fix: idempotent ladder_rank migration for legacy gate_evidence tables

app/harness/observation.py

@@ -358,6 +358,12 @@ def write_gate_evidence(
 
     with HarnessLog(db_path, run_id) as log:
         log.create_table("gate_evidence", _GATE_EVIDENCE_COLS)
+        # 幂等迁移(对齐 question_gen/run_store 先例):块序贯时代的旧表只有
+        # block_idx 列,CREATE TABLE IF NOT EXISTS 不补列,直接插 ladder_rank
+        # 会 OperationalError——为旧 workspace 复用补列,新表恒为 no-op。
+        cols = {r["name"] for r in log.query("PRAGMA table_info(gate_evidence)")}
+        if "ladder_rank" not in cols:
+            log.execute("ALTER TABLE gate_evidence ADD COLUMN ladder_rank INTEGER")
         for row in rows:
             log.insert("gate_evidence", {"epoch": epoch, "step": step, **row})
 

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