feat: add view history navigation and drill-into-cell

Add view navigation history with back/forward stacks (bound to < and >).

Introduce CategoryKind enum to distinguish regular categories from
virtual ones (_Index, _Dim) that are synthesized at query time.

Add DrillIntoCell command that creates a drill view showing raw data
for an aggregated cell, expanding categories on Axis::None into Row
and Column axes while filtering by the cell's fixed coordinates.

Virtual categories default to Axis::None and are automatically added
to all views when the model is initialized.

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

src/view/types.rs

@@ -41,15 +41,20 @@ impl View {
 
     pub fn on_category_added(&mut self, cat_name: &str) {
         if !self.category_axes.contains_key(cat_name) {
-            // Auto-assign: first → Row, second → Column, rest → Page
-            let rows = self.categories_on(Axis::Row).len();
-            let cols = self.categories_on(Axis::Column).len();
-            let axis = if rows == 0 {
-                Axis::Row
-            } else if cols == 0 {
-                Axis::Column
+            // Virtual categories (names starting with `_`) default to Axis::None.
+            // Regular categories auto-assign: first → Row, second → Column, rest → Page.
+            let axis = if cat_name.starts_with('_') {
+                Axis::None
             } else {
-                Axis::Page
+                let rows = self.categories_on(Axis::Row).len();
+                let cols = self.categories_on(Axis::Column).len();
+                if rows == 0 {
+                    Axis::Row
+                } else if cols == 0 {
+                    Axis::Column
+                } else {
+                    Axis::Page
+                }
             };
             self.category_axes.insert(cat_name.to_string(), axis);
         }