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/axis.rs

@@ -6,6 +6,7 @@ pub enum Axis {
     Row,
     Column,
     Page,
+    None,
 }
 
 impl std::fmt::Display for Axis {
@@ -14,6 +15,7 @@ impl std::fmt::Display for Axis {
             Axis::Row => write!(f, "Row ↕"),
             Axis::Column => write!(f, "Col ↔"),
             Axis::Page => write!(f, "Page ☰"),
+            Axis::None => write!(f, "None ∅"),
         }
     }
 }