feat: 2D multi-level grid headers with repeat suppression

Column headers now render one row per column category instead of
joining with '/'. Row headers render one sub-column per row category.
Repeat suppression hides labels when the prefix is unchanged from
the previous row/column.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/app.rs

@@ -219,9 +219,11 @@ impl App {
             (KeyCode::Char('d'), KeyModifiers::CONTROL) => { self.scroll_rows(5); }
             (KeyCode::Char('u'), KeyModifiers::CONTROL) => { self.scroll_rows(-5); }
 
+            // Enter = advance (down, wrapping to top of next column)
+            (KeyCode::Enter, _) => { self.enter_advance(); }
+
             // ── Editing ────────────────────────────────────────────────────
-            (KeyCode::Enter, _)
-            | (KeyCode::Char('i'), KeyModifiers::NONE)
+            (KeyCode::Char('i'), KeyModifiers::NONE)
             | (KeyCode::Char('a'), KeyModifiers::NONE) => {
                 let current = self.selected_cell_key()
                     .and_then(|k| self.model.get_cell(&k).cloned())
@@ -1107,10 +1109,33 @@ impl App {
         self.mode = AppMode::ImportWizard;
     }
 
+    /// Advance selection down one row; when at the last row, wrap to row 0 of
+    /// the next column (typewriter-style). Does nothing if the grid is empty.
+    pub fn enter_advance(&mut self) {
+        let (row_max, col_max) = {
+            let layout = GridLayout::new(&self.model, self.model.active_view());
+            (layout.row_count().saturating_sub(1), layout.col_count().saturating_sub(1))
+        };
+        let view = self.model.active_view_mut();
+        let (r, c) = view.selected;
+        let (nr, nc) = if r < row_max {
+            (r + 1, c)
+        } else if c < col_max {
+            (0, c + 1)
+        } else {
+            (r, c) // already at bottom-right; stay
+        };
+        view.selected = (nr, nc);
+        if nr < view.row_offset { view.row_offset = nr; }
+        if nr >= view.row_offset + 20 { view.row_offset = nr.saturating_sub(19); }
+        if nc < view.col_offset { view.col_offset = nc; }
+        if nc >= view.col_offset + 8 { view.col_offset = nc.saturating_sub(7); }
+    }
+
     /// Hint text for the status bar (context-sensitive)
     pub fn hint_text(&self) -> &'static str {
         match &self.mode {
-            AppMode::Normal => "hjkl:nav  i:edit  x:clear  /:search  F/C/V:panels  T:tiles  [:]:page  ::cmd",
+            AppMode::Normal => "hjkl:nav  Enter:advance  i:edit  x:clear  /:search  F/C/V:panels  T:tiles  [:]:page  ::cmd",
             AppMode::Editing { .. } => "Enter:commit  Esc:cancel",
             AppMode::FormulaPanel => "n:new  d:delete  jk:nav  Esc:back",
             AppMode::FormulaEdit { .. } => "Enter:save  Esc:cancel  — type: Name = expression",
@@ -1126,3 +1151,45 @@ impl App {
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::model::Model;
+
+    fn two_col_model() -> App {
+        let mut m = Model::new("T");
+        m.add_category("Row").unwrap();    // → Row axis
+        m.add_category("Col").unwrap();    // → Column axis
+        m.category_mut("Row").unwrap().add_item("A");
+        m.category_mut("Row").unwrap().add_item("B");
+        m.category_mut("Row").unwrap().add_item("C");
+        m.category_mut("Col").unwrap().add_item("X");
+        m.category_mut("Col").unwrap().add_item("Y");
+        App::new(m, None)
+    }
+
+    #[test]
+    fn enter_advance_moves_down_within_column() {
+        let mut app = two_col_model();
+        app.model.active_view_mut().selected = (0, 0);
+        app.enter_advance();
+        assert_eq!(app.model.active_view().selected, (1, 0));
+    }
+
+    #[test]
+    fn enter_advance_wraps_to_top_of_next_column() {
+        let mut app = two_col_model();
+        // row_max = 2 (A,B,C), col 0 → should wrap to (0, 1)
+        app.model.active_view_mut().selected = (2, 0);
+        app.enter_advance();
+        assert_eq!(app.model.active_view().selected, (0, 1));
+    }
+
+    #[test]
+    fn enter_advance_stays_at_bottom_right() {
+        let mut app = two_col_model();
+        app.model.active_view_mut().selected = (2, 1);
+        app.enter_advance();
+        assert_eq!(app.model.active_view().selected, (2, 1));
+    }
+}