fix: AddFormula now adds target item to category

When adding a formula interactively, the target (e.g. "Margin") was
registered as a formula but never added as an item to the target
category. The grid layout never created cells for it, making the
formula invisible. Now AddFormula::apply calls add_item before
registering the formula.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ui/effect.rs

@@ -82,6 +82,11 @@ impl Effect for AddFormula {
     fn apply(&self, app: &mut App) {
         match crate::formula::parse_formula(&self.raw, &self.target_category) {
             Ok(formula) => {
+                // Ensure the formula target exists as an item in the target category
+                // so the grid layout includes cells for it.
+                if let Some(cat) = app.model.category_mut(&formula.target_category) {
+                    cat.add_item(&formula.target);
+                }
                 app.model.add_formula(formula);
             }
             Err(e) => {
@@ -1021,6 +1026,38 @@ mod tests {
         assert!(!app.model.formulas().is_empty());
     }
 
+    /// Regression: AddFormula must add the target item to the target category
+    /// so the grid layout includes cells for it. Without this, a formula like
+    /// `Margin = Profit / Revenue` would be registered but invisible in the grid.
+    #[test]
+    fn add_formula_adds_target_item_to_category() {
+        let mut app = test_app();
+        // "Margin" does not exist as an item in "Type" before adding the formula
+        assert!(!app
+            .model
+            .category("Type")
+            .unwrap()
+            .ordered_item_names()
+            .contains(&"Margin"));
+        AddFormula {
+            raw: "Margin = Food * 2".to_string(),
+            target_category: "Type".to_string(),
+        }
+        .apply(&mut app);
+        let items: Vec<&str> = app
+            .model
+            .category("Type")
+            .unwrap()
+            .ordered_item_names()
+            .into_iter()
+            .collect();
+        assert!(
+            items.contains(&"Margin"),
+            "formula target 'Margin' should be added as item to 'Type' category, got: {:?}",
+            items
+        );
+    }
+
     #[test]
     fn add_formula_invalid_sets_error_status() {
         let mut app = test_app();