Tooltip to always display at top of graph / ability to round corners for ALL rod items

Currently we use `BarChartRodData` with a height of `maxY` and color invisible, and then we add a single `BarChartRodStackItem` with the actual Y value and color. This forces our tooltip to display on top of the graph.

HOWEVER - we want rounded corners on our stack item, but because its y != the bar y, the painter doesn't round the top corners

so either we need to be able to force the tooltip to display outside the graph (then we won't need the stack items) or we need to be able to apply rounding to individual stack items

Start by tracing BarChartRodData and BarChartRodStackItem through the bar painter and tooltip positioning behavior. Compare the two requested approaches: allowing tooltips outside the graph or supporting rounded corners on individual stack items. Done means the selected behavior works for stacked bars without relying on an invisible full-height rod.

Written by the indexing model from the issue text.