Datums: drawn at the model's extent, never framed, the selected one shown

A datum plane is drawn as the model's extent within it (where it cuts the
model's box, in the plane's axes) plus a 5 % margin, a datum axis along the
model's extent plus the margin, instead of squares 1.2 diagonals wide.
Fit, Home, the standard views and headless pictures frame the model
without the datums (they stay inside the clipping radius). The datum
selected in the browser is drawn highlighted and on top, even when hidden.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/menu.rs

@@ -233,7 +233,7 @@ mod tests {
     fn on_datum(app: &mut ModelApp, name: &str) -> egui::Pos2 {
         let bounds = app.display().bounds;
         let mut s = fieldcraft_render::Scene::default();
-        fieldcraft_render::glyphs::datums(&app.session.model, &bounds, &|n| n == name, &crate::theme::Tokens::dark().glyph_colors(), &mut s);
+        fieldcraft_render::glyphs::datums(&app.session.model, &bounds, &|n| n == name, None, &crate::theme::Tokens::dark().glyph_colors(), &mut s);
         let l = s.lines[0];
         viewport::to_screen(app, (l.a + l.b) * 0.5).unwrap()
     }