refactor: use data_col_to_visual via group_for helpers, add column group toggle

Add row_group_for/col_group_for to GridLayout, replacing inline
backward-search logic. Refactor grid renderer to use col_group_for
instead of pre-filtering col_items. Add gz keybinding for column
group collapse toggle, symmetric with z for rows.

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

src/view/layout.rs

@@ -188,6 +188,40 @@ impl GridLayout {
         }
         None
     }
+
+    /// Find the group containing the Nth data row.
+    /// Returns `(cat_name, group_name)` of the nearest preceding GroupHeader.
+    pub fn row_group_for(&self, data_row: usize) -> Option<(String, String)> {
+        let vi = self.data_row_to_visual(data_row)?;
+        self.row_items[..vi].iter().rev().find_map(|e| {
+            if let AxisEntry::GroupHeader {
+                cat_name,
+                group_name,
+            } = e
+            {
+                Some((cat_name.clone(), group_name.clone()))
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Find the group containing the Nth data column.
+    /// Returns `(cat_name, group_name)` of the nearest preceding GroupHeader.
+    pub fn col_group_for(&self, data_col: usize) -> Option<(String, String)> {
+        let vi = self.data_col_to_visual(data_col)?;
+        self.col_items[..vi].iter().rev().find_map(|e| {
+            if let AxisEntry::GroupHeader {
+                cat_name,
+                group_name,
+            } = e
+            {
+                Some((cat_name.clone(), group_name.clone()))
+            } else {
+                None
+            }
+        })
+    }
 }
 
 /// Expand a single category into `AxisEntry` values, given a coordinate prefix.
@@ -483,4 +517,91 @@ mod tests {
         assert_eq!(layout.data_row_to_visual(1), Some(3)); // Apr is at visual index 3
         assert_eq!(layout.data_row_to_visual(2), None);
     }
+
+    #[test]
+    fn data_col_to_visual_skips_headers() {
+        let mut m = Model::new("T");
+        m.add_category("Type").unwrap(); // Row
+        m.add_category("Month").unwrap(); // Column
+        m.category_mut("Type").unwrap().add_item("Food");
+        m.category_mut("Month")
+            .unwrap()
+            .add_item_in_group("Jan", "Q1");
+        m.category_mut("Month")
+            .unwrap()
+            .add_item_in_group("Apr", "Q2");
+        let layout = GridLayout::new(&m, m.active_view());
+        // col_items: [GroupHeader(Q1), DataItem(Jan), GroupHeader(Q2), DataItem(Apr)]
+        assert_eq!(layout.data_col_to_visual(0), Some(1));
+        assert_eq!(layout.data_col_to_visual(1), Some(3));
+        assert_eq!(layout.data_col_to_visual(2), None);
+    }
+
+    #[test]
+    fn row_group_for_finds_enclosing_group() {
+        let mut m = Model::new("T");
+        m.add_category("Month").unwrap();
+        m.add_category("Type").unwrap();
+        m.category_mut("Month")
+            .unwrap()
+            .add_item_in_group("Jan", "Q1");
+        m.category_mut("Month")
+            .unwrap()
+            .add_item_in_group("Apr", "Q2");
+        m.category_mut("Type").unwrap().add_item("Food");
+        let layout = GridLayout::new(&m, m.active_view());
+        assert_eq!(
+            layout.row_group_for(0),
+            Some(("Month".to_string(), "Q1".to_string()))
+        );
+        assert_eq!(
+            layout.row_group_for(1),
+            Some(("Month".to_string(), "Q2".to_string()))
+        );
+    }
+
+    #[test]
+    fn row_group_for_returns_none_for_ungrouped() {
+        let mut m = Model::new("T");
+        m.add_category("Type").unwrap();
+        m.add_category("Month").unwrap();
+        m.category_mut("Type").unwrap().add_item("Food");
+        m.category_mut("Month").unwrap().add_item("Jan");
+        let layout = GridLayout::new(&m, m.active_view());
+        assert_eq!(layout.row_group_for(0), None);
+    }
+
+    #[test]
+    fn col_group_for_finds_enclosing_group() {
+        let mut m = Model::new("T");
+        m.add_category("Type").unwrap(); // Row
+        m.add_category("Month").unwrap(); // Column
+        m.category_mut("Type").unwrap().add_item("Food");
+        m.category_mut("Month")
+            .unwrap()
+            .add_item_in_group("Jan", "Q1");
+        m.category_mut("Month")
+            .unwrap()
+            .add_item_in_group("Apr", "Q2");
+        let layout = GridLayout::new(&m, m.active_view());
+        assert_eq!(
+            layout.col_group_for(0),
+            Some(("Month".to_string(), "Q1".to_string()))
+        );
+        assert_eq!(
+            layout.col_group_for(1),
+            Some(("Month".to_string(), "Q2".to_string()))
+        );
+    }
+
+    #[test]
+    fn col_group_for_returns_none_for_ungrouped() {
+        let mut m = Model::new("T");
+        m.add_category("Type").unwrap();
+        m.add_category("Month").unwrap();
+        m.category_mut("Type").unwrap().add_item("Food");
+        m.category_mut("Month").unwrap().add_item("Jan");
+        let layout = GridLayout::new(&m, m.active_view());
+        assert_eq!(layout.col_group_for(0), None);
+    }
 }