fix: Model::add_formula auto-adds target item; fix formula panel display

Model::add_formula now ensures the formula target exists as an item in
the target category (same fix as AddFormula effect, but at the model
level — covers file loading and headless paths).

Formula panel now shows raw formula text instead of debug-formatted
string with redundant target name and quotes.

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

src/model/types.rs

@@ -172,6 +172,11 @@ impl Model {
     }
 
     pub fn add_formula(&mut self, formula: 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) = self.categories.get_mut(&formula.target_category) {
+            cat.add_item(&formula.target);
+        }
         // Replace if same target within the same category
         if let Some(pos) = self.formulas.iter().position(|f| {
             f.target == formula.target && f.target_category == formula.target_category
@@ -731,6 +736,30 @@ mod model_tests {
         assert_eq!(result, Some(CellValue::Number(150.0)));
     }
 
+    /// Model::add_formula should add the formula target as an item to the
+    /// target category so the grid layout includes cells for it.
+    #[test]
+    fn add_formula_adds_target_item() {
+        use crate::formula::parse_formula;
+
+        let mut m = Model::new("Test");
+        m.add_category("Measure").unwrap();
+        m.add_category("Region").unwrap();
+        m.category_mut("Region").unwrap().add_item("East");
+        m.category_mut("Measure").unwrap().add_item("Revenue");
+        m.category_mut("Measure").unwrap().add_item("Cost");
+
+        m.add_formula(parse_formula("Profit = Revenue - Cost", "Measure").unwrap());
+
+        assert!(
+            m.category("Measure")
+                .unwrap()
+                .ordered_item_names()
+                .contains(&"Profit"),
+            "add_formula should add target item to category"
+        );
+    }
+
     #[test]
     fn evaluate_aggregated_no_hidden_delegates_to_evaluate() {
         let mut m = Model::new("Test");