feat(model): add regular_category_names to Model

Updates `Model` to include `regular_category_names` , which returns
category names excluding virtual categories (_Index, _Dim). This allows
other parts of the application to distinguish between user-defined
categories and system-internal ones.

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

src/model/types.rs

@@ -250,6 +250,15 @@ impl Model {
         self.categories.keys().map(|s| s.as_str()).collect()
     }
 
+    /// Category names excluding virtual categories (_Index, _Dim).
+    pub fn regular_category_names(&self) -> Vec<&str> {
+        self.categories
+            .iter()
+            .filter(|(_, c)| c.kind.is_regular())
+            .map(|(name, _)| name.as_str())
+            .collect()
+    }
+
     /// Evaluate a computed value at a given key, considering formulas.
     /// Returns None when the cell is empty (no stored value, no applicable formula).
     pub fn evaluate(&self, key: &CellKey) -> Option<CellValue> {