.fig export: warn when a set's variants disagree on a property default

The format keeps one default per property of a set; the first variant's
is written (CodeRabbit review on #1152).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/format/src/fig/export.rs

@@ -2002,13 +2002,23 @@ impl<'d> Exporter<'d> {
                 ]));
             }
         }
-        let mut seen = std::collections::BTreeSet::new();
+        // The format keeps one default per property of a set (a variant's copy holds none): the first
+        // variant's, with a warning when another variant's differs.
+        let mut seen: BTreeMap<String, PropDef> = BTreeMap::new();
         for v in &variants {
             let NodeKind::Component { props, .. } = &v.kind else { continue };
             for (name, d) in props {
-                if prop_type(d).is_some() && seen.insert(name.clone()) {
-                    let index = defs.len();
-                    defs.extend(self.prop_def(set.id, name, d, index));
+                if prop_type(d).is_none() {
+                    continue;
+                }
+                match seen.get(name) {
+                    Some(first) if first != d => self.warn("variants with different defaults for one property were written with the first variant's"),
+                    Some(_) => {}
+                    None => {
+                        seen.insert(name.clone(), d.clone());
+                        let index = defs.len();
+                        defs.extend(self.prop_def(set.id, name, d, index));
+                    }
                 }
             }
         }

crates/format/src/fig/export_tests.rs

@@ -1382,3 +1382,14 @@ fn grid_track_sizes_are_written_and_read_back() {
     assert_eq!(l.column_tracks, columns);
     assert_eq!(l.row_tracks, vec![GridTrack::fixed(69.0)]);
 }
+
+#[test]
+fn variants_that_disagree_on_a_default_are_a_warning() {
+    let (mut doc, _) = set_with_properties();
+    let large = doc.nodes().find(|n| n.name == "Size=Large").unwrap().id;
+    if let NodeKind::Component { props, .. } = &mut doc.node_mut(large).unwrap().kind {
+        props.insert("Label".into(), PropDef::Text { default: "Large".into() });
+    }
+    let (_, warnings) = export(&doc, None).unwrap();
+    assert!(warnings.iter().any(|w| w.contains("different defaults")), "{warnings:?}");
+}