feat: add pair atomic accepted-bank writer helper

PairPendingBuffer collects pair members by pair_id and emits units;
write_accepted does tmp+os.replace atomic write; read_accepted rebuilds
units and drops disk-dangling orphans. Pure helper; on_accept wiring is
Phase 2 (real write point: adversarial_filter.write_final_bank).

tests/unit/test_pair_atomic_write.py

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+"""pair_atomic_writer 纯 helper 单元测试。
+
+覆盖三件核心行为：
+- PairPendingBuffer：single 直通成单元、pair 按 pair_id 收齐才 emit、
+  未配对孤儿可查询、绑定不一致（unit_hash 语义代偿）拒绝。
+- write_accepted：全量 tmp + os.replace 原子写，落盘无残留 .tmp。
+- read_accepted：build_units + validate_units 剔除磁盘悬挂孤儿（warn+drop）。
+
+测试用真实 GeneratedQuestion 构造（single + 合法 AR pair + 故意缺 mirror 的孤儿），
+跑完整 add → write → read 往返。
+"""
+
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from app.harness.pools import _q_to_dict
+from app.question_gen.pair_atomic_writer import (
+    PairPendingBuffer,
+    read_accepted,
+    write_accepted,
+)
+from core.types import GeneratedQuestion
+
+
+def _make_single(qid: str = "s1") -> GeneratedQuestion:
+    """构造一条真实 single 题（pair_id=None，question_role='single'）。"""
+    return GeneratedQuestion(
+        question_id=qid,
+        video_id="vid_A",
+        task_type="Action Reasoning",
+        question=f"{qid} 问题文本？",
+        options=("A. 甲", "B. 乙", "C. 丙", "D. 丁"),
+        answer="B",
+        source_nodes=("L2_0", "L3_1"),
+        difficulty="medium",
+    )
+
+
+def _make_pair(
+    pair_id: str = "p1",
+    *,
+    video_id: str = "vid_B",
+    flip_axis: str = "before_after",
+) -> tuple[GeneratedQuestion, GeneratedQuestion]:
+    """构造一对合法 AR 孪生对（original + mirror，共享 pair_id/flip_axis）。"""
+    original = GeneratedQuestion(
+        question_id=f"{pair_id}_o",
+        video_id=video_id,
+        task_type="Action Reasoning",
+        question="事件 X 发生在事件 Y 之前吗？",
+        options=("A. 是", "B. 否", "C. 无关", "D. 无法判断"),
+        answer="A",
+        source_nodes=("L2_2",),
+        difficulty="hard",
+        pair_id=pair_id,
+        question_role="pair_original",
+        flip_axis=flip_axis,
+    )
+    mirror = GeneratedQuestion(
+        question_id=f"{pair_id}_m",
+        video_id=video_id,
+        task_type="Action Reasoning",
+        question="事件 Y 发生在事件 X 之前吗？",
+        options=("A. 是", "B. 否", "C. 无关", "D. 无法判断"),
+        answer="B",
+        source_nodes=("L2_2",),
+        difficulty="hard",
+        pair_id=pair_id,
+        question_role="pair_mirror",
+        flip_axis=flip_axis,
+    )
+    return original, mirror
+
+
+def test_single_add_emits_unit_immediately() -> None:
+    """single 题 add 后立即返回 kind='single' 单元，不进 pending。"""
+    buf = PairPendingBuffer()
+    unit = buf.add(_make_single())
+    assert unit is not None
+    assert unit.kind == "single"
+    assert unit.size == 1
+    assert buf.pending_orphans() == []
+
+
+def test_pair_emits_only_when_both_arrived() -> None:
+    """pair 首个成员进 pending 返回 None，第二个到齐才 emit 一个 pair 单元。"""
+    buf = PairPendingBuffer()
+    original, mirror = _make_pair()
+
+    assert buf.add(original) is None
+    assert len(buf.pending_orphans()) == 1
+
+    unit = buf.add(mirror)
+    assert unit is not None
+    assert unit.kind == "pair"
+    assert unit.size == 2
+    assert buf.pending_orphans() == []
+
+
+def test_pair_completes_regardless_of_arrival_order() -> None:
+    """mirror 先到、original 后到也能正确成对。"""
+    buf = PairPendingBuffer()
+    original, mirror = _make_pair()
+
+    assert buf.add(mirror) is None
+    unit = buf.add(original)
+    assert unit is not None and unit.kind == "pair" and unit.size == 2
+
+
+def test_dangling_original_stays_in_pending() -> None:
+    """只落 original 未落 mirror → 悬挂在 pending，供调用方批次末尾检测。"""
+    buf = PairPendingBuffer()
+    original, _ = _make_pair()
+    assert buf.add(original) is None
+    orphans = buf.pending_orphans()
+    assert len(orphans) == 1
+    assert orphans[0].question_id == original.question_id
+
+
+def test_inconsistent_pair_binding_rejected() -> None:
+    """同 pair_id 两成员 flip_axis 不一致 → 拒绝（unit_hash 语义代偿）。"""
+    buf = PairPendingBuffer()
+    original, _ = _make_pair(flip_axis="before_after")
+    _, bad_mirror = _make_pair(flip_axis="left_right")
+
+    buf.add(original)
+    with pytest.raises(ValueError, match="绑定不一致"):
+        buf.add(bad_mirror)
+
+
+def test_same_role_pair_rejected() -> None:
+    """同 pair_id 两成员角色相同（两个 original）→ fail-fast 拒绝。"""
+    buf = PairPendingBuffer()
+    o1, _ = _make_pair()
+    o2, _ = _make_pair()
+    buf.add(o1)
+    with pytest.raises(ValueError):
+        buf.add(o2)
+
+
+def test_write_read_roundtrip_keeps_pair_and_single(tmp_path) -> None:
+    """add → write_accepted → read_accepted 往返：single 直通、pair 不拆、内容等价。"""
+    buf = PairPendingBuffer()
+    single = _make_single()
+    original, mirror = _make_pair()
+
+    units = []
+    for q in (single, original, mirror):
+        emitted = buf.add(q)
+        if emitted is not None:
+            units.append(emitted)
+    assert buf.pending_orphans() == []
+    assert len(units) == 2  # 1 single + 1 pair
+
+    out = tmp_path / "accepted" / "bank.json"
+    write_accepted(out, units)
+
+    assert out.exists()
+    assert not out.with_suffix(".tmp").exists()  # 原子写无残留
+
+    loaded = read_accepted(out)
+    kinds = sorted(u.kind for u in loaded)
+    assert kinds == ["pair", "single"]
+
+    loaded_pair = next(u for u in loaded if u.kind == "pair")
+    assert loaded_pair.size == 2
+    assert {q.question_id for q in loaded_pair.questions} == {
+        original.question_id,
+        mirror.question_id,
+    }
+    assert {q.flip_axis for q in loaded_pair.questions} == {"before_after"}
+
+
+def test_read_drops_disk_dangling_orphan(tmp_path, caplog) -> None:
+    """磁盘上某 pair 只有 original（缺 mirror）→ read_accepted warn+drop，不 raise。"""
+    single = _make_single()
+    original, _ = _make_pair()
+    # 手工写入"只落 P 未落 Q"的记录：single + 孤零 original
+    records = [_q_to_dict(single), _q_to_dict(original)]
+    out = tmp_path / "bank.json"
+    out.write_text(json.dumps(records, ensure_ascii=False, indent=2), encoding="utf-8")
+
+    loaded = read_accepted(out)
+
+    assert len(loaded) == 1
+    assert loaded[0].kind == "single"
+    assert loaded[0].questions[0].question_id == single.question_id
+
+
+def test_write_accepted_rejects_malformed_unit(tmp_path) -> None:
+    """validate_units 闸门：结构非法的 pair 单元（size≠2）落盘前 fail-fast。"""
+    from core.types import QuestionUnit
+
+    original, _ = _make_pair()
+    bad_unit = QuestionUnit("pair", "p1", "Action Reasoning", (original,))
+    with pytest.raises(ValueError):
+        write_accepted(tmp_path / "bad.json", [bad_unit])