Add StackedBarChart component

[islubee]

Summary

• Adds a StackedBarChart component using d3.stack() to layer multiple numeric series per category
• Accepts a required keys array — each key must be a numeric property on every datum
• Tooltip on hover shows category name, hovered series value, and the bar total
• Non-hovered segments dim to 35% opacity via CSS :has() rules
• Legend is shown by default (one swatch per key); supports all four legendPosition values
• README updated with usage example and full prop table

Test plan

• Run npm test — all 84 tests pass (12 new StackedBarChart tests)
• Verify segments render: data.length × keys.length rects total
• Hover a segment — tooltip shows category, series value, and bar total
• Non-hovered segments dim; moving to another segment updates tooltip
• showLegend={false} hides the legend
• Empty data or empty keys renders nothing