feat: loosen multi_true gate to qualifier-scoped correctness

store/prompts/question_gen/gate_multi_true.md

@@ -16,10 +16,17 @@ You are checking whether a multiple-choice question has **more than one plausibl
 
 ## Instructions
 
-1. Read the source material and the question carefully.
-2. For each option, assess whether it could be considered a correct or plausible answer given the source.
-3. If exactly ONE option is clearly correct → verdict "pass".
-4. If TWO or more options are plausibly correct → verdict "fail".
+1. Read the source material and the question carefully. Note the question's
+   explicit qualifiers (subject, timing, manner, object).
+2. For each option, assess whether it is **fully correct under the question's
+   qualifiers** — not merely whether it has some partial or local support in
+   the source.
+3. A good hard-negative distractor MAY have partial/local evidence in the video
+   yet be FALSE under the question's constraints. Such an option is NOT a
+   second correct answer — do NOT fail the question for it.
+4. Verdict "fail" ONLY IF two or more options are each fully correct under the
+   question's qualifiers (a genuine ambiguity).
+5. Otherwise verdict "pass".
 
 ## Response Format (strict JSON)
 

tests/unit/test_gate_multi_true_rubric.py

@@ -0,0 +1,13 @@
+"""multi_true rubric 已松绑为题干限定判据。"""
+
+from pathlib import Path
+
+_PROMPT = Path(__file__).resolve().parents[2] / "store" / "prompts" / "question_gen" / "gate_multi_true.md"
+
+
+def test_rubric_uses_qualifier_constraint():
+    text = _PROMPT.read_text(encoding="utf-8")
+    # 新 rubric 必须提到"题干限定下同时为真才 fail"（qualifier / under the question's constraints）
+    assert "under the question" in text.lower() or "qualifier" in text.lower()
+    # 必须显式放行"有局部真实证据但在限定下为假"的干扰项
+    assert "partial" in text.lower() or "locally" in text.lower()