Test: add unit tests co-located with the code they cover

- model/cell.rs: CellKey (sorting, get, with, without, matches_partial,
  display) and DataStore (set/get, overwrite, empty-removes-key,
  sum_matching, matching_cells, text exclusion)
- model/category.rs: item ids, deduplication, group assignment,
  top_level_groups, item_index insertion order
- formula/parser.rs: subtraction, WHERE clause, SUM/AVG, IF,
  numeric literal, chained arithmetic, error on missing =
- view/view.rs: auto-axis assignment, set_axis, categories_on,
  page_selection, group collapse toggle, hide/show, cycle_axis
  (all transitions + scroll/selection reset)

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

src/model/category.rs

@@ -119,3 +119,76 @@ impl Category {
         seen
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{Category, Group};
+
+    fn cat() -> Category {
+        Category::new(0, "Region")
+    }
+
+    #[test]
+    fn add_item_returns_sequential_ids() {
+        let mut c = cat();
+        let id0 = c.add_item("East");
+        let id1 = c.add_item("West");
+        assert_eq!(id0, 0);
+        assert_eq!(id1, 1);
+    }
+
+    #[test]
+    fn add_item_duplicate_returns_same_id() {
+        let mut c = cat();
+        let id1 = c.add_item("East");
+        let id2 = c.add_item("East");
+        assert_eq!(id1, id2);
+        assert_eq!(c.items.len(), 1);
+    }
+
+    #[test]
+    fn add_item_in_group_sets_group() {
+        let mut c = cat();
+        c.add_item_in_group("Jan", "Q1");
+        let item = c.item_by_name("Jan").unwrap();
+        assert_eq!(item.group.as_deref(), Some("Q1"));
+    }
+
+    #[test]
+    fn add_item_in_group_duplicate_returns_same_id() {
+        let mut c = cat();
+        let id1 = c.add_item_in_group("Jan", "Q1");
+        let id2 = c.add_item_in_group("Jan", "Q1");
+        assert_eq!(id1, id2);
+        assert_eq!(c.items.len(), 1);
+    }
+
+    #[test]
+    fn add_group_deduplicates() {
+        let mut c = cat();
+        c.add_group(Group::new("Q1"));
+        c.add_group(Group::new("Q1"));
+        assert_eq!(c.groups.len(), 1);
+    }
+
+    #[test]
+    fn top_level_groups_returns_unique_groups_in_insertion_order() {
+        let mut c = cat();
+        c.add_item_in_group("Jan", "Q1");
+        c.add_item_in_group("Feb", "Q1");
+        c.add_item_in_group("Apr", "Q2");
+        let groups = c.top_level_groups();
+        assert_eq!(groups, vec!["Q1", "Q2"]);
+    }
+
+    #[test]
+    fn item_index_reflects_insertion_order() {
+        let mut c = cat();
+        c.add_item("East");
+        c.add_item("West");
+        c.add_item("North");
+        assert_eq!(c.item_index("East"),  Some(0));
+        assert_eq!(c.item_index("West"),  Some(1));
+        assert_eq!(c.item_index("North"), Some(2));
+    }
+}