Test: add unit tests co-located with the code they cover

- model/cell.rs: CellKey (sorting, get, with, without, matches_partial,
  display) and DataStore (set/get, overwrite, empty-removes-key,
  sum_matching, matching_cells, text exclusion)
- model/category.rs: item ids, deduplication, group assignment,
  top_level_groups, item_index insertion order
- formula/parser.rs: subtraction, WHERE clause, SUM/AVG, IF,
  numeric literal, chained arithmetic, error on missing =
- view/view.rs: auto-axis assignment, set_axis, categories_on,
  page_selection, group collapse toggle, hide/show, cycle_axis
  (all transitions + scroll/selection reset)

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

src/formula/parser.rs

@@ -304,3 +304,60 @@ fn parse_comparison(tokens: &[Token], pos: &mut usize) -> Result<Expr> {
     let right = parse_add_sub(tokens, pos)?;
     Ok(Expr::BinOp(op.to_string(), Box::new(left), Box::new(right)))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::parse_formula;
+    use crate::formula::{Expr, AggFunc};
+
+    #[test]
+    fn parse_simple_subtraction() {
+        let f = parse_formula("Profit = Revenue - Cost", "Measure").unwrap();
+        assert_eq!(f.target, "Profit");
+        assert_eq!(f.target_category, "Measure");
+        assert!(matches!(f.expr, Expr::BinOp(ref op, _, _) if op == "-"));
+    }
+
+    #[test]
+    fn parse_where_clause() {
+        let f = parse_formula("EastRev = Revenue WHERE Region = \"East\"", "Measure").unwrap();
+        assert_eq!(f.target, "EastRev");
+        let filter = f.filter.as_ref().unwrap();
+        assert_eq!(filter.category, "Region");
+        assert_eq!(filter.item, "East");
+    }
+
+    #[test]
+    fn parse_sum_aggregation() {
+        let f = parse_formula("Total = SUM(Revenue)", "Measure").unwrap();
+        assert!(matches!(f.expr, Expr::Agg(AggFunc::Sum, _, _)));
+    }
+
+    #[test]
+    fn parse_avg_aggregation() {
+        let f = parse_formula("Avg = AVG(Revenue)", "Measure").unwrap();
+        assert!(matches!(f.expr, Expr::Agg(AggFunc::Avg, _, _)));
+    }
+
+    #[test]
+    fn parse_if_expression() {
+        let f = parse_formula("Capped = IF(Revenue > 1000, 1000, Revenue)", "Measure").unwrap();
+        assert!(matches!(f.expr, Expr::If(_, _, _)));
+    }
+
+    #[test]
+    fn parse_numeric_literal() {
+        let f = parse_formula("Fixed = 42", "Measure").unwrap();
+        assert!(matches!(f.expr, Expr::Number(n) if (n - 42.0).abs() < 1e-10));
+    }
+
+    #[test]
+    fn parse_chained_arithmetic() {
+        parse_formula("X = (A + B) * (C - D)", "Cat").unwrap();
+    }
+
+    #[test]
+    fn parse_missing_equals_returns_error() {
+        assert!(parse_formula("BadFormula Revenue Cost", "Cat").is_err());
+    }
+}