feat(chart): add solid (thick) pie and donut charts

Add HMSolidPieChart and HMSolidDonutChart: a tilted, SVG-drawn disc with an
outer wall and, for the donut, an inner wall seen through the hole. Same data,
slice angles, order, legend and tooltip as the 2D charts; the tooltip is the
same on the top face and the walls and follows the pointer. The existing 2D
charts and their props are unchanged.

- Side walls use darker HM tokens (--hm-chart-1-side .. --hm-chart-5-side) and
  a color-mix darkening for custom colors; shared chart tokens are untouched.
- Add a "입체형" example to the Pie Chart and Donut Chart stories
  (PieSolidExample, DonutSolidExample); ExampleCard description is now optional.
- Document usage and constraints in the chart package README.

src/chart/donut-chart.stories.tsx

@@ -4,6 +4,7 @@ import {
   DonutCenterExample,
   DonutInteractiveExample,
   DonutPlainExample,
+  DonutSolidExample,
 } from "@hm-design/chart/examples"
 import { StoryPage, StoryPageHeader, StorySection } from "@/components/story-page"
 
@@ -34,6 +35,9 @@ export const Donut: Story = {
         <StorySection title="Interactive">
           <DonutInteractiveExample />
         </StorySection>
+        <StorySection title="입체형">
+          <DonutSolidExample />
+        </StorySection>
       </div>
     </StoryPage>
   ),

src/chart/pie-chart.stories.tsx

@@ -1,5 +1,5 @@
 import type { Meta, StoryObj } from "@storybook/react-vite"
-import { PieLabelExample, PieLabelListExample, PiePlainExample, PieStackedExample } from "@hm-design/chart/examples"
+import { PieLabelExample, PieLabelListExample, PiePlainExample, PieSolidExample, PieStackedExample } from "@hm-design/chart/examples"
 import { StoryPage, StoryPageHeader, StorySection } from "@/components/story-page"
 
 // Sample data only. All pies share one item list and color mapping; the stacked pie draws two series as concentric rings.
@@ -29,6 +29,9 @@ export const Pie: Story = {
         <StorySection title="Pie + Stacked">
           <PieStackedExample />
         </StorySection>
+        <StorySection title="입체형">
+          <PieSolidExample />
+        </StorySection>
       </div>
     </StoryPage>
   ),