Support rounded bar and histogram corners #506

Support layout-wide and per-trace corner radii in pixels or percentages using a shared StyleParam.CornerRadius, including stacked bars and histograms. Decision: accept replacing the existing CornerRadius: int API with this type as a source/binary breaking change; do not add a parallel marker parameter. Include tests and migration documentation, preserving pixel rendering after migration (including treemaps).

Release notes · Upstream implementation PR #6761.