refactor: assert warn sink, rename disk-pair sifter, trim WHAT docstrings

Capture loguru warning via project sink pattern and assert the dangling-orphan
warning is emitted; rename _sift_disk_pairs to _keep_complete_disk_pairs; drop
pure-WHAT docstrings on __init__/pending_orphans while keeping WHY notes.

tests/unit/test_pair_atomic_write.py

@@ -15,6 +15,7 @@ from __future__ import annotations
 import json
 
 import pytest
+from loguru import logger
 
 from app.harness.pools import _q_to_dict
 from app.question_gen.pair_atomic_writer import (
@@ -179,7 +180,7 @@ def test_write_read_roundtrip_keeps_pair_and_single(tmp_path) -> None:
     assert {q.flip_axis for q in loaded_pair.questions} == {"before_after"}
 
 
-def test_read_drops_disk_dangling_orphan(tmp_path, caplog) -> None:
+def test_read_drops_disk_dangling_orphan(tmp_path) -> None:
     """磁盘上某 pair 只有 original（缺 mirror）→ read_accepted warn+drop，不 raise。"""
     single = _make_single()
     original, _ = _make_pair()
@@ -188,11 +189,19 @@ def test_read_drops_disk_dangling_orphan(tmp_path, caplog) -> None:
     out = tmp_path / "bank.json"
     out.write_text(json.dumps(records, ensure_ascii=False, indent=2), encoding="utf-8")
 
-    loaded = read_accepted(out)
+    # loguru 不走标准 logging，用项目既定 sink 捕获模式（见 test_inference_pair_aggregate）
+    captured: list[str] = []
+    sink_id = logger.add(captured.append, level="WARNING", format="{message}")
+    try:
+        loaded = read_accepted(out)
+    finally:
+        logger.remove(sink_id)
 
     assert len(loaded) == 1
     assert loaded[0].kind == "single"
     assert loaded[0].questions[0].question_id == single.question_id
+    # warn+drop 契约的 "warn" 半：悬挂孤儿必须告警，不静默丢弃
+    assert any("悬挂孤儿" in msg for msg in captured), "悬挂孤儿未告警（静默丢弃）"
 
 
 def test_read_raises_on_disk_corrupt_duplicate_role(tmp_path) -> None: