fix: idempotent ladder_rank migration for legacy gate_evidence tables

tests/unit/test_harness_observation.py

@@ -383,3 +383,39 @@ def test_write_epoch_report(tmp_path: Path) -> None:
     assert data["system_tool_action"] == "updated"
     assert data["momentum_updated_task_types"] == ["temporal", "causal"]
     assert data["best_val_acc"] == pytest.approx(0.88)
+
+
+def test_write_gate_evidence_migrates_legacy_block_idx_table(tmp_path) -> None:
+    """旧块序贯表(含 block_idx 无 ladder_rank)复用:幂等补列后写入成功(终审 C1 回归锁)。"""
+    import sqlite3
+
+    db = tmp_path / "harness.db"
+    conn = sqlite3.connect(db)
+    conn.execute(
+        "CREATE TABLE gate_evidence (run_id TEXT, timestamp TEXT, epoch INTEGER,"
+        " step INTEGER, question_id TEXT, task_type TEXT, block_idx INTEGER,"
+        " baseline_correct INTEGER, candidate_correct INTEGER, e_value REAL,"
+        " stop_reason TEXT)"
+    )
+    conn.commit()
+    conn.close()
+
+    write_gate_evidence(
+        str(db),
+        run_id="r1",
+        epoch=1,
+        step=0,
+        rows=[
+            {
+                "question_id": "q1",
+                "task_type": "Action Reasoning",
+                "ladder_rank": 0,
+                "baseline_correct": False,
+                "candidate_correct": True,
+                "e_value": 1.5,
+                "stop_reason": "",
+            }
+        ],
+    )
+    got = read_gate_evidence(str(db), run_id="r1")
+    assert len(got) == 1 and got[0]["ladder_rank"] == 0