feat: rename Measure to _Measure (virtual), add fixed-point formula eval

_Measure is now a virtual category (CategoryKind::VirtualMeasure),
created automatically in Model::new() alongside _Index and _Dim.
Formula targets are added as items automatically by add_formula.

Formula evaluation uses a fixed-point cache: recompute_formulas()
iterates evaluation of all formula cells until values stabilize,
resolving refs through the cache for formula values and raw data
aggregation for non-formula values. This fixes formulas that
reference other measures when hidden dimensions are present.

evaluate_aggregated now checks the formula cache instead of
recursively evaluating formulas, breaking the dependency between
formula evaluation and aggregation.

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

src/ui/app.rs

@@ -239,6 +239,15 @@ impl App {
     /// Rebuild the grid layout from current model, view, and drill state.
     /// Note: `with_frozen_records` already handles pruning internally.
     pub fn rebuild_layout(&mut self) {
+        // Gather none_cats before mutable borrow for formula recomputation
+        let none_cats: Vec<String> = self
+            .model
+            .active_view()
+            .categories_on(crate::view::Axis::None)
+            .into_iter()
+            .map(String::from)
+            .collect();
+        self.model.recompute_formulas(&none_cats);
         let view = self.model.active_view();
         let frozen = self.drill_state.as_ref().map(|s| Rc::clone(&s.records));
         self.layout = GridLayout::with_frozen_records(&self.model, view, frozen);
@@ -314,13 +323,13 @@ impl App {
     }
 
     /// True when the model has no user-defined categories (show welcome/help).
-    /// Virtual categories (_Index, _Dim) are always present and don't count.
+    /// Virtual categories (_Index, _Dim, _Measure) are always present and don't count.
     pub fn is_empty_model(&self) -> bool {
         use crate::model::category::CategoryKind;
         self.model.categories.values().all(|c| {
             matches!(
                 c.kind,
-                CategoryKind::VirtualIndex | CategoryKind::VirtualDim
+                CategoryKind::VirtualIndex | CategoryKind::VirtualDim | CategoryKind::VirtualMeasure
             )
         })
     }
@@ -412,8 +421,8 @@ mod tests {
         app.apply_effects(effects);
     }
 
-    fn enter_advance_cmd(app: &App) -> crate::command::cmd::EnterAdvance {
-        use crate::command::cmd::CursorState;
+    fn enter_advance_cmd(app: &App) -> crate::command::cmd::navigation::EnterAdvance {
+        use crate::command::cmd::navigation::CursorState;
         let view = app.model.active_view();
         let cursor = CursorState {
             row: view.selected.0,
@@ -425,7 +434,7 @@ mod tests {
             visible_rows: 20,
             visible_cols: 8,
         };
-        crate::command::cmd::EnterAdvance { cursor }
+        crate::command::cmd::navigation::EnterAdvance { cursor }
     }
 
     #[test]