Presentational legend atom - a row (or column) of coloured swatches
paired with labels. Decoupled from <cngx-chart> and the layer
atoms by design: the consumer drives the items array, so the
legend stays a pure rendering surface with zero opinions about
series-discovery, visibility-toggling, or interaction.
Compose alongside a chart wherever a multi-layer composition needs
a label key:
Theming via --cngx-chart-legend-gap,
--cngx-chart-legend-swatch-size, --cngx-chart-legend-swatch-radius,
--cngx-chart-legend-font-size. [orientation] flips between row
and column; [align] aligns along the main axis (start, center,
end).