Function plot_rd_svg 

pub fn plot_rd_svg(
    curve: &[(f64, f64, f64)],
    calibration: &RDCalibration,
    title: &str,
) -> String

Generate an SVG plot of the R-D curve with corner angle grid and knee markers.

Plots (bpp, s2) with angle reference lines radiating from the worst corner, and marks the knee positions.