feat: add Axis::None for hidden dimensions with implicit aggregation

Categories on the None axis are excluded from the grid and cell keys.
When evaluating cells, values across hidden dimensions are aggregated
using a per-measure function (default SUM). Adds evaluate_aggregated
to Model, none_cats to GridLayout, and 'n' shortcut in TileSelect.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/view/layout.rs

@@ -27,6 +27,8 @@ pub struct GridLayout {
     pub page_coords: Vec<(String, String)>,
     pub row_items: Vec<AxisEntry>,
     pub col_items: Vec<AxisEntry>,
+    /// Categories on `Axis::None` — hidden, implicitly aggregated.
+    pub none_cats: Vec<String>,
 }
 
 impl GridLayout {
@@ -46,6 +48,11 @@ impl GridLayout {
             .into_iter()
             .map(String::from)
             .collect();
+        let none_cats: Vec<String> = view
+            .categories_on(Axis::None)
+            .into_iter()
+            .map(String::from)
+            .collect();
 
         let page_coords = page_cats
             .iter()
@@ -77,6 +84,7 @@ impl GridLayout {
             page_coords,
             row_items,
             col_items,
+            none_cats,
         }
     }