feat(domain): restore must_use message; test TermRef accessors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/domain/src/id.rs

@@ -10,7 +10,7 @@ macro_rules! id_newtype {
 
         impl $name {
             /// Generate a fresh random id.
-            #[must_use]
+            #[must_use = "generating an id and discarding it is almost certainly a mistake"]
             pub fn new() -> Self {
                 Self(uuid::Uuid::new_v4())
             }

crates/domain/src/vocabulary.rs

@@ -50,3 +50,18 @@ impl TermRef {
         self.vocabulary_id
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{TermId, VocabularyId};
+
+    #[test]
+    fn term_ref_exposes_its_parts() {
+        let term_id = TermId::new();
+        let vocabulary_id = VocabularyId::new();
+        let r = TermRef::new(term_id, vocabulary_id);
+        assert_eq!(r.term_id(), term_id);
+        assert_eq!(r.vocabulary_id(), vocabulary_id);
+    }
+}