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/ui/tile_bar.rs

@@ -14,6 +14,7 @@ fn axis_display(axis: Axis) -> (&'static str, Color) {
         Axis::Row => ("↕", Color::Green),
         Axis::Column => ("↔", Color::Blue),
         Axis::Page => ("☰", Color::Magenta),
+        Axis::None => ("∅", Color::DarkGray),
     }
 }