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/model/types.rs

@@ -181,10 +181,13 @@ 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);
+        // For non-_Measure target categories, add the target as a category item
+        // so it appears in the grid. _Measure targets are dynamically included
+        // via measure_item_names().
+        if formula.target_category != "_Measure" {
+            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| {
@@ -264,6 +267,36 @@ impl Model {
         self.categories.keys().map(|s| s.as_str()).collect()
     }
 
+    /// Effective item names for the _Measure category: the union of items
+    /// explicitly in the category plus all formula targets (for formulas
+    /// targeting _Measure). Preserves insertion order of explicit items,
+    /// then appends formula targets not already present.
+    pub fn measure_item_names(&self) -> Vec<String> {
+        let mut names: Vec<String> = self
+            .category("_Measure")
+            .map(|c| c.ordered_item_names().iter().map(|s| s.to_string()).collect())
+            .unwrap_or_default();
+        for f in &self.formulas {
+            if f.target_category == "_Measure" && !names.iter().any(|n| n == &f.target) {
+                names.push(f.target.clone());
+            }
+        }
+        names
+    }
+
+    /// Effective item names for any category. For _Measure, this includes
+    /// formula targets dynamically. For all others, delegates to
+    /// `ordered_item_names()`.
+    pub fn effective_item_names(&self, cat_name: &str) -> Vec<String> {
+        if cat_name == "_Measure" {
+            self.measure_item_names()
+        } else {
+            self.category(cat_name)
+                .map(|c| c.ordered_item_names().iter().map(|s| s.to_string()).collect())
+                .unwrap_or_default()
+        }
+    }
+
     /// Category names excluding virtual categories (_Index, _Dim).
     pub fn regular_category_names(&self) -> Vec<&str> {
         self.categories
@@ -450,6 +483,13 @@ impl Model {
                     return Some(cat_name.as_str());
                 }
             }
+            // Fall back to formula targets: if a formula defines this item
+            // in _Measure, resolve it there.
+            for f in model.formulas() {
+                if f.target == item_name && f.target_category == "_Measure" {
+                    return Some("_Measure");
+                }
+            }
             None
         }
 
@@ -616,6 +656,13 @@ impl Model {
                     return Some(cat_name.as_str());
                 }
             }
+            // Fall back to formula targets: if a formula defines this item
+            // in _Measure, resolve it there.
+            for f in model.formulas() {
+                if f.target == item_name && f.target_category == "_Measure" {
+                    return Some("_Measure");
+                }
+            }
             None
         }
 
@@ -982,14 +1029,14 @@ 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.
+    /// Formula targets should appear in measure_item_names() without being
+    /// explicitly added to the _Measure category. _Measure is dynamically
+    /// computed from data items + formula targets.
     #[test]
-    fn add_formula_adds_target_item() {
+    fn measure_item_names_includes_formula_targets() {
         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");
@@ -997,12 +1044,22 @@ mod model_tests {
 
         m.add_formula(parse_formula("Profit = Revenue - Cost", "_Measure").unwrap());
 
+        let names = m.measure_item_names();
         assert!(
-            m.category("_Measure")
+            names.contains(&"Revenue".to_string()),
+            "measure_item_names should include data items"
+        );
+        assert!(
+            names.contains(&"Profit".to_string()),
+            "measure_item_names should include formula targets"
+        );
+        // Formula target should NOT be in the category's own items
+        assert!(
+            !m.category("_Measure")
                 .unwrap()
                 .ordered_item_names()
                 .contains(&"Profit"),
-            "add_formula should add target item to category"
+            "formula targets should not be added to the category directly"
         );
     }