Summary

• Extracts the repeated D3 label rendering block (10+ copies) into a single
  renderDataLabels function in src/core/shapes/data-labels.ts
• Covers: bar-x, bar-y, line, area, waterfall, radar, funnel, heatmap, sankey, x-range
• Returns a typed D3 selection so callers can chain extra attributes and use it
  in hover handlers

Bug fixes (found during refactor)

• funnel, heatmap — text-anchor was never applied; now consistent with all other shapes
• sankey — font-weight was missing; fill was set via attr() instead of style()
  (CSS wins over SVG attribute, so style() is correct everywhere)
• heatmap — added textAnchor field to HeatmapLabel type and populated it in
  prepare-data ('start', since x is already manually offset to center the text)

Not included

treemap — uses a custom TreemapLabelData type without per-label style,
reads style from series.dataLabels.style directly. Aligning it is a separate task.