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/sets.rs

@@ -134,7 +134,7 @@ impl Model {
     }
 
     /// The definitions that refer to a set, as `kind \`name\`` strings.
-    pub(crate) fn set_users(&self, set: &str) -> Vec<String> {
+    pub fn set_users(&self, set: &str) -> Vec<String> {
         let mut users = Vec::new();
         users.extend(self.sections.iter().filter(|(_, s)| s.set == set).map(|(n, _)| format!("section `{n}`")));
         users.extend(self.couplings.iter().filter(|(_, c)| c.set == set).map(|(n, _)| format!("coupling `{n}`")));