Fix Rose Chart area encoding: use sqrt radius on ECharts and Chart.js to match Vega-Lite

[Copilot]

The same Rose Chart ChartAssemblyInput compiled to different perceptual encodings per backend: Vega-Lite set radius.scale.type = 'sqrt' (wedge area ∝ value), while ECharts and Chart.js placed raw values on linear radial scales (area ∝ value²). A 4:1 input rendered as 4× area in Vega-Lite but 16× in the other two.

Neither ECharts' polar radiusAxis nor Chart.js' radial r scale supports a sqrt type, so the transform is applied to the series data itself, with the true value preserved for the tooltip.

Changes

• ECharts (echarts/templates/rose.ts)
  • Single-series data items now carry value: sqrt(raw) plus _rawValue.
  • Stacked series use incremental radii sqrt(cumsum) − sqrt(prevCumsum), so each stacked segment's area — not just the outer edge — is proportional to its value (mirrors Vega-Lite's stacked sqrt scale).
  • Tooltip formatter reports _rawValue; sqrt-space radial tick labels are hidden.
• Chart.js (chartjs/templates/rose.ts)
  • Dataset data becomes sqrt(value); a tooltip label callback reports the true value (kept in a rawValues closure aligned after slice sorting); sqrt-space radial ticks are hidden.
• Tests (tests/rose-area-truth.test.ts) — asserts the sqrt/area invariant across all three backends, including the stacked cumulative-radius case and raw-value preservation.

Result

For values 9:4:1, all three backends now render radii 3:2:1 (area 9:4:1):

assembleECharts(input).series[0].data;
// [{ value: 3, name: 'C', _rawValue: 9 }, { value: 2, name: 'A', _rawValue: 4 }, { value: 1, name: 'B', _rawValue: 1 }]

assembleChartjs(input).data.datasets[0].data; // [3, 2, 1]  (tooltips still show 9, 4, 1)

The stacked case stays area-true per segment: D1 = {S1: 9, S2: 16} → outer edge at sqrt(25) = 5, with the S1 segment at radius 3 and the S2 increment at 2.