Function render_footprint_studies 

pub fn render_footprint_studies(
    painter: &Painter,
    mapping: &ChartMapping,
    bars: &[Bar],
    flow: &Footprint,
    studies: &[FootprintStudy],
    config: &FootprintConfig,
)

Paints footprint study levels and zones. Compute studies only after executions change. Stacks, naked POCs and unfinished levels extend to their first revisit, their expiry or the last displayed bar, in full colour while unvisited and dimmed once revisited or expired; local annotations stay anchored to their originating bar. Delta percentage and ratio markers are triangles outside the bar: buy markers above it, sell markers below, the delta marker nearest and the ratio marker beyond it. Captions avoid every footprint and its marker rows; details are available on hover when a caption has no room. Overview marks shrink with bar spacing and zones become faint centre lines. Prices use the aggregation’s row step.