fix(HEL-356): preserve derived limit streaks

Co-authored-by: multica-agent <github@multica.ai>

tests/test_hel494_regressions.py

@@ -80,6 +80,50 @@ class LimitOverlayTests(unittest.TestCase):
         self.assertEqual(rows[0]["fd_amount"], 82000000)
         self.assertEqual(rows[0]["turnover_ratio"], 18.4)
 
+    def test_overlay_replaces_provisional_first_board_with_observed_streak(self) -> None:
+        mixin = DailyMarketMixin()
+        mixin._load_limit_lists = lambda trade_date: []
+        mixin.try_limit_pool = lambda trade_date: [
+            {"ts_code": "000737.SZ", "limit_times": 3}
+        ]
+
+        rows = mixin._overlay_board_fields(
+            [{"ts_code": "000737.SZ", "limit_times": 1, "limit_type": "U"}],
+            "20260909",
+        )
+
+        self.assertEqual(rows[0]["limit_times"], 3)
+
+    def test_daily_fallback_extends_yesterday_streak(self) -> None:
+        mixin = DailyMarketMixin()
+        mixin.query = lambda *args, **kwargs: []
+        mixin._load_limit_lists = lambda trade_date: []
+
+        rows = mixin._derive_limits(
+            "20260909",
+            [
+                {
+                    "ts_code": "000737.SZ",
+                    "trade_date": "20260909",
+                    "close": 11.0,
+                    "high": 11.0,
+                    "pct_chg": 10.0,
+                    "amount": 100000,
+                }
+            ],
+            price_limits=[
+                {"ts_code": "000737.SZ", "up_limit": 11.0, "down_limit": 9.0}
+            ],
+            basic_rows=[
+                {"ts_code": "000737.SZ", "name": "北方铜业", "industry": "有色"}
+            ],
+            previous_limit_rows=[
+                {"ts_code": "000737.SZ", "limit_times": 2}
+            ],
+        )
+
+        self.assertEqual(rows[0]["limit_times"], 3)
+
 
 class ShenwanRealtimeSourceTests(unittest.TestCase):
     def test_transport_refuses_rt_sw_k(self) -> None: