Add BarChart for categorical data comparison

[islubee]

Summary

• New BarChart component — compares categorical values side by side using d3.scaleBand for the x-axis and d3.scaleLinear for the y-axis
• Axis improvement — AxisHorizontal now supports band/ordinal scales (falls back to scale.domain() when scale.ticks() is unavailable, centers labels with scale.bandwidth() / 2)
• Reuses existing Bars and Axis primitives — no new SVG primitives needed
• Consistent API with the rest of the library (xLabel/yLabel, empty data returns null, default accessors)

<BarChart
  data={[{ category: 'Mon', value: 12 }, { category: 'Tue', value: 8 }]}
  xAccessor={d => d.category}
  yAccessor={d => d.value}
  xLabel="Day"
  yLabel="Sales"
/>

Test plan

• npm test passes (55 tests, 7 new for BarChart)
• npm run build-lib succeeds
• CI checks pass on Node 18, 20, 22
• Verify bars render one rect per data point
• Verify x-axis tick labels are centered within each bar
• Verify empty data renders nothing

https://claude.ai/code/session_01Bf9veNX9mqfMqrCEGnujAo