feat(model): introduce virtual '_Measure' category for formulas

Refactor formula handling to use a virtual '_Measure' category.

- Formulas now default to targeting '_Measure' if no category is specified.
- '_Measure' items are dynamically computed from existing data items and
  formula targets, preventing redundant item storage.
- Updated persistence to handle '_Measure' formulas and items correctly in
  Markdown.
- Updated UI and model to use 'effective_item_names' for layout and item
  resolution.
- Updated tests to reflect the new '_Measure' behavior.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

src/ui/effect.rs

@@ -82,10 +82,13 @@ 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);
+                // For non-_Measure targets, add the item to the category so it
+                // appears in the grid. _Measure targets are dynamically included
+                // via Model::measure_item_names().
+                if formula.target_category != "_Measure" {
+                    if let Some(cat) = app.model.category_mut(&formula.target_category) {
+                        cat.add_item(&formula.target);
+                    }
                 }
                 app.model.add_formula(formula);
             }
@@ -1058,6 +1061,34 @@ mod tests {
         );
     }
 
+    /// Formula targets in _Measure should appear in measure_item_names()
+    /// dynamically, without being added to the category's own items.
+    #[test]
+    fn add_formula_to_measure_shows_in_effective_items() {
+        let mut app = test_app();
+        AddFormula {
+            raw: "Margin = Food * 2".to_string(),
+            target_category: "_Measure".to_string(),
+        }
+        .apply(&mut app);
+        // Should appear in effective_item_names (used by layout)
+        let effective = app.model.effective_item_names("_Measure");
+        assert!(
+            effective.contains(&"Margin".to_string()),
+            "formula target 'Margin' should appear in effective _Measure items, got: {:?}",
+            effective
+        );
+        // Should NOT be in the category's own items
+        assert!(
+            !app.model
+                .category("_Measure")
+                .unwrap()
+                .ordered_item_names()
+                .contains(&"Margin"),
+            "formula target should not be added directly to _Measure category items"
+        );
+    }
+
     #[test]
     fn add_formula_invalid_sets_error_status() {
         let mut app = test_app();