Model: datum points, drawn as a small cross; each definition's users are public

Datum::Point joins planes and axes (checked like them, refused by
partitions with the kind named). The users each remove_* refuses with
(sets, materials, boundary conditions, loads, reference points, fluid
domains, datums) are public functions, so a caller can list them before
deleting.

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

crates/ui-egui/src/browser.rs

@@ -658,8 +658,7 @@ fn rows(app: &mut ModelApp, ui: &mut egui::Ui, acts: &mut Vec<Act>) {
             );
         }
         // Datum planes and axes, then the partitions in the order they cut.
-        let datums: Vec<(String, &'static str)> =
-            app.session.model.datums().iter().map(|(n, d)| (n.clone(), if d.plane().is_some() { "plane" } else { "axis" })).collect();
+        let datums: Vec<(String, &'static str)> = app.session.model.datums().iter().map(|(n, d)| (n.clone(), d.kind())).collect();
         for (i, (n, kind)) in datums.iter().enumerate() {
             draw_row(
                 app,