fix(data): judge late newest rows as sparse; require SPY print only in the ETF panel

Tickers count as alive for 10 rows after their last bar, so a newest row
printed by a few early tickers is dropped until the rest arrive. The
benchmark-print session rule now applies only to the mixed US/HK ETF panel:
Yahoo's 510300.SS lags CN stocks by days and would stall the CN book.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/test_live_pipeline.py

@@ -42,6 +42,13 @@ class PanelRefreshTests(unittest.TestCase):
         self.assertTrue((returns > 0).all(), returns)
         self.assertAlmostEqual(merged["AAA"].iloc[0], 50.0)
 
+    def test_newest_row_printed_by_few_tickers_is_dropped(self):
+        panel = _panel({
+            "A": [1, 1, 1, np.nan], "B": [1, 1, 1, np.nan],
+            "C": [1, 1, 1, np.nan], "D": [1, 1, 1, 2],
+        })
+        self.assertNotIn(panel.index[-1], data_manager._drop_sparse_rows(panel).index)
+
     def test_rows_printed_by_a_minority_of_tickers_are_dropped(self):
         panel = _panel({
             "SPY": [1, 1, np.nan, 1, 1],