm-sem-bg-surface)"
+              stroke="var(--hm-chart-surface)"
               strokeWidth={2}
             />
           ))}