test(model,ui): add tests for precision and cat tree

Add unit tests for model precision and category tree rendering:

- `src/model/types.rs` : Added `formula_chain_preserves_full_precision` to
  ensure formulas use full `f64` precision for calculations, even when
  display is rounded.
- `src/ui/cat_tree.rs` : Added comprehensive tests for `build_cat_tree` ,
  covering empty models (virtual categories), expanded/collapsed states,
  and item rendering.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

src/model/types.rs

@@ -1136,6 +1136,54 @@ mod formula_tests {
         );
     }
 
+    // ── Precision guarantee: formulas compute at full f64 precision ────────
+
+    /// Display rounds to the view's decimal setting, but the underlying
+    /// model must keep full f64 precision so chained calculations don't
+    /// accumulate rounding errors.
+    #[test]
+    fn formula_chain_preserves_full_precision() {
+        let mut m = Model::new("Test");
+        m.add_category("Measure").unwrap();
+        if let Some(cat) = m.category_mut("Measure") {
+            cat.add_item("Price");
+            cat.add_item("Tax");
+            cat.add_item("Total");
+        }
+        // Price = 10.0, Tax = Price * 0.075 = 0.75, Total = Price + Tax = 10.75
+        m.set_cell(coord(&[("Measure", "Price")]), CellValue::Number(10.0));
+        m.add_formula(parse_formula("Tax = Price * 0.075", "Measure").unwrap());
+        m.add_formula(parse_formula("Total = Price + Tax", "Measure").unwrap());
+
+        let tax = m
+            .evaluate(&coord(&[("Measure", "Tax")]))
+            .and_then(|v| v.as_f64())
+            .unwrap();
+        let total = m
+            .evaluate(&coord(&[("Measure", "Total")]))
+            .and_then(|v| v.as_f64())
+            .unwrap();
+
+        // Full precision: Tax is exactly 0.75, Total is exactly 10.75
+        assert!(approx_eq(tax, 0.75), "Tax should be 0.75 (full prec), got {tax}");
+        assert!(
+            approx_eq(total, 10.75),
+            "Total should be 10.75 (full prec), got {total}"
+        );
+
+        // If display rounded Tax to 0 decimals (showing "1"), and the formula
+        // used that rounded value, Total would be 11 instead of 10.75.
+        // This proves the formula uses the raw f64, not the display string.
+        use crate::format::format_f64;
+        let tax_display = format_f64(tax, true, 0);
+        assert_eq!(tax_display, "1", "display rounds 0.75 → 1");
+        // But the computed Total is 10.75, not 10 + 1 = 11
+        assert!(
+            approx_eq(total, 10.75),
+            "Total must use full-precision Tax (0.75), not display-rounded (1)"
+        );
+    }
+
     /// Bug: remove_formula matches by target name alone, so removing "Profit"
     /// in "Measure" also destroys the "Profit" formula in "KPI".
     /// After targeted removal, the other category's formula must survive.