test(harness): 补 _ladder_units 直测 + 澄清 gate 观测表 unit_id 口径注释

M1：quadrant_pair / gate_evidence 的 question_id 列注释与 write_* docstring
更正为承载 unit_id（single=question_id、pair=pair_id），提示逐题明细在
predictions 表溯源、按 pair_id join 真实 question 表会 join 不上。

M2：给 _ladder_units 补直接单测——纯非 AR 恒等（unit 序==原题序、
unit_id==question_id）、混格交错保持信息阶梯序（按单元最早出现下标重排、
pair 整锁）、且与 build_units 的 single-first 默认序显式区分（防阶梯序被污染）。

tests/unit/test_gate_block_unit.py

@@ -15,7 +15,7 @@ import pytest
 from app.harness.gate_ladder import BaselineCache, skill_hash
 from app.harness.inference import PREDICTIONS_SCHEMA, InferenceResult
 from app.harness.log import HarnessLog
-from app.harness.validate import validate_skill_local
+from app.harness.validate import _ladder_units, validate_skill_local
 from core.evolution import GateParams
 from core.types import GeneratedQuestion
 
@@ -136,6 +136,45 @@ def _make_mock_run_inference(
     return mock_fn, call_log
 
 
+class TestLadderUnits:
+    """_ladder_units：阶梯题序聚合为单元并保持信息阶梯序。"""
+
+    def test_pure_non_ar_is_identity(self) -> None:
+        """纯非 AR 输入 → 单元序恒等（unit 序==原题序、unit_id==question_id）。"""
+        ladder = [_single("s0"), _single("s1"), _single("s2")]
+        units = _ladder_units(ladder)
+        assert [u.kind for u in units] == ["single", "single", "single"]
+        # unit_id 逐一等于原 question_id，且顺序与输入完全一致
+        assert [u.unit_id for u in units] == ["s0", "s1", "s2"]
+        assert [q.question_id for u in units for q in u.questions] == ["s0", "s1", "s2"]
+
+    def test_mixed_preserves_ladder_order(self) -> None:
+        """混格交错输入 → 按单元最早出现下标重排，pair 整锁、不被 single-first 污染。"""
+        p1o, p1m = _pair("p1")
+        # 交错布置：pair 两成员分居 idx 1、3；single 分居 idx 0、2、4
+        ladder = [_single("s0"), p1o, _single("s1"), p1m, _single("s2")]
+        units = _ladder_units(ladder)
+        # 最早出现下标：s0=0, p1=min(1,3)=1, s1=2, s2=4 → 阶梯序 [s0,p1,s1,s2]
+        assert [u.unit_id for u in units] == ["s0", "p1", "s1", "s2"]
+        # pair 整锁为一个单元（含两成员），不被拆
+        p1_unit = next(u for u in units if u.unit_id == "p1")
+        assert p1_unit.kind == "pair"
+        assert {q.question_id for q in p1_unit.questions} == {"p1_o", "p1_m"}
+
+    def test_mixed_differs_from_build_units_default(self) -> None:
+        """混格重排必须纠正 build_units 的 single-first 顺序（否则阶梯序被污染）。"""
+        from app.harness.question_units import build_units
+
+        p1o, p1m = _pair("p1")
+        ladder = [_single("s0"), p1o, _single("s1"), p1m, _single("s2")]
+        default_order = [u.unit_id for u in build_units(ladder)]
+        ladder_order = [u.unit_id for u in _ladder_units(ladder)]
+        # build_units 把 pair 排到 single 之后；_ladder_units 恢复信息阶梯序
+        assert default_order == ["s0", "s1", "s2", "p1"]
+        assert ladder_order == ["s0", "p1", "s1", "s2"]
+        assert ladder_order != default_order
+
+
 @pytest.mark.asyncio
 async def test_gate_n_used_counts_units_not_questions(tmp_path: Path) -> None:
     """混格阶梯（1 pair + 2 single）→ n_used=3 单元，非 4 题。"""