test(io): add csv, format and table tests

test/csv_test.dart

@@ -0,0 +1,60 @@
+import 'package:cellar/cellar.dart';
+import 'package:test/test.dart';
+
+double numberOf(CellValue value) => (value as NumberValue).value;
+
+void main() {
+  group('csv parsing', () {
+    test('parses quoted fields with embedded commas and quotes', () {
+      final rows = parseCsv('a,"b,c","he said ""hi"""\n1,2,3');
+      expect(rows, [
+        ['a', 'b,c', 'he said "hi"'],
+        ['1', '2', '3'],
+      ]);
+    });
+
+    test('a trailing newline does not create an empty record', () {
+      final rows = parseCsv('a,b\nc,d\n');
+      expect(rows.length, 2);
+    });
+  });
+
+  group('csv import and compute', () {
+    test('imports literals and formulas then computes', () {
+      final sheet = Sheet();
+      importCsv(sheet, 'Name,Qty,Price,Total\nApple,3,2,=B2*C2\nPear,4,5,=B3*C3\n,,,=SUM(D2:D3)');
+      expect(numberOf(sheet.valueOf('D2')), 6);
+      expect(numberOf(sheet.valueOf('D3')), 20);
+      expect(numberOf(sheet.valueOf('D4')), 26);
+    });
+  });
+
+  group('csv round trip', () {
+    test('formulas survive export and re-import with equal results', () {
+      final original = Sheet();
+      original.setValue('A1', '10');
+      original.setValue('A2', '20');
+      original.setValue('A3', '30');
+      original.setValue('B1', '=SUM(A1:A3)');
+      original.setValue('B2', '=AVERAGE(A1:A3)');
+      original.setValue('B3', '=IF(B1>50, "high", "low")');
+
+      final formulaCsv = exportFormulas(original);
+      final reloaded = Sheet();
+      importCsv(reloaded, formulaCsv);
+
+      expect(exportFormulas(reloaded), formulaCsv);
+      expect(numberOf(reloaded.valueOf('B1')), 60);
+      expect(numberOf(reloaded.valueOf('B2')), 20);
+      expect(reloaded.valueOf('B3'), const TextValue('high'));
+    });
+
+    test('value export renders computed numbers', () {
+      final sheet = Sheet();
+      sheet.setValue('A1', '4');
+      sheet.setValue('B1', '=A1*A1');
+      final valueCsv = exportValues(sheet);
+      expect(valueCsv, '4,16');
+    });
+  });
+}