Function render_structured_diagnostics

Render diagnostics with a clickable header, source line + caret, and note. A click on a span-bearing diagnostic sets `goto_line` to jump the editor.

(
    ui: &mut egui::Ui,
    panel_id: egui::Id,
    panel_height: f32,
    diags: &[Diagnostic],
    goto_line: &mut Option<u32>,
)