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/persistence/mod.rs

@@ -117,6 +117,7 @@ pub fn format_md(model: &Model) -> String {
                     Some(sel) => writeln!(out, "{}: page, {}", cat, sel).unwrap(),
                     None => writeln!(out, "{}: page", cat).unwrap(),
                 },
+                Axis::None => writeln!(out, "{}: none", cat).unwrap(),
             }
         }
         if !view.number_format.is_empty() {
@@ -315,6 +316,7 @@ pub fn parse_md(text: &str) -> Result<Model> {
                         let axis = match rest {
                             "row" => Axis::Row,
                             "column" => Axis::Column,
+                            "none" => Axis::None,
                             _ => continue,
                         };
                         view.axes.push((cat.to_string(), axis));