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/ui/grid.rs

@@ -291,16 +291,27 @@ impl<'a> GridWidget<'a> {
                             break;
                         }
 
-                        let key = match layout.cell_key(ri, ci) {
-                            Some(k) => k,
-                            None => {
-                                x += COL_WIDTH;
-                                continue;
-                            }
+                        let (cell_str, value) = if layout.is_records_mode() {
+                            let s = layout.records_display(ri, ci).unwrap_or_default();
+                            // In records mode the value is a string, not aggregated
+                            let v = if !s.is_empty() {
+                                Some(crate::model::cell::CellValue::Text(s.clone()))
+                            } else {
+                                None
+                            };
+                            (s, v)
+                        } else {
+                            let key = match layout.cell_key(ri, ci) {
+                                Some(k) => k,
+                                None => {
+                                    x += COL_WIDTH;
+                                    continue;
+                                }
+                            };
+                            let value = self.model.evaluate_aggregated(&key, &layout.none_cats);
+                            let s = format_value(value.as_ref(), fmt_comma, fmt_decimals);
+                            (s, value)
                         };
-                        let value = self.model.evaluate_aggregated(&key, &layout.none_cats);
-
-                        let cell_str = format_value(value.as_ref(), fmt_comma, fmt_decimals);
                         let is_selected = ri == sel_row && ci == sel_col;
                         let is_search_match = !self.search_query.is_empty()
                             && cell_str