Function pick 

pub fn pick<'a>(
    model: &'a GraphModel,
    camera: &Camera,
    rect: WidgetRect,
    probe_x: f32,
    probe_y: f32,
) -> Option<&'a str>

STAGE: picking — resolve a screen-space probe to the node whose chip contains it, honouring the widget rect (a probe outside the rect picks nothing). The same affine the render used; returns the picked node id (borrowed from model).

Return-asserted (the EMIT-DOCTRINE): a probe at a drawn node’s centre resolves that node; a probe in a culled/empty region resolves None.