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/persistence/mod.rs

@@ -459,11 +459,15 @@ pub fn export_csv(model: &Model, view_name: &str, path: &Path) -> Result<()> {
         }
         let row_values: Vec<String> = (0..layout.col_count())
             .map(|ci| {
-                layout
-                    .cell_key(ri, ci)
-                    .and_then(|key| model.evaluate_aggregated(&key, &layout.none_cats))
-                    .map(|v| v.to_string())
-                    .unwrap_or_default()
+                if layout.is_records_mode() {
+                    layout.records_display(ri, ci).unwrap_or_default()
+                } else {
+                    layout
+                        .cell_key(ri, ci)
+                        .and_then(|key| model.evaluate_aggregated(&key, &layout.none_cats))
+                        .map(|v| v.to_string())
+                        .unwrap_or_default()
+                }
             })
             .collect();
         out.push_str(&row_values.join(","));