feat: add records mode (long-format view) for drill-down

Implement records mode (long-format view) when drilling into aggregated cells.

Key changes:
- DrillIntoCell now creates views with _Index on Row and _Dim on Column
- GridLayout detects records mode and builds a records list instead of
  cross-product row/col items
- Added records_display() to render individual cell values in records mode
- GridWidget and CSV export updated to handle records mode rendering
- category_names() now includes virtual categories (_Index, _Dim)
- Tests updated to reflect virtual categories counting toward limits

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)

src/model/types.rs

@@ -63,7 +63,7 @@ impl Model {
 
     pub fn add_category(&mut self, name: impl Into<String>) -> Result<CategoryId> {
         let name = name.into();
-        // Count only regular categories for the limit
+        // Virtuals don't count against the regular category limit
         let regular_count = self
             .categories
             .values()
@@ -180,17 +180,8 @@ impl Model {
     }
 
     /// Return all category names
-    /// Names of all regular (non-virtual) categories.
+    /// Names of all categories (including virtual ones).
     pub fn category_names(&self) -> Vec<&str> {
-        self.categories
-            .iter()
-            .filter(|(_, c)| !c.kind.is_virtual())
-            .map(|(s, _)| s.as_str())
-            .collect()
-    }
-
-    /// Names of all categories including virtual ones.
-    pub fn all_category_names(&self) -> Vec<&str> {
         self.categories.keys().map(|s| s.as_str()).collect()
     }
 
@@ -432,7 +423,8 @@ mod model_tests {
         let id1 = m.add_category("Region").unwrap();
         let id2 = m.add_category("Region").unwrap();
         assert_eq!(id1, id2);
-        assert_eq!(m.category_names().len(), 1);
+        // Region + 2 virtuals (_Index, _Dim)
+        assert_eq!(m.category_names().len(), 3);
     }
 
     #[test]
@@ -1400,12 +1392,14 @@ mod five_category {
     #[test]
     fn five_categories_well_within_limit() {
         let m = build_model();
-        assert_eq!(m.category_names().len(), 5);
+        // 5 regular + 2 virtual (_Index, _Dim)
+        assert_eq!(m.category_names().len(), 7);
         let mut m2 = build_model();
         for i in 0..7 {
             m2.add_category(format!("Extra{i}")).unwrap();
         }
-        assert_eq!(m2.category_names().len(), 12);
+        // 12 regular + 2 virtuals = 14
+        assert_eq!(m2.category_names().len(), 14);
         assert!(m2.add_category("OneMore").is_err());
     }
 }