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/model/src/bcs.rs

@@ -232,10 +232,14 @@ impl Model {
         replace_existing(&mut self.boundary_conditions, "boundary condition", name, bc)
     }
 
+    /// The steps that list a boundary condition, as `step \`name\`` strings.
+    pub fn boundary_condition_users(&self, name: &str) -> Vec<String> {
+        self.steps.iter().filter(|s| s.boundary_conditions.contains(name)).map(|s| format!("step `{}`", s.name)).collect()
+    }
+
     /// Remove a boundary condition that no step lists.
     pub fn remove_boundary_condition(&mut self, name: &str) -> Result<()> {
-        let users = self.steps.iter().filter(|s| s.boundary_conditions.contains(name)).map(|s| format!("step `{}`", s.name)).collect();
-        not_in_use("boundary condition", name, users)?;
+        not_in_use("boundary condition", name, self.boundary_condition_users(name))?;
         remove_existing(&mut self.boundary_conditions, "boundary condition", name).map(|_| ())
     }
 }