improvise-kb1d: refuse renaming a formula outside _Measure

A regular category's formula target is a real item, and replace_formula added
the new target without removing the old, stranding it as an empty item and
appending the new one. Until formulas have a stable identity (improvise-sboz),
a target change outside _Measure is refused and the editor stays open.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

crates/improvise-command/src/ui/app.rs

@@ -4541,4 +4541,39 @@ mod formula_edit_tests {
             vec![PROFIT, TAX, MARGIN, "Levy = Revenue * 0.3"]
         );
     }
+
+    /// improvise-kb1d: a Region-targeted formula at index 1. Renaming it would
+    /// strand `Total` as an empty item, so the edit refuses and stays open.
+    #[test]
+    fn renaming_a_formula_outside_measure_refuses_and_keeps_the_editor() {
+        let mut wb = Workbook::new("T");
+        wb.add_item("_Measure", "Revenue").unwrap();
+        wb.add_item("_Measure", "Cost").unwrap();
+        wb.add_category("Region").unwrap();
+        wb.add_item("Region", "East").unwrap();
+        wb.add_item("Region", "West").unwrap();
+        for (raw, cat) in [
+            (PROFIT, "_Measure"),
+            ("Total = East + West", "Region"),
+            (MARGIN, "_Measure"),
+        ] {
+            wb.model.add_formula(parse_formula(raw, cat).unwrap());
+        }
+        let mut app = App::new(wb, None);
+        app.view_state.formula_panel_open = true;
+        app.view_state.mode = AppMode::FormulaPanel;
+        app.view_state.formula_cursor = 1;
+        let regions = |app: &App| {
+            let region = app.model_state.workbook.model.category("Region").unwrap();
+            region.ordered_item_names().iter().map(|s| s.to_string()).collect::<Vec<_>>()
+        };
+        let before = regions(&app);
+        open_editor(&mut app);
+        retype(&mut app, "Sum = East + West");
+        press(&mut app, KeyCode::Enter);
+        assert_eq!(raws(&app), vec![PROFIT, "Total = East + West", MARGIN]);
+        assert_eq!(regions(&app), before, "no ghost item, nothing appended");
+        assert!(in_editor(&app), "{:?}", app.view_state.mode);
+        assert_eq!(buffer(&app), Some("Sum = East + West"));
+    }
 }

crates/improvise-core/src/model/types.rs

@@ -425,7 +425,14 @@ impl Model {
     /// let a new target displace another formula — `context/code/core.md` §18.
     pub fn replace_formula(&mut self, id: &FormulaId, formula: Formula) -> Result<()> {
         let new_id = formula.id();
-        if new_id != *id && self.formula(&new_id).is_some() {
+        let renames = new_id != *id;
+        // A regular category's target is a real item; renaming it is sboz's (kb1d).
+        if renames && !CategoryKind::of_name(&formula.target_category).is_measure() {
+            return Err(anyhow!(
+                "Renaming a formula outside _Measure is not supported yet; delete and re-add it"
+            ));
+        }
+        if renames && self.formula(&new_id).is_some() {
             return Err(anyhow!("A formula for {} already exists", formula.target));
         }
         let pos = self
@@ -1935,6 +1942,27 @@ mod formula_tests {
         assert_eq!(targets(&m), vec!["Profit", "Margin"]);
     }
 
+    /// improvise-kb1d: a Region-targeted formula at index 1. A rename would
+    /// leave `Total` as an empty item and append `Sum`, so it is refused.
+    #[test]
+    fn replace_formula_refuses_to_rename_a_target_outside_measure() {
+        let mut m = three_formulas();
+        m.formulas.insert(1, parse_formula("Total = East + West", "Region").unwrap());
+        let shape = |m: &Model| {
+            let region = m.category("Region").unwrap().ordered_item_names();
+            let owned: Vec<String> = region.iter().map(|s| s.to_string()).collect();
+            (targets(m).iter().map(|s| s.to_string()).collect::<Vec<_>>(), owned)
+        };
+        let before = shape(&m);
+        let id = m.formulas[1].id();
+        let sum = parse_formula("Sum = East + West", "Region").unwrap();
+        assert!(m.replace_formula(&id, sum).is_err());
+        assert_eq!(shape(&m), before, "nothing moved, nothing added");
+        let edit = parse_formula("Total = East - West", "Region").unwrap();
+        m.replace_formula(&id, edit).unwrap();
+        assert_eq!(targets(&m), vec!["Profit", "Total", "Tax", "Margin"]);
+    }
+
     #[test]
     fn a_formula_mentions_what_its_expression_or_its_where_names() {
         let margin = parse_formula("Margin = Profit / Revenue", "_Measure").unwrap();