CngxChartLegendItem

Single legend entry. color is optional - when omitted the swatch falls back to the chart-level primary token. value is rendered as a trailing .cngx-chart-legend__value node when present (!= null, so a literal 0 still shows), turning a legend into a readout. It stays intentionally unconstrained so the same field a consumer weights in the row can still carry their domain key (id, layer name, accessor return type, ...) for later interactions without a second lookup.