HEL-206: 修正公司间期末余额方向为转出=应收增加

翻转 _pair_net_change 符号，与往来层/页面口径一致；同步修正旧断言并新增期初200+转出100=期末300的双边守恒与浏览器方向冒烟。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

tests/test_calculation.py

@@ -470,8 +470,59 @@ class BalanceBasisTests(CalculationBase):
         balance = calculation.compute_pair_balance(
             self.connection, self.company_a, self.company_b, cutoff="2026-01-31"
         )
-        self.assertEqual("-80.00", balance["net_change"])
-        self.assertEqual("-80.00", balance["closing"])
+        # 甲转出 80 → 应收方向为正
+        self.assertEqual("80.00", balance["net_change"])
+        self.assertEqual("80.00", balance["closing"])
+
+    def test_opening_plus_outflow_increases_receivable(self) -> None:
+        """期初应收 200 + 本期转出垫付 100 → 期末应收 300；乙方对称为应付 300。"""
+        from test_matching import MatchingBase
+
+        item = calculation.create_opening_balance(
+            self.connection, self.company_a, self.company_b, "200", "期初应收", self.admin
+        )
+        calculation.confirm_opening_balance(
+            self.connection, item["id"], "确认", self.admin
+        )
+        helper = object.__new__(MatchingBase)
+        helper.connection = self.connection
+        row_a = helper.add_row(
+            self.company_a,
+            own_account="6222000000000001",
+            cp_account="6222000000000002",
+            expense="100.00",
+            at="2026-01-15T10:00:00",
+        )
+        matching.reconcile_rows(self.connection, [row_a])
+        row_b = helper.add_row(
+            self.company_b,
+            own_account="6222000000000002",
+            cp_account="6222000000000001",
+            income="100.00",
+            at="2026-01-15T11:00:00",
+        )
+        matching.reconcile_rows(self.connection, [row_b])
+
+        bal_a = calculation.compute_pair_balance(
+            self.connection, self.company_a, self.company_b, cutoff="2026-01-31"
+        )
+        self.assertEqual("full", bal_a["basis"])
+        self.assertEqual("200", bal_a["opening"])
+        self.assertEqual("100.00", bal_a["net_change"])
+        self.assertEqual("300.00", bal_a["closing"])
+
+        bal_b = calculation.compute_pair_balance(
+            self.connection, self.company_b, self.company_a, cutoff="2026-01-31"
+        )
+        self.assertEqual("full", bal_b["basis"])
+        self.assertEqual("-200", bal_b["opening"])
+        self.assertEqual("-100.00", bal_b["net_change"])
+        self.assertEqual("-300.00", bal_b["closing"])
+        # 双边守恒
+        self.assertEqual(
+            Decimal(bal_a["closing"]) + Decimal(bal_b["closing"]),
+            Decimal("0"),
+        )
 
 
 class CalculationApiTests(unittest.TestCase):