UI: datum planes and axes in the viewport, datums and partitions in the browser

Datum planes draw as a hatched square around the model, axes as a line through
it, each with its name, in the desktop viewport and the headless render. The
browser's Geometry node lists the datums and the partitions with the datum
each cuts by.

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

crates/ui-egui/src/lib.rs

@@ -750,7 +750,10 @@ impl ModelApp {
             };
             // Symbols follow the drawn nodes: the deformed shape of a results plot.
             let drawn = self.plot.1.as_ref().map_or(&self.display.1, |p| &p.display);
-            let symbols = scene::symbols(&mut s, &self.glyphs, |n| drawn.node_position(n), show, self.plot.1.is_some(), &t);
+            let mut symbols = scene::symbols(&mut s, &self.glyphs, |n| drawn.node_position(n), show, self.plot.1.is_some(), &t);
+            if show.geometry && self.plot.1.is_none() {
+                symbols.union(scene::datums(&mut s, &self.session.model, &self.display.1.bounds, &t));
+            }
             self.scene = (Some(key), s, symbols);
         }
         let mut bounds = self.display.1.bounds;