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/view/view.rs

@@ -125,3 +125,117 @@ impl View {
         self.col_offset = 0;
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::View;
+    use crate::view::Axis;
+
+    fn view_with_cats(cats: &[&str]) -> View {
+        let mut v = View::new("Test");
+        for &c in cats { v.on_category_added(c); }
+        v
+    }
+
+    #[test]
+    fn first_category_assigned_to_row() {
+        let v = view_with_cats(&["Region"]);
+        assert_eq!(v.axis_of("Region"), Axis::Row);
+    }
+
+    #[test]
+    fn second_category_assigned_to_column() {
+        let v = view_with_cats(&["Region", "Product"]);
+        assert_eq!(v.axis_of("Product"), Axis::Column);
+    }
+
+    #[test]
+    fn third_and_later_categories_assigned_to_page() {
+        let v = view_with_cats(&["Region", "Product", "Time", "Scenario"]);
+        assert_eq!(v.axis_of("Time"),     Axis::Page);
+        assert_eq!(v.axis_of("Scenario"), Axis::Page);
+    }
+
+    #[test]
+    fn set_axis_changes_assignment() {
+        let mut v = view_with_cats(&["Region", "Product"]);
+        v.set_axis("Region", Axis::Column);
+        assert_eq!(v.axis_of("Region"), Axis::Column);
+    }
+
+    #[test]
+    fn categories_on_returns_correct_list() {
+        let v = view_with_cats(&["Region", "Product", "Time"]);
+        assert_eq!(v.categories_on(Axis::Row),    vec!["Region"]);
+        assert_eq!(v.categories_on(Axis::Column), vec!["Product"]);
+        assert_eq!(v.categories_on(Axis::Page),   vec!["Time"]);
+    }
+
+    #[test]
+    fn axis_of_unknown_category_returns_unassigned() {
+        let v = View::new("Test");
+        assert_eq!(v.axis_of("Ghost"), Axis::Unassigned);
+    }
+
+    #[test]
+    fn page_selection_set_and_get() {
+        let mut v = view_with_cats(&["Region", "Product", "Time"]);
+        v.set_page_selection("Time", "Q1");
+        assert_eq!(v.page_selection("Time"),   Some("Q1"));
+        assert_eq!(v.page_selection("Region"), None);
+    }
+
+    #[test]
+    fn toggle_group_collapse_toggles_twice() {
+        let mut v = View::new("Test");
+        assert!(!v.is_group_collapsed("Time", "Q1"));
+        v.toggle_group_collapse("Time", "Q1");
+        assert!(v.is_group_collapsed("Time", "Q1"));
+        v.toggle_group_collapse("Time", "Q1");
+        assert!(!v.is_group_collapsed("Time", "Q1"));
+    }
+
+    #[test]
+    fn hide_and_show_item() {
+        let mut v = View::new("Test");
+        assert!(!v.is_hidden("Region", "East"));
+        v.hide_item("Region", "East");
+        assert!(v.is_hidden("Region", "East"));
+        v.show_item("Region", "East");
+        assert!(!v.is_hidden("Region", "East"));
+    }
+
+    #[test]
+    fn cycle_axis_row_to_column() {
+        let mut v = view_with_cats(&["Region", "Product"]);
+        v.cycle_axis("Region");
+        assert_eq!(v.axis_of("Region"), Axis::Column);
+    }
+
+    #[test]
+    fn cycle_axis_column_to_page() {
+        let mut v = view_with_cats(&["Region", "Product"]);
+        v.set_axis("Product", Axis::Column);
+        v.cycle_axis("Product");
+        assert_eq!(v.axis_of("Product"), Axis::Page);
+    }
+
+    #[test]
+    fn cycle_axis_page_to_row() {
+        let mut v = view_with_cats(&["Region", "Product", "Time"]);
+        v.cycle_axis("Time");
+        assert_eq!(v.axis_of("Time"), Axis::Row);
+    }
+
+    #[test]
+    fn cycle_axis_resets_scroll_and_selection() {
+        let mut v = view_with_cats(&["Region"]);
+        v.row_offset = 5;
+        v.col_offset = 3;
+        v.selected = (2, 2);
+        v.cycle_axis("Region");
+        assert_eq!(v.row_offset, 0);
+        assert_eq!(v.col_offset, 0);
+        assert_eq!(v.selected,   (0, 0));
+    }
+}