Function build_visuals 

pub fn build_visuals(
    field: &Field,
    _vp: &Viewport,
    params: VisualParams,
) -> Vec<NodeVisual>

Build a render-friendly list of visuals from the current Field + Viewport.

• Skips nodes that are not experience-visible.
• Emits label scaling that grows as you zoom out.
• Marks Core nodes as cluster cores (badge is optional; uses cluster lookup if available).